1. Overview of Animal Diversity

2. General Features of Animals
Animals are a diverse group of consumers that share major characteristics
All are heterotrophs
All are multicellular
Cells do not have cell walls
Most are able to move
All are very diverse in form and habitat
Most reproduce sexually
Have a characteristic patterns of embryonic development
Cells of all animals (except sponges) are organized into tissues

3. Evolution of the Animal Body Plan
Five key transitions in animal evolution
Tissues
Symmetry
Body cavity
Development
Segmentation

4. 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

5. Evolution of the Animal Body Plan
2. Evolution of symmetry
Parazoa (sponges) lack any definite symmetry
Eumetazoa have a symmetry defined along imaginary axes drawn through the animal’s body
There are two main types of symmetry
Radial symmetry
Bilateral symmetry

6. Evolution of the Animal Body Plan
Radial symmetry
Body parts arranged around central axis
Can be bisected into two equal halves in any 2D plane perpendicular to that axis

7. Evolution of the Animal Body Plan
Bilateral symmetry
Body has right and left halves that are mirror images
Body has distinct anterior/posterior and dorsal/ventral divisions

8. Evolution of the Animal Body Plan
Bilaterally symmetrical animals have two main advantages over radially symmetrical animals
Cephalization
Evolution of a definite brain area
Greater mobility

9. Evolution of the Animal Body Plan
3. Evolution of a body cavity
Eumetazoa produce three germ layers
Outer ectoderm (body coverings and nervous system)
Middle mesoderm (skeleton and muscles)
Inner endoderm (digestive organs and intestines)

10. Evolution of the Animal Body Plan
3. Evolution of a body cavity
Three basic kinds of body plans
Acoelomates have no body cavity

11. Evolution of the Animal Body Plan
Pseudocoelomates have a body cavity between mesoderm and endoderm
Called the pseudocoel

12. Evolution of the Animal Body Plan
Coelomates have a body cavity entirely within the mesoderm
Called the coelom

13. 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. Internal body cavity is a hemocoel or blood chamber. Ex: Arthropods, mollusks.
Closed circulatory system: blood moves continuously through vessels that are separated from body fluids.

14. 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 2-layer-thick ball called a gastrula with:
Blastopore - Opening to outside
Archenteron - Primitive body cavity

15. 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

16. Evolution of the Animal Body Plan
Deuterostomes differ from protostomes in three other embryological features:
Cleaveage pattern of embryonic cells
Protostomes - Spiral cleavage
Deuterostomes - Radial cleavage
Developmental fate of cells
Protostomes - Determinate development
Deuterostomes - Indeterminate development
Origination of coelom
Protostomes - Forms simply and directly from the mesoderm
Deuterostomes - Forms indirectly from the archenteron

17. Evolution of the Animal Body Plan

18. Evolution of the Animal Body Plan
5. Evolution of segmentation
Segmentation provides two advantages
1. Allows redundant organ systems in adults such as 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

19. 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

20. 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

21. 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)

22. A New Look At Metazoans
It seems that key morphological characters used in traditional classification are not necessarily correct
Molecular systematics uses unique sequences within certain genes to identify clusters of related groups

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

24. Examples can be found in Table 32.2 of Raven et al.
A New Look At Metazoans
Examples can be found in Table 32.2 of Raven et al.

25. 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

26. 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