1. Overview of Animal Diversity
Chapter 32

2. General Features of Animals
Animals are the consumers of the Earth
They are a very diverse group
-However, they share major characteristics
-Are heterotrophs
-Are multicellular
-Have cells without cell walls
-Most are able to move

3. General Features of Animals
-Are very diverse in form and habitat
-Most reproduce sexually
-Have a characteristic pattern of embryonic development
-Cells of all animals (except sponges) are organized into tissues

7. Evolution of the Animal Body Plan
Five key transitions can be noted in animal evolution
1. Tissues
2. Symmetry
3. Body cavity
4. Development
5. Segmentation

8. Evolution of the Animal Body Plan
1. Evolution of tissues
-Parazoa (Sponges - the simplest animals) lack defined tissues and organs
-Have the ability to disaggregate and aggregate their cells
-Eumetazoa (all other animals) have distinct and well-defined tissues
-Have irreversible differentiation for most cell types

9. Evolution of the Animal Body Plan
2. Evolution of symmetry
-Sponges also lack any definite symmetry
-Eumetazoa have a symmetry defined along an imaginary axis drawn through the animal’s body
-There are two main types of symmetry

10. Evolution of the Animal Body Plan
-Radial symmetry
-Body parts arranged around central axis
-Can be bisected into two equal halves in any 2-D plane
-Bilateral symmetry
-Body has right and left halves that are mirror images
-Only the sagittal plane bisects the animal into two equal halves

11. Top
Back
Front
Bottom

12. Evolution of the Animal Body Plan
Bilaterally symmetrical animals have two main advantages over radially symmetrical ones
1. Cephalization
-Evolution of a definite brain area
2. Greater mobility

13. Evolution of the Animal Body Plan
3. Evolution of a body cavity
-Eumetazoa produce two or three germ layers
-Outer ectoderm
-Middle mesoderm (not found in Cnidaria & Ctenophora)
-Inner endoderm
Body cavity = Space surrounded by mesoderm tissue that is formed during development

14. Evolution of the Animal Body Plan
3. Evolution of a body cavity
-Three basic kinds of body plans
-Acoelomates = No body cavity
-Pseudocoelomates = Body cavity between mesoderm and endoderm
-Called the pseudocoelom
-Coelomates = Body cavity entirely within the mesoderm
-Called the coelom

16. Evolution of the Animal Body Plan
-The body cavity made possible the development of advanced organs systems
-Coelomates developed a circulatory system to flow nutrients and remove wastes
-Open circulatory system: blood passes from vessels into sinuses, mixes with body fluids and reenters the vessels
-Closed circulatory system: blood moves continuously through vessels that are separated from body fluids

17. Evolution of the Animal Body Plan
4. Evolution of different patterns of development
-The basic Bilaterian pattern of development:
-Mitotic cell divisions of the egg form a hollow ball of cells, called the blastula
-Blastula indents to form a two-layer- thick ball with:
-Blastopore = Opening to outside
-Archenteron = Primitive gut

18. Evolution of the Animal Body Plan
Bilaterians can be divided into two groups:
-Protostomes develop the mouth first from or near the blastopore
-Anus (if present) develops either from blastopore or another region of embryo
-Deuterostomes develop the anus first from the blastopore
-Mouth develops later from another region of the embryo

19. Evolution of the Animal Body Plan
Deuterostomes differ from protostomes in three other fundamental embryological features:
-1. Cleaveage pattern of embryonic cells
-Protostomes = Spiral cleavage
-Deuterostomes = Radial cleavage
-2. Developmental fate of cells
-Protostomes = Determinate development
-Deuterostomes = Indeterminate development

20. Evolution of the Animal Body Plan
-3. Origination of coelom
-Protostomes = schizocoely
-Deuterostomes = enterocoely
Deuterostomes evolved from protostomes more than 500 MYA

22. Evolution of the Animal Body Plan
5. Evolution of segmentation
-Segmentation provides two advantages
-1. Allows redundant organ systems in adults such as occurs in the annelids
-2. Allows for more efficient and flexible movement because each segment can move independently
Segmentation appeared several times in the evolution of animals

23. Traditional Classification of Animals
Multicellular animals, or metazoans, are traditionally divided into 36 or so distinct phyla based on shared anatomy and embryology
Metazoans are divided into two main branches:
-Parazoa = Lack symmetry and tissues
-Eumetazoa = Have symmetry and tissues
-Diploblastic = Have two germ layers
-Triploblastic = Have three germ layers

24. A New Look At Metazoans
The traditional animal phylogeny is being reevaluated using molecular data
Myzostomids are marine animals that are parasites of echinoderms
-Have no body cavity and only incomplete segmentation
-And so have been allied with annelids

25. A New Look At Metazoans
Recent analysis of the translation machinery revealed that myzostomids have no close link to the annelids at all
-Instead, they are more closely allied with the flatworms (planaria and tapeworms)

26. A New Look At Metazoans
Therefore, key morphological characters used in traditional classification are not necessarily conservative
Molecular systematics uses unique sequences within certain genes to identify clusters of related groups

27. A New Look At Metazoans
Most new phylogenies agree on two revolutionary features:
1. Separation of annelids and arthropods into different clades
2. Division of the protostome group into Ecdysozoa and Spiralia
-The latter is then broken down into Lophotrochozoa and Platyzoa

28. A New Look At Metazoans

29. Evolutionary Developmental Biology
Most taxonomists agree that the animal kingdom is monophyletic
Three prominent hypotheses have been proposed for the origin of metazoans from single-celled protists

30. Evolutionary Developmental Biology
1. The multinucleate hypothesis
2. The colonial flagellate hypothesis
3. The polyphyletic origin hypothesis
Molecular systematics using rRNA sequences settles this argument in favor of the colonial flagellate hypothesis

31. Evolutionary Developmental Biology
Molecular analysis may also explain the Cambrian explosion
-The enormous expansion of animal diversity in the Cambrian period (543 to 525 MYA)
-The homeobox (Hox) developmental gene complex evolved
-Provided a tool that can produce rapid changes in body plan

32. Evolutionary Developmental Biology