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 three main types of symmetry
Radial symmetry
Bilateral symmetry
Asymmetry

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 head/brain area
Greater mobility

9. Evolution of the Animal Body Plan
Asymmetry
Body without symmetry
Body has no distinct halves or segments that can be made that would be identical

10. Evolution of the Animal Body Plan
3. Evolution of a body cavity
Eumetazoa produce three germ layers
Outer ectoderm (body coverings and nervous system) – nervous system is different due to the way it forms
Middle mesoderm (skeleton and muscles) – completely enclosed in the body
Inner endoderm (digestive organs and intestines) – open to the outside via tracts

11. Evolution of the Animal Body Plan
3. Evolution of a body cavity
Three basic kinds of body plans
Acoelomates have no body cavity
Ex. Flatworms (Planeria)

12. Evolution of the Animal Body Plan
Pseudocoelomates have a body cavity between mesoderm and endoderm
Called the pseudocoel
Ex. Roundworms (Nematode)

13. Evolution of the Animal Body Plan
Coelomates have a body cavity entirely within the mesoderm
Called the coelom
Ex. Segmented Worms (Earthworms)

14. 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

15. 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

16. Evolution of the Animal Body Plan

17. Evolution of the Animal Body Plan
Bilaterians can be divided into two groups
Protostomes develop the mouth first from 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

18. Evolution of the Animal Body Plan
Deuterostomes differ from protostomes in three other embryological features:
Cleavage pattern of embryonic cells
Protostomes - Spiral cleavage
Deuterostomes - Radial cleavage
Developmental fate of cells
Protostomes – Development determined; the removal of one cell stops development
Deuterostomes – Development indetermined; the removal of one cell does nothing to development; the removed cell can create an additional organism
Origination of coelom
Protostomes - Forms simply and directly from the mesoderm; the mesoderm forms near the mouth and splits to form the coelom
Deuterostomes - Forms indirectly from the archenteron; the mesodermal pouches form from the endodermal lining which eventually forms the coelom

19. Evolution of the Animal Body Plan

20. Protostomes vs. Deuterostomes
The mouth develops from the first opening in the gastrula.
Removal of one cell stops embryonic development.
8-cell stage has cells that are shaped like a spiral
Mesoderm forms and splits down the middle to eventually form the coelom.
The anus develops from the first opening in the gastrula and the mouth develops later.
Removal of one cell does not stop embryonic development.
8-cell stage has cells that are directly aligned.
Mesoderm forms pouches from the endoderm to eventually form coelom.

21. 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

22. DEVELOPMENT OF TISSUES
Eventually the germ layers (ectoderm, endoderm, and mesoderm) begin to specialize to form tissues. All of the body tissues of animals are comprised of one of the four basic tissue types.
Epithelial tissue
Connective tissue
Muscular tissue
Nervous tissue 22

23. Evolutionary Developmental Biology
Most taxonomists agree that the animal kingdom is monophyletic – descending from one ancestor or group of organisms (clade).
Three prominent hypotheses have been proposed for the origin of metazoans from single-celled protists.

24. Evolutionary Developmental Biology
1. The multinucleate hypothesis – animals descended from ciliated protists but are closely related to algae
2. The colonial flagellate hypothesis - animals are descended from an ancestor that resembled a hollow spherical colony of flagellated cells called choanoflagellates
3. The polyphyletic origin hypothesis – animals descended from more than one ancestor or clade
Molecular systematics using rRNA sequences settles this argument in favor of the colonial flagellate hypothesis