2. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Snails, slugs, oysters, clams, scallops, octopuses, and squids are all mollusks. Mollusks and annelids share a feature during the larval stage called a trochophore which develops from the fertilized egg. Section 1 Mollusks Chapter 29 Phylum Mollusca

3. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu In some species, the trochophore is free- swimming and propels itself through the water by movement of cilia on its surface. The presence of a trochophore larva in mollusks and annelids suggests that they share a common ancestor. Section 1 Mollusks Chapter 29

4. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks 1.The body cavity in mollusks is a true coelom, although in most species it is reduced to a small area immediately surrounding the heart. 2.Most mollusks exhibit bilateral symmetry. 3.Mollusks have organ systems for excretion, circulation, respiration, digestion, and reproduction. Section 1 Mollusks Chapter 29

5. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks 4. The body of every mollusk has three distinct parts: the visceral mass, the mantle, and the muscular foot. Section 1 Mollusks Chapter 29

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks The visceral mass is a central section that contains the mollusk’s organs. The mantle is a heavy fold of tissue that forms the outer layer of the body. Finally, every mollusk has a muscular region called a foot, which is used primarily for locomotion. Section 1 Mollusks Chapter 29

7. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks 5. Many mollusks have either one or two shells that serve as an exoskeleton, protecting their soft body. 6. All mollusks except bivalves have a radula, a tongue-like organ located in their mouth. The radula has thousands of pointed, backward-curving teeth arranged in rows. Section 1 Mollusks Chapter 29

8. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks Organ Systems: Excretion A mollusk’s coelom is a collecting place for waste- laden body fluids. The beating of cilia pulls the fluid from the coelom into tiny tubular structures called nephridia. The nephridia recover useful molecules (sugars, salts, and water) from the coelomic fluid. Section 1 Mollusks Chapter 29

9. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks Organ Systems: Circulation In a circulatory system, blood carries nutrients and oxygen to tissues and removes waste and carbon dioxide. Most mollusks have a three-chambered heart and an open circulatory system. Octopuses and squids are exceptions because they each have a closed circulatory system. Section 1 Mollusks Chapter 29

10. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks Organ Systems: Respiration Most mollusks respire with gills, which are located in the mantle cavity. Most terrestrial snails have no gills. Instead, the thin membrane that lines their empty mantle cavity functions like a primitive lung. Sea snails also lack gills, and gas exchange takes place directly through their skin. Section 1 Mollusks Chapter 29

11. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks Organ Systems: Reproduction Most species of mollusks have distinct male and female individuals, although some snails and slugs are hermaphrodites. Certain species of oysters and sea slugs are able to change from one sex to the other and back again. Many marine mollusks are moved from place to place as their trochophore larvae drift in the ocean currents. Section 1 Mollusks Chapter 29

12. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Body Plans of Mollusks Gastropods- Stomach footed Gastropods—snails and slugs—are primarily a marine group that has successfully invaded freshwater and terrestrial habitats. Most gastropods have a pair of tentacles on their head with eyes often located at the tips. Gastropods display varied feeding habits. Many are herbivores that scrape algae off rocks using their radula. Section 1 Mollusks Chapter 29

13. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Characteristics of Gastropods Section 1 Mollusks Chapter 29

14. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Body Plans of Mollusks Bivalves All bivalves have a two-part hinged shell. The valves, or shells, of a bivalve are secreted by the mantle. Two thick muscles, the adductor muscles, connect the valves. When these muscles are contracted, they cause the valves to close tightly. Bivalves are unique among the mollusks because they do not have a distinct head region or a radula. Section 1 Mollusks Chapter 29

15. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Body Plans of Mollusks Bivalves Many bivalves use their muscular foot to dig down into the sand. Sexual Reproduction occurs by gametes being released into open water. (external fertilization) Section 1 Mollusks Chapter 29

16. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Characteristics of Bivalves Section 1 Mollusks Chapter 29

17. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Body Plans of Mollusks Cephalopods - Head footed Squids, octopuses, cuttlefish, and nautiluses (only one to have an external shell) are all cephalopods. Most of their body is made up of a large head attached to tentacles. Cephalopods are the most intelligent of all invertebrates. They have a complex nervous system that includes a well-developed brain. The structure of a cephalopod eye is similar in many ways to that of a vertebrate eye, and some species have color vision. Section 1 Mollusks Chapter 29

18. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Characteristics of Cephalopods Section 1 Mollusks Chapter 29 Most have a small internal shell or no shell at all The Nautilus has only one external shell. Move by jet propulsion Most have a dark- colored foul tasting ink they can release when frightened.

19. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The First Segmented Animals Annelids are easily recognized by their segments, which are visible as a series of ringlike structures along the length of their body. Some of the segments are modified for specific functions, such as reproduction, feeding, or sensation. A well-developed cerebral ganglion, or primitive brain, is located in one anterior segment. Internal body walls, called septa, separate the segments of most annelids. Section 2 Annelids Chapter 29

20. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The First Segmented Animals Characteristics of Annelids 1.The fluid-filled coelom is large and is located entirely within the mesoderm. 2.The organ systems of annelids show a high degree of specialization and include a closed circulatory system and excretory structures called nephridia. 3.Most annelids have external bristles called setae. Some annelids also have fleshy appendages called parapodia. Section 2 Annelids Chapter 29

21. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Annelid Groups Marine Worms Marine segmented worms are members of class Polychaeta, the largest group of annelids. Polychaetes live in virtually all ocean habitats. A distinctive characteristic of polychaetes is the pair of fleshy, paddle-like parapodia that occur on most of their segments. The parapodia, which usually have setae, are used to swim, burrow, or crawl. Section 2 Annelids Chapter 29

22. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Annelid Groups Marine Worms Nereis, a polychaete worm, grasps its prey in its jaws, which open when it thrusts out its pharynx. Section 2 Annelids Chapter 29

23. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Annelid Groups Earthworms Earthworms and some related freshwater worms are members of the class Oligochaeta. Oligochaetes have no parapodia and only a few setae on each segment. Earthworms lack the distinctive head region and have no eyes. Their clitellum is involved in sexual reproduction. Earthworms are highly specialized scavengers. They literally eat their way through the soil, consuming their own weight in soil every day. Section 2 Annelids Chapter 29

24. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Benefits of Earthworms Their castings contain nutrients which are returned to the soil. They aerate the soil as they move and burrow through it. They break up the soil in which they live allowing plant roots to penetrate the soil.

25. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Anatomy of an Earthworm Section 2 Annelids Chapter 29

26. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Annelid Groups Hydrostatic Skeleton The fluid within the coelom of each body segment creates a hydrostatic skeleton that supports the segment. Each segment contains muscles that pull against this hydrostatic skeleton. Circular muscles wrap around the segment, while longitudinal muscles span its length. Section 2 Annelids Chapter 29

27. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Annelid Groups Class Hirudinea Leeches Leeches are the only members of class Hirudinea. Leeches lack both setae and parapodia. Most live in moist tropical countries Usually no more than 6cm long and are somewhat flattened. Most are freshwater that exist as external parasites few are marine or terrestrial ¼ are carnivores not parasites and feed on soft-bodied invertebrates such as snails, worms, and insect larva Drink blood and body fluids of their host Section 2 Annelids Chapter 29

28. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Hirudinea characteristics The body of a leech is flattened, and unlike other annelids, its segments are not separated internally. A leech has suckers at both ends of its body. Suckers at the anterior end attach the leech to the host. Suckers at the posterior end attach the leech to an object as it waits for a host to come by

29. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Leech Attachment Leeches penetrate the skin of their host in one of two ways Some have a muscular proboscis, a tubular organ that they force into the tissue of the host. Other leeches slice the tissue with razor-sharp jaws. The use a muscular pharynx to suck blood out of the area. Some can anesthetize the wound so the host does not know they have been bitten Leeches release an anti-clotting agent to prevent the hosts blood from clotting

30. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Feeding A meal can be as much as 10 times the weight of the leech It can take the leech up to 200 days to digest its food. A leech may need to feed only once a year