ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Chapter Outline		AP BIOLOGY: Chapter Thirty-Eight Review Answers 1. Animals are multicellular heterotrophs that digest their food internally. This distinguishes them from the other kingdoms of living organisms. Animal cells are distinct from plant cells in that they do not have cellulose-containing cell walls. 2. Vertebrates possess a backbone-a dorsal vertebral column enclosing a dorsal hollow nerve cord. Invertebrates do not, yet they are by far more numerous than the vertebrates, containing among them all the arthropod and molluscan phyla, among others. 3. Sponges can be put through a sieve and reduced to single cells, which will then reaggregate to form a new individual. Sponges evolved from the choanoflagellate protists, as evidenced by the presence of choanoflagellate-like cells (choanocytes) in the body of the sponge. 4. Eumetazoans possess three tissue layers: ectoderm, mesoderm, and endoderm. 5. An asymmetrical animal has no recognizable symmetry, except perhaps by chance; this lack of symmetry is characteristic of the sponges. Radially symmetrical animals can be bisected in any direction within a single plane, and includes the jellyfishes and ctenophores. Bilaterally symmetrical animals comprise the rest of the invertebrates and invertebrates and can be bisected only in one plane along a single axis, producing two equal halves. Bilateral symmetry allows for cephalization-a concentration of sensory organs at the anterior end of the animal. 6. Cnidarians have a tissue level of organization, whereas sponges do not. Tissue-level organization allows for greater coordination within an animal. 7. An acoelomate animal possesses no coelom (body cavity). A pseudocoelomate animal possesses no internal space between its gut and its external musculature. A coelomate animal possesses a true body cavity between the external surface of the gut and the internal surface of the body wall, in which many of the organs are suspended by connective tissues. Coelomates have as an advantage over pseudocoelomates and acoelomates the potential space for digestive tracts that are longer than their own body lengths, permitting a wider range of feeding strategy, as well as internal spaces for the development and storage of gametes. 8. Cells are organized into functional units called tissues, which aggregate to form organs. The simplest group in which organ level of development is seen is the Platyhelminthes. 9. Protostome animals exhibit spiral determinate cleavage, with the blastopore becoming the mouth. Deuterostomes exhibit radial indeterminate cleavage, with the blastopore becoming the anus. The major coelomate protostome phyla include the annelids, molluscs, and arthropods. Echinoderms and chordates are deuterostomes. In protostomes, the coelom arises as an enlarging space within the mesoderm. In deuterostomes, the coelom is formed from an evagination of the archenteron. 10. Flatworms are thin because they have no body cavity through which gases can diffuse; gas transport must occur through the skin, so the organism cannot attain great thicknesses or oxygen will not reach all of the tissues. With the roundworms came the advent of a body cavity, permitting some degree of internal circulation of gases. 11. Segmentation allows for greater evolutionary potential and flexibility. Segments can be duplicated or modified into specialized regions of the body. 12. Jointed appendages confer a great degree of motility upon an animal. Arthropods and chordates possess jointed appendages. 13. The evolutionary milestones in animal body plan evolution are multicellularity (sponges); tissues (cnidarians); organs (flatworms); body cavity (roundworms); segmentation (annelids); jointed appendages (arthropods); deuterostomic development (echinoderms); and development of the notochord (chordates).