Overview of Animal Diversity

1. Overview of Animal Diversity Chapter 32

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 • 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 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

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. 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