The Evolution of Animals

The Evolution of Animals
Chapter 17 The Evolution of Animals

What Is an Animal? Animals are:
Eukaryotic Multicellular Heterotrophic organisms that obtain nutrients by ingestion Able to digest their food within their bodies Animal cells lack the cell walls that provide strong support in the bodies of plants and fungi. Student Misconceptions and Concerns 1. When asked to think about animals in general, students are typically biased to think of vertebrate examples. As an exercise to generate thought and attention, consider starting your lectures on animal diversity (at the very beginning of the first class on that subject) by asking students to write down the name of the first type of animal that comes to mind. Don’t give it any thought, just what first comes to mind, and then pass it in. Depending upon the size of your class, either tabulate their responses quickly or have students raise their hands depending upon what they indicated. In general, fewer than 5% (often fewer than 1%) of my students have indicated an invertebrate. This exercise makes the point that what we tend to think of, when we think of animals, is vertebrates. Many of us have had a dog, cat, or other vertebrate for a pet. Yet, more than 95% of all known species of animals are invertebrates. The challenge to the students is to expand their mental image and expectations of what it means to be an animal. The content in Chapter 17 should facilitate that understanding. 2. Students struggle to think through the requirements of invertebrates, and to consider them as animals with basic needs. It is intellectually challenging to see the similarities between an earthworm and a bird or a tick and a cat. The common features of animals addressed at the start of Chapter 17 can be expanded to include the needs for oxygen, nourishing food, a tolerable environment, and a suitable habitat to reproduce, applied to animals representing all of the major phyla. Illustrating these common demands can help to build the intellectual foundations that can be so difficult to fully comprehend. Teaching Tips 1. Depending upon what chapters you have included to this point in your course, you might consider carefully examining the defining traits common to all animals. Consider challenging your students to identify at least one characteristic of each of the other kingdoms that is distinctly different from animals. 2. You might wish to share the now somewhat famous quote of Lewis Wolpert, who in 1986 said, it is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life. The development and arrangement of the basic embryonic layers (ectoderm forming skin & nervous system, mesoderm forming muscle & bone, and endoderm forming the digestive tract) establishes the basic body plan. 3. Your students might enjoy discussing whether or not they are larvae and if they can be said to go through metamorphosis. (No, to both questions.) 4. Before addressing the subject of animal symmetry, you might wish to have your students speculate about the adaptive advantages of radial or bilateral symmetry found in animals. This sort of comparison raises an opportunity to make some larger points about biology. There is no one best animal. Each form, each adaptation, each body plan has advantages and disadvantages. The value of adaptations is relative to the organism’s environment and most adaptations represent a compromise. 5. The web site of the University of California Museum of Paleontology is an excellent resource in support of evolution and the history of life. The following portion of that web site specifically addresses the Cambrian period. (

Figure 17.5 Sponges No true tissues Cnidarians Radial symmetry
Ancestral protist Molluscs Flatworms Tissues Annelids Roundworms Figure 17.5 An overview of animal phylogeny based on body features and genetic data Arthropods Bilateral symmetry Echinoderms Chordates Figure 17.5

