Chapter 30

1. Most animals are multicellular heterotrophs with internal digestion, muscular systems that allow movement, and nervous systems. This combination of features generally allows us to distinguish animals from other groups, although none of these features is diagnostic (by itself) for all animals. Other groups (such as protists, fungi, and plants) include multicellular species; many protists and fungi are heterotrophs; some protists have internal digestion; and not all animals have muscular systems and nervous systems. Evidence for the monophyly of animals comes from gene sequences, as well as a few microstructural features: a common set of extracellular matrix molecules, including collagen and proteoglycans; and unique types of junctions between cells (tight junctions, desmosomes, and gap junctions).

2. 1. In radial symmetry, body parts are symmetrical across multiple planes that run through a single axis at the body’s center. Animals with radial symmetry have no front or rear ends, and they are often sessile or drift freely with currents. If they move under their own power, they can typically move slowly equally well in any direction. In contrast, bilaterally symmetrical animals have mirror-image right and left halves divided by a single plane that runs along an anterior–posterior midline. They have front ends that usually contain a concentration of sensory systems and nervous tissues in a distinct head. Bilaterally symmetrical animals usually move forward in the direction of the head, so that the head encounters new environments first.

   2. Among the bilaterian animals, there are two distinct forms of gastrulation—the initial indentation of a hollow sphere of cells early in development that forms the blastopore. In protostomes the blastopore eventually develops into the mouth of the animal, whereas in deuterostomes the blastopore becomes the anus.

   3. Diploblastic animals have embryos with two cell layers (an outer ectoderm and an inner endoderm). The embryos of triploblastic animals have an additional cell layer between the ectoderm and the endoderm, known as mesoderm.

1. Bilaterally symmetrical organisms have an anterior and a posterior end. As the animal moves through the environment, its anterior end encounters potential food or predators first. It is therefore advantageous for the sensory organs and central nervous system to be concentrated at the anterior end.

2. The body cavities of many animals function as hydrostatic skeletons. As the muscles that surround a cavity contract, the fluids must shift to another part of the cavity. In this way, animals can extend parts of their bodies and move specific body parts. Segmentation allows specialization of the body parts, and soft-bodied animals can change the shape of each part independently, thereby increasing the precision of movement. In animals with a hard external skeleton (such as the arthropods), segmentation and the accompanying appendages (controlled by muscles attached to the exoskeleton) allow even greater specialization of movement. Some arthropod appendages are used for walking, swimming, and even flying. The central nervous system is used to sense the environment (including food, appropriate temperatures, and potential predators) and to coordinate movement.

   Acoelomate animals lack a body cavity enclosed in mesoderm. Pseudocoelomate animals have a body cavity enclosed in mesoderm; this body cavity contains the gut and internal organs composed of endoderm, but these latter organs are not lined with mesoderm. Coelomate animals have a body cavity that is enclosed in mesoderm, and the internal organs are also lined with mesoderm.

1. Filter feeders filter water or air and trap small food particles they contain. These particles may include small animals, but the filter feeder does not typically actively chase and feed on individual prey. Predators, in contrast, actively seek out and feed on other individual animals, killing them in the process. Parasites also feed on other animals, usually without killing them; a parasite may reside inside another animal or feed on its parts from the outside (such as a mosquito or a tick does).

2. Herbivores must digest relatively fibrous, tough plant material. So they usually need a longer gut (compared with carnivores) or a digestive system that permits fermentation of plant material. They usually need mouthparts that allow chewing of leaves or sucking of plant fluids. A predator needs to be able to move quickly enough to catch its prey and needs adaptations for subduing the prey (such as teeth, jaws, claws, venom, constriction, etc.).

1. In a multicellular organism, every cell has the same genotype, and the cells are all physiologically interconnected and interdependent to form one functioning organism. In a colonial species, the individuals are more loosely integrated with one another, and individuals that make up a colony may also be able to exist independently of one another. In some cases, different individuals in a colony have different genotypes, although some colonies may be composed of clonally duplicated organisms that function as an integrated whole. Every individual in a colony is typically multicellular, unlike the single cells in a multicellular individual.

2. Typically the characteristics of an animal in one life stage are beneficial under certain conditions but detrimental under others. A change that improves one characteristic, such as a thicker shell for protection against predation, usually comes at a cost of some kind, such as less mobility. In a life cycle, an animal may evolve to produce more eggs, but that usually comes at a cost of the investment of resources in each egg, which leads to lower survivorship of the average offspring. For this reason, there are limits on the changes that can occur in the evolution of life cycles.

1. First, phylogenetic analysis showed that ctenophores were the sister group of other animals. Ctenophