1. Chapter 30: Comparing Invertebrates
Section 1: Evolution of the Invertebrates

2. Evolution of the Invertebrates
The evolutionary relationships between different groups of organisms can be shown in the form of a diagram called a phylogenetic tree
Shows our best understanding of which phyla originate from a common ancestor and approximately when evolutionary lines diverged

3. Evolution of the Invertebrates
The base of the tree represents the common ancestor of all the groups shown on the tree
Branches that originate close to the bottom of the tree represent groups that evolved long ago
Branches that originate near the top of the tree represent groups that evolved relatively recently
The tips of the branches represent living groups
Some phylogenetic trees show “dead” branches that do not reach the outside of the tree
Dead branches represent extinct evolutionary lines
There are no living groups from these lines

4. Evolution of the Invertebrates
There are several major branches on a phylogenetic tree
Figure 30 – 3
Protostomes, deuterostomes, acoelomates, pseudocoelomates, and coelomates
These branches represent basic evolutionary lines in animals with bilateral symmetry

5. Evolution of the Invertebrates
The division of animals into deuterostomes and protostomes is based on events in early development
The division of animals into acoelomates, pseudocoelomates, and coelomates is based on the structure of the body cavity

6. Early Development
Protostomes include flatworms, roundworms, annelids, mollusks, arthropods, and the members of most of the minor invertebrate phyla
Deuterostomes include echinoderms, several small phyla of strange-looking marine animals we have not discussed, and all members of our own phylum, Chordata
To understand the reasons for dividing animals into protostomes and deuterostomes, we must examine the earliest stages in the development of animals

7. Early Development
Soon after an egg has been fertilized, it begins a series of divisions
These divisions lead first to a two-cell stage and then to a four-cell stage
When the embryo grows from four cells to eight cells, the new cells can be arranged in different ways
In spiral cleavage, which occurs in almost all protostomes, the four new cells sit in between the four older cells
In radial cleavage, which occurs in almost all deuterostomes, the four new cells sit directly on top of the four older cells

8. Early Development
In both protostomes and deuterostomes, the cells of the embryo continue to divide until they form a hollow ball
Then the ball becomes flattened on one side and folds in on itself
The layer of cells on the outside of the ball is called the ectoderm
The layer of cells that has folded inside the ball is called the endoderm
Both the endoderm and ectoderm eventually develop into several different kinds of tissue

9. Early Development
The round central cavity enclosed by the endoderm will become the digestive tract of the developing embryo
The opening of this cavity to the outside is called the blastopore
It is the blastopore that determines whether an animal is a protostome or a deuterostome
If the blastopore becomes the mouth, the animal is a protostome
If the blastopore becomes the anus and an opening that appears later becomes the mouth, the animal is a deuterostome

10. Early Development
There is a third cell layer in embryos, called the mesoderm, which is located between the endoderm and the ectoderm
Many important tissues, including muscles, develop from the mesoderm

11. Body Cavities Body cavities are important for several reasons
Provide a space in which internal organs can be suspended so that they are not pressed on by muscles and twisted out of shape by body movements
Allow room for internal organs to develop and expand
Contain fluids that may be involved with internal transport, or the carrying of food, wastes, and other materials from one part of the body to another

12. Body Cavities
Some phyla, such as flatworms, have no body cavity at all
Acoelomates
Other phyla, such as roundworms, have a body cavity that is partially lined with mesoderm
Pseudocoelomates
Still other phyla have a true coelom, or body cavity that is completely lined with mesoderm
Coelomates
More advanced than the other two
The complete mesoderm lining makes it possible for the digestive tract to develop specialized regions and organs, allows for the formation of blood vessels, and makes it easier for complex organ systems to develop

13. Chapter 30: Comparing Invertebrates
Section 2: Form and Function in Invertebrates

14. Form and Function in Invertebrates
Each animal phylum represents an experiment in the design of body structures to perform the tasks necessary for survival
The appearance of each phylum in the fossil record represents the random evolutionary development of a basic body plan that is different in some way from other body plans
Evolution is random and undirected

15. Form and Function in Invertebrates
Organisms are not better or worse than one another – they are simply different
The body systems that perform the vital functions of life have taken many different forms in different phyla
Some are complex, others are simple
Some are efficient, others are not
More complicated and efficient systems are not necessarily “better” than simpler systems

16. Movement
Almost all animals use specialized tissues called muscles to move
Without muscles, animals could not swim, fly, burrow, or run
Muscles work only by contracting
When muscles are stimulated, they generate force by getting shorter
When they are not stimulated, they relax
In most animals, muscles work together with some sort of skeletal system that provides firm support
There are three main kinds of skeletal systems: hydrostatic skeletons, exoskeletons, and endoskeletons

17. Hydrostatic Skeletons
Hydrostatic skeletons do not contain hard structures, such as bones or chitin plates, for muscles to pull against
Instead, the muscles surround and are supported by a water-filled body cavity
When the muscles contract, they push against the water in the body cavity
Cnidarians, some flatworms, roundworms, some mollusks, and annelids have hydrostatic skeletons

18. Exoskeletons
Exoskeletons usually refer to the hard nonliving coating that encloses an arthropod’s internal organs and muscles
However, the shells of mollusks can also be considered exoskeletons
Muscles attached to the inside of an arthropod’s exoskeleton are used to bend and straighten the joints
Muscles attached to the shell in mollusks make it possible for snails to withdraw into their shell and for bivalves to close their two-part shell