A second major evolutionary split is based on body symmetry.
Radial symmetry refers to animals that are identical all around a central axis. Bilateral symmetry exists where there is only one way to split the animal into equal halves. Student Misconceptions and Concerns 1. When asked to think about animals in general, students are typically biased to think of vertebrate examples. As an exercise to generate thought and attention, consider starting your lectures on animal diversity (at the very beginning of the first class on that subject) by asking students to write down the name of the first type of animal that comes to mind. Don’t give it any thought, just what first comes to mind, and then pass it in. Depending upon the size of your class, either tabulate their responses quickly or have students raise their hands depending upon what they indicated. In general, fewer than 5% (often fewer than 1%) of my students have indicated an invertebrate. This exercise makes the point that what we tend to think of, when we think of animals, is vertebrates. Many of us have had a dog, cat, or other vertebrate for a pet. Yet, more than 95% of all known species of animals are invertebrates. The challenge to the students is to expand their mental image and expectations of what it means to be an animal. The content in Chapter 17 should facilitate that understanding. 2. Students struggle to think through the requirements of invertebrates, and to consider them as animals with basic needs. It is intellectually challenging to see the similarities between an earthworm and a bird or a tick and a cat. The common features of animals addressed at the start of Chapter 17 can be expanded to include the needs for oxygen, nourishing food, a tolerable environment, and a suitable habitat to reproduce, applied to animals representing all of the major phyla. Illustrating these common demands can help to build the intellectual foundations that can be so difficult to fully comprehend. Teaching Tips 1. Depending upon what chapters you have included to this point in your course, you might consider carefully examining the defining traits common to all animals. Consider challenging your students to identify at least one characteristic of each of the other kingdoms that is distinctly different from animals. 2. You might wish to share the now somewhat famous quote of Lewis Wolpert, who in 1986 said, it is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life. The development and arrangement of the basic embryonic layers (ectoderm forming skin & nervous system, mesoderm forming muscle & bone, and endoderm forming the digestive tract) establishes the basic body plan. 3. Your students might enjoy discussing whether or not they are larvae and if they can be said to go through metamorphosis. (No, to both questions.) 4. Before addressing the subject of animal symmetry, you might wish to have your students speculate about the adaptive advantages of radial or bilateral symmetry found in animals. This sort of comparison raises an opportunity to make some larger points about biology. There is no one best animal. Each form, each adaptation, each body plan has advantages and disadvantages. The value of adaptations is relative to the organism’s environment and most adaptations represent a compromise. 5. The web site of the University of California Museum of Paleontology is an excellent resource in support of evolution and the history of life. The following portion of that web site specifically addresses the Cambrian period. (

Radial symmetry. Parts radiate from the center, so any slice
through the central axis divides into mirror images. Figure 17.6 Body symmetry Bilateral symmetry. Only one slice can divide left and right sides into mirror-image halves. Figure 17.6

There are differences in how the body cavity forms.
Animals also vary according to the presence and type of body cavity, a fluid-filled space separating the digestive tract from the outer body wall. There are differences in how the body cavity forms. If the body cavity is not completely lined by tissue derived from mesoderm, it is a pseudocoelom. A true coelom is completely lined by tissue derived from mesoderm. Student Misconceptions and Concerns 1. When asked to think about animals in general, students are typically biased to think of vertebrate examples. As an exercise to generate thought and attention, consider starting your lectures on animal diversity (at the very beginning of the first class on that subject) by asking students to write down the name of the first type of animal that comes to mind. Don’t give it any thought, just what first comes to mind, and then pass it in. Depending upon the size of your class, either tabulate their responses quickly or have students raise their hands depending upon what they indicated. In general, fewer than 5% (often fewer than 1%) of my students have indicated an invertebrate. This exercise makes the point that what we tend to think of, when we think of animals, is vertebrates. Many of us have had a dog, cat, or other vertebrate for a pet. Yet, more than 95% of all known species of animals are invertebrates. The challenge to the students is to expand their mental image and expectations of what it means to be an animal. The content in Chapter 17 should facilitate that understanding. 2. Students struggle to think through the requirements of invertebrates, and to consider them as animals with basic needs. It is intellectually challenging to see the similarities between an earthworm and a bird or a tick and a cat. The common features of animals addressed at the start of Chapter 17 can be expanded to include the needs for oxygen, nourishing food, a tolerable environment, and a suitable habitat to reproduce, applied to animals representing all of the major phyla. Illustrating these common demands can help to build the intellectual foundations that can be so difficult to fully comprehend. Teaching Tips 1. Depending upon what chapters you have included to this point in your course, you might consider carefully examining the defining traits common to all animals. Consider challenging your students to identify at least one characteristic of each of the other kingdoms that is distinctly different from animals. 2. You might wish to share the now somewhat famous quote of Lewis Wolpert, who in 1986 said, it is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life. The development and arrangement of the basic embryonic layers (ectoderm forming skin & nervous system, mesoderm forming muscle & bone, and endoderm forming the digestive tract) establishes the basic body plan. 3. Your students might enjoy discussing whether or not they are larvae and if they can be said to go through metamorphosis. (No, to both questions.) 4. Before addressing the subject of animal symmetry, you might wish to have your students speculate about the adaptive advantages of radial or bilateral symmetry found in animals. This sort of comparison raises an opportunity to make some larger points about biology. There is no one best animal. Each form, each adaptation, each body plan has advantages and disadvantages. The value of adaptations is relative to the organism’s environment and most adaptations represent a compromise. 5. The web site of the University of California Museum of Paleontology is an excellent resource in support of evolution and the history of life. The following portion of that web site specifically addresses the Cambrian period. (

