Solution Manual for Principles of Animal Physiology, 3rd Edition

Chapter 2 Physiological Evolution of Animals Answers to Review Questions 1. What is the significance of the similarity between choanoflagellates and choanocytes? Answer It is likely that choanoflagellates are the modern protist that is most closely related to the earliest metazoan. There is a striking similarity in the general structure of choanoflagellates and choanocytes of sponges. 2. Why aren’t protozoans considered animals? Answer Protozoans are a polyphyletic group of single-celled eukarytotes, which include groups that are plantlike, fungilike, and animal-like. Animals are, by definition, multicellular, so a single-celled eukaryotic organism could not be an animal. Some protists have the animal-like trait of locomotion, but that is not an indicator of how closely a protist is related to the ancestors of animals. 3. Which animals are diploblasts? Answer Diploblasts have only two embryonic cell layers, endoderm and ectoderm. Sponges lack cell layers, and most animals have three embryonic cell layers. Only cnidarians and probably ctenophores are diploblasts. 4. Explain why arthropods are considered Ecdysozoans. Answer Ecdysozoans share the ability to moult at some stage in their development. Arthropods moult. 5. Did all jawed animals evolve from the same agnathan ancestors? Answer No. Agnathans are a large group of jawless fish that gave rise to multiple lineages of fish and their descendants. It appears likely that sharks and bony fish arose from different agnathan ancestors, which were distinct from those that gave rise to lamprey and hagfish. It now appears likely that hagfish and lamprey, despite their divergence, shared a common ancestor. 6. How many times did terrestriality arise in animal lineages? Answer Many times. Land was invaded by multiple arthropod lineages, including myriapods, hexapods, and chelicerates. Amphibians were the first tetrapod group that had a major terrestrial presence. Many invertebrate phyla have groups that live on land, although in many cases they live in wet environments, such as moss, or live as endoparasites in the fluids of land animals. Of course, select groups of fish can spend periods of time on land. Copyright © 2016 Pearson Canada Inc. 1 7. What is meant by the term “a new head”? Answer The “new head” hypothesis suggests that the evolutionary diversification and success of vertebrates was made possible by the development of structures anterior to the notochord. In different lineages, these structures become important musculoskeletal features such as the jaw. 8. What is the significance of the evolution of the Na +/K+ATPase? Answer The evolution of the Na+ /K+ ATPase gave animals a mechanism by which to pump Na+ and K+ across their cell membranes, creating the electrochemical potential that is used to drive many transport processes and permit the formation of excitable tissues. The transporters linked indirectly to Na +/K+ ATPase help animals regulate their cell volume, which was important in this group of organisms because they lack cell walls. 9. When did endothermy arise in animal evolution? Answer Endothermy arose several times in animals. Endothermy arose independently in the ancestors of two groups of vertebrates, birds and mammals. Birds arose about 150 million years ago and mammals about 300 million years ago. Other groups have select species that are partially endothermic, such as bees, tuna, billfish, and large sharks. The acquisition of endothermy in these groups is much more recent. 10. Which came first, hormones or hormone receptors? Answer It is likely that the answer may differ between hormones and hormone receptors, but in the case of the aldosterone receptor discussed, it is likely that the receptor was present and capable of responding to other steroids before aldosterone came on the scene. Copyright © 2016 Pearson Canada Inc. 2