19. Endoskeletons
Endoskeletons are frameworks located inside the body of animals
Sponges, echinoderms, and vertebrates have endoskeletons
Animals with endoskeletons typically have muscles that attach to the outside surface of the endoskeleton

20. Feeding
As you move through the invertebrate phyla from simpler animals such as sponges to more complex animals such as arthropods, you can observe three major evolutionary trends
First, simpler animals such as sponges, cnidarians, and flatworms break down their food primarily through intracellular digestion
More complex animals use extracellular digestion

21. Feeding
In intracellular digestion food is digested, or broken down, inside the cells
In extracellular digestion, food is broken down outside the cells – specifically, in a digestive tract
Mollusks, annelids, arthropods, echinoderms, and chordates typically rely on extracellular digestion

22. Feeding
Second, cnidarians and some flatworms have a simple digestive system that has a single opening through which food enters and through which solid wastes are expelled
More advanced digestive systems, such as those found in roundworms, mollusks, annelids, arthropods, echinoderms, and chordates, have two openings – a mouth at one end and an anus at the other

23. Feeding
Third, the digestive tract tends to acquire more and more specialized regions
The digestive system is not the only system to become more specialized as you move from simpler animals to more complex animals
This evolutionary trend is seen in most of the other systems responsible for performing essential life functions

24. Internal Transport
All cells of multicellular animals must be supplied with oxygen and nutrients and must dispose of metabolic wastes
The smallest and thinnest multicellular animals manage to fulfill their internal transport needs through diffusion between their body surface and the environment
Most complex multicellular animals have a collection of pumps and tubes called a circulatory system
There are two basic types of circulatory systems: open and closed

25. Respiration
In order to supply oxygen to and remove carbon dioxide from their tissues, animals must exchange these gases with the environment
Two features are common to all respiratory systems
First, they almost always have structures that maximize the amount of surface area in contact with air or water
Second, they have some way of keeping the gas exchange surfaces moist so that diffusion can occur

26. Respiration
Some animals that live in water or in very moist soil, such as cnidarians and flatworms, respire through their skin
Aquatic organisms – mollusks, crustaceans, some insects, and many annelids, for example – have gills that help them exchange gases with the water around them
Terrestrial invertebrates have evolved several organs for breathing air
These include the highly modified mantle cavities of land snails, the book lungs of spiders, and the tracheal tubes of insects

27. Excretion
Multicellular animals, whether they live in water or on land, must control the amount of water in their tissues
At the same time, all animals must get rid of toxic nitrogenous wastes produced as a result of cellular metabolism
Excretory systems in invertebrates have evolved in ways that enable these animals to both regulate the amount of water in the body and get rid of nitrogenous wastes

28. Excretion
In all animals, the breakdown of amino acids during cellular metabolism produces ammonia
Many aquatic animals simply allow ammonia to diffuse through their body tissues and out into the surrounding water, which immediately dilutes it and carries it away

29. Excretion
Terrestrial animals must do two things: conserve body water and get rid of nitrogenous wastes at the same time
In order to do this, many invertebrates convert ammonia into urea
Urea is soluble in water and is much less toxic than ammonia
The waste product produced by the excretory system, which is called urine, is expelled from the body
Terrestrial animals can get rid of more wastes in less water than their aquatic counterparts

30. Excretion Sponges, cnidarians, and roundworms
Diffusion through body surfaces
Freshwater flatworms
Flame cells
Insects and some arachnids
Malpighian tubules
Annelids, mollusks, and chordates
Nephridia

31. Response
Nervous systems gather information from the environment, process information, and allow animals to respond to it
Invertebrates show three obvious trends in the evolution of the nervous system: centralization, cephalization, and specialization

32. Response Cnidarians and some flatworms Nerve nets Other flatworms
Ganglia
Mollusks and arthropods
Ganglia are organized into a brain

33. Reproduction and Development
Many simple invertebrates reproduce asexually through fragmentation or budding
Asexual reproduction allows animals to produce offspring rapidly from a single individual
Sexual reproduction maintains genetic diversity in a population
Although sexual reproduction does not create new genes, it does result in new combinations of genes
Most of the more complex animals reproduce sexually

34. Fertilization
There are two basic ways in which sperm cells and egg cells are brought together in sexual reproduction: external fertilization and internal fertilization
External fertilization is generally associated with less complex animals
The eggs are fertilized outside the body
Internal fertilization is associated with more complex animals
The egg is fertilized inside the female’s body

35. Parental Care
Many invertebrates do not take care of their fertilized eggs or young
The eggs are ignored as soon as they are laid
Most of the young are eaten or are exposed to adverse environmental conditions and die
Some invertebrates take care of their offspring

36. Parental Care
Some of the ways in which invertebrates care for their offspring may seem horrifying to humans
For example, the eggs of some species of mites hatch within the female’s body
The larvae immediately begin to devour their mother from the inside!
Within two days – while still inside their mother’s nearly empty exoskeleton – the young mites mature, mate, and eat their way to the outside
The males die within a few hours
The females seek out prey in the form of insect eggs and begin to feed – even as their own offspring start chewing on their internal organs