(a) No body cavity (b) Pseudocoelom (c) True coelom Figure 17.7
Body covering (from ectoderm) Tissue-filled region (from mesoderm) (a) No body cavity Digestive tract (from endoderm) Body covering (from ectoderm) (b) Pseudocoelom Muscle layer (from mesoderm) Pseudocoelom Digestive tract (from endoderm) Figure 17.7 Body plans of bilateral animals (c) True coelom Coelom Body covering (from ectoderm) Tissue layer lining coelom and suspending internal organs (from mesoderm) Digestive tract (from endoderm) Figure 17.7

MAJOR INVERTEBRATE PHYLA
Invertebrates: Are animals without backbones Represent 95% of the animal kingdom Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Sponges Sponges include sessile animals that lack true tissues and that were once believed to be plants. Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Figure 17.8b Anatomy of a sponge: photo

Cnidarians Cnidarians (phylum Cnidaria) are characterized by:
The presence of body tissues Radial symmetry Tentacles with stinging cells The basic body plan of a cnidarian is a sac with a gastrovascular cavity, a central digestive compartment with only one opening. The body plan has two variations: The sessile polyp The floating medusa Cnidarians are carnivores that use tentacles, armed with nematocysts (or cnidocytes) (“stinging cells”), to capture prey. Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Figure 17.9 Mouth/anus Tentacle Gastrovascular cavity Polyp form Coral
Hydra Sea anemone Gastrovascular cavity Figure 17.9 Polyp and medusa forms of cnidarians Mouth/anus Tentacle Medusa form Jelly Figure 17.9

Tentacle Capsule Coiled thread Trigger Discharge of thread Prey
Figure Cnidocyte action Discharge of thread Prey Cnidocyte Figure 17.10

Molluscs Molluscs (phylum Mollusca) are represented by soft-bodied animals, usually protected by a hard shell. Many molluscs feed by using a file-like organ called a radula to scrape up food. The body of a mollusc has three main parts: A muscular foot used for movement A visceral mass housing most of the internal organs A mantle, which secretes the shell if present Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Visceral mass Mantle Foot
Coelom Reproductive organs Kidney Heart Digestive tract Mantle Shell Mantle cavity Radula Anus Gill Mouth Digestive tract Radula Foot Nerve cords Figure The general body plan of a mollusc Mouth Figure 17.11

The three major groups of molluscs are:
Gastropods, protected by a single, spiraled shell Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Gastropods Snail (spiraled shell) Sea slug (no shell) Figure 17.12a
Figure 17.12a Mollusc diversity: gastropods Sea slug (no shell) Figure 17.12a

Bivalves, with a shell divided into two halves hinged together
Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Bivalves (hinged shell)
Figure 17.12b Mollusc diversity: bivalves Scallop Figure 17.12b

And cephalopods Typically lacking an external shell
Built for speed and agility Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

(large brain and tentacles)
Cephalopods (large brain and tentacles) Figure 17.12c Mollusc diversity: cephalopods Octopus Squid Figure 17.12c

MAJOR GROUPS OF MOLLUSCS
Gastropods Bivalves (hinged shell) Cephalopods (large brain and tentacles) Snail (spiraled shell) Figure Mollusc diversity Scallop Octopus Squid Sea slug (no shell) Figure 17.12

Flatworms Flatworms (phylum Platyhelminthes) are the simplest bilateral animals. Flatworms include forms that are: Parasites or Free-living in marine, freshwater, or damp habitats Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Digestive tract (gastrovascular cavity) Nerve cords Mouth Eyespots
(detect light) Blood fluke Nervous tissue clusters (simple brain) Bilateral symmetry Planarian Reproductive unit with skin removed Hooks Head Figure Flatworm diversity Suckers Tapeworm Figure 17.13

Annelids Annelids (phylum Annelida) have:
Body segmentation, a subdivision of the body along its length into a series of repeated parts A coelom A complete digestive tract with Two openings, a mouth and anus One-way movement of food Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Anus Brain Main heart Coelom Digestive tract Segment Mouth walls
Figure Segmented anatomy of an earthworm Accessory hearts Nerve cord Waste disposal organ Blood vessels Figure 17.15

The three main groups of annelids are:
Earthworms, which eat their way through soil Polychaetes, marine worms with segmental appendages for movement and gas exchange Leeches, typically free-living carnivores but with some bloodsucking forms Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

MAJOR GROUPS OF ANNELIDS Earthworms Polychaetes Leeches
Figure Annelid diversity Giant Australian earthworm Christmas tree worm European freshwater leech Figure 17.14

Roundworms Roundworms (phylum Nematoda) are:
Cylindrical in shape, tapered at both ends The most diverse and widespread of all animals Roundworms (also called nematodes) are: Important decomposers Dangerous parasites in plants, humans, and other animals Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Infected with parasitic roundworms
(a) A free-living roundworm (b) Parasitic roundworms in pork (c) Canine heart Infected with parasitic roundworms Figure Roundworm diversity Figure 17.16

Arthropods Arthropods (phylum Arthropoda) are named for their jointed appendages. There are about one million arthropod species identified, mostly insects. Arthropods are a very diverse and successful group, occurring in nearly all habitats in the biosphere. There are four main groups of arthropods. Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

MAJOR GROUPS OF ARTHROPODS
Arachnids Crustaceans Millipedes and Centipedes Figure Arthropod diversity Insects Figure 17.17

General Characteristics of Arthropods
Arthropods are segmented animals with specialized segments and appendages for an efficient division of labor among body regions. The body of arthropods is completely covered by an exoskeleton, an external skeleton that provides: Protection Points of attachment for the muscles that move appendages Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Figure 17.18 Cephalothorax (head and thorax) Abdomen Antenna
(sensory reception) Eyes on movable stalks Mouthparts (feeding) Walking leg Swimming appendage Figure Anatomy of a lobster, a crustacean Walking legs Figure 17.18

Arachnids: Arachnids Live on land
Usually have four pairs of walking legs and a specialized pair of feeding appendages Include spiders, scorpions, ticks, and mites Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Two feeding appendages Leg (four pairs) Scorpion Dust mite
Figure Arachnid characteristics and diversity Scorpion Dust mite Black widow spider Wood tick Figure 17.19

Crustaceans: Crustaceans Are nearly all aquatic
Have multiple pairs of specialized appendages Include crabs, lobsters, crayfish, shrimps, and barnacles Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Figure 17.20 Two feeding appendages Leg (three or more pairs) Antennae
Figure Crustacean characteristics and diversity Crab Pill bug Shrimp Crayfish Barnacles Figure 17.20

Millipedes and Centipedes
Millipedes and centipedes have similar segments over most of the body. Millipedes: Eat decaying plant matter Have two pairs of short legs per body segment Centipedes: Are terrestrial carnivores with poison claws Have one pair of short legs per body segment Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

One pair of legs per segment
Two pairs of legs per segment Figure Millipede and centipede Millipede Centipede Figure 17.21

Insects typically have a three-part body:
Insect Anatomy Insects typically have a three-part body: Head Thorax Abdomen The insect head usually bears: A pair of sensory antennae A pair of eyes The mouthparts are adapted for particular kinds of eating. Flight is one key to the great success of insects. Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Head Thorax Abdomen Mouthparts
Antenna Eye Figure Anatomy of a grasshopper, an insect Mouthparts Figure 17.22

Insects outnumber all other forms of life combined. Insects live in:
Insect Diversity Insects outnumber all other forms of life combined. Insects live in: Almost every terrestrial habitat Freshwater The air Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Leaf roller Banded Orange Heliconian Praying mantis Giraffe weevil
Figure Insect diversity Giraffe weevil Peacock katydid Yellow jacket wasp Leaf beetle Longhorn beetle Figure 17.23

Many insects undergo metamorphosis in their development.
Young insects may: Appear to be smaller forms of the adult or Change from a larval form to something much different as an adult Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Figure 17.24-5 The larva (caterpillar) spends
its time eating and growing, molting as it grows. After several molts, the larva becomes a pupa encased in a cocoon. Within the pupa, the larval organs break down and adult organs develop from cells that were dormant in the larva. Figure Metamorphosis of a monarch butterfly (Step 5) Finally, the adult emerges from the cocoon. The butterfly flies off and reproduces, nourished mainly by calories stored when it was a caterpillar. Figure

Figure 17.24 Monarch butterflies
Figure 17.24a

Echinoderms Echinoderms (phylum Echinodermata):
Lack body segments Typically show radial symmetry as adults but bilateral symmetry as larvae Have an endoskeleton Have a water vascular system that facilitates movement and gas exchange Echinoderms are a very diverse group. Student Misconceptions and Concerns 1. Students typically expect that every animal has a head. The subject of a head might have already been introduced when discussing body plans. As you proceed through the animal phyla, have students consider how sponges and cnidarians meet their basic needs without the benefit of a well-defined head. 2. As students survey the major animal phyla, they might perceive the diversity of animals as spread somewhat evenly across the nine major phyla of invertebrates. Yet, two thirds of all known species of life (and at least 80% all described animal species) are arthropods. Although the number of described species of life varies by the source of information, there is widespread agreement that the number of undescribed species is many times more than the number of known species. By examining the number of described species, some amazing proportions emerge. You might consider this exercise to make the point. Calculate quickly the number of students attending class and determine how many students represent 1% of the class. Then have the entire class stand. Tell the students that collectively, they represent all of the animal species known to exist today. Have 5% of the class sit, representing the proportion of known animal species that are vertebrates. Next, have an additional 15% of the class sit, representing all of the remaining types of animals except arthropods. At this point, everyone standing (80% of the class) represents the proportion of known animal species that are arthropods (a conservative fraction). Finally, have an additional 50% of the entire class sit, leaving 30% of the entire class still standing. Have the class guess what group of animals is represented now (clearly some subgroup of arthropods). This final 30% represents the known number of species of beetles! (The numbers used are rounded. Even higher percentages for beetles and invertebrates may be more accurate. The point of this exercise is the relative proportions and not precise percentages.) Teaching Tips 1. Imagine a country cabin with a roaring fire in a fireplace. The windows are partially opened to permit air to rush into the house to feed the fire. This flow of air through the windows, through the home, to the fireplace, and then out the chimney is analogous to the flow of water through a sponge. Side note: Many sponges, especially commercial bath sponges, have outer body walls that are highly folded. Simpler vase sponges, with straighter walls, are good examples to show students when discussing this basic water flow pattern. 2. Students may be challenged to fully appreciate why sponges are animals. It might be helpful to return to the distinctions between animals, plants, and fungi when introducing sponges. 3. Just about any glass bottle with a narrow neck makes a good model of the cnidarian body plan. The narrowed neck of the body represents the constricted region of a cnidarian that regulates what enters and leaves the gastrovascular cavity. 4. The name gastrovascular represents the dual function of the cavity. It must serve as the site of digestion and the method of delivery of the nutrients. The cavity extends, like a primitive circulatory system, throughout the body—thus vascular, in nature. 5. If students have been stung by a jellyfish, it was a toxin on the cnidocytes that caused the reaction. (Nematocysts are the firing part of the cnidocyte cell. The toxin is delivered by the firing of the nematocysts.) 6. When we eat clams, we usually eat strips of the muscular foot. If the clams are mushy and / or contain sand, they may also include portions of the intestines. 7. A simple demonstration of the bivalve body plan can be obtained by purchasing smoked oysters in a grocery store. Each smoked oyster (ready to be consumed) is the soft body removed from the paired shells. For a startling demonstration, discuss the anatomy of a specimen and then enjoy it for lunch. 8. When discussing cephalopods, the subject of animal intelligence may be noted. Defining and identifying animal intelligence is not a simple task. Consider discussing the correlation of intelligence and predation. Why might these two traits be linked? What other characteristics correlate with intelligence (perhaps sophistication of communication)? 9. A planarian, indicated in Figure 17.13, ingests food in the middle of its body. Challenge students to think of other animals that have a mouth located distantly from the head. The discussion that will follow this challenge will likely include the defining features of a head. Such challenges as this invoke critical analysis of animal body plans, an important exercise for this chapter. 10. The undercooking of meat contributes to the spread of parasitic diseases. If you addressed the denaturation of proteins in Chapter 3, here is a chance to reinforce the points about the effects of heat. Proteins denature and discolor when heated (pink meat turns brown). This same process denatures proteins in parasites and can kill them (although some parasites can survive relatively high temperatures). If any portion of a steak is still pink, the meat has not been cooked enough to denature the meat or parasitic proteins. 11. Annelids and nematodes have the advantage of a digestive tract with openings at both ends. This permits the efficiency and specialization of the digestive tract for one-way flow of ingested materials. Assembly lines enjoy this same advantage, but in the reverse process of construction, using a sort of disassembly line. These worms (and all animals with a mouth separate from the anus) disassemble food as it moves through the digestive tract. 12. Conceptually, and very generally speaking, the exoskeleton of an arthropod is like the hard outside of an M & M. The exoskeleton prevents physical damage to the internal anatomy and prevents desiccation. The outside of an M & M prevents damage to the chocolate and keeps the chocolate from melting or drying out. 13. Many students assume that all insects possess six legs and two pairs of wings. As common examples of variation on this theme, challenge students with images of beetles (in which the outer wings, called elytra, are hardened) and flies (in which one pair of wings is reduced to a pair of structures called halteres). 14. Beetles have a long history in human culture, used in textiles, ornamentation, and jewelry. A quick image search of the Internet will reveal many examples. In particular, note the significance of scarab beetles in ancient Egypt. 15. The bodies of adult echinoderms reveal degrees of radial symmetry, which may cause students to wonder why echinoderms are not grouped with cnidarians. As discussed in the text, the bilateral symmetry of embryonic echinoderms demonstrates the acquisition of radial symmetry from a bilaterally symmetrical ancestor. Thus, echinoderms are more closely associated with other bilaterally symmetrical organisms. 16. Radial symmetry, such as that seen in many adult echinoderms, permits an organism to respond well in any direction. You cannot sneak up behind their back. This ability is especially adaptive in sedentary or relatively sedentary organisms, such as sea urchins, sea stars, and cnidarians. Ask your students if these same advantages also occur in radially symmetrical plants.

Sea star Tube feet Sea urchin Sea cucumber Sand dollar Figure 17.25
Figure Echinoderm diversity Sea cucumber Sand dollar Figure 17.25

The Evolution of Animals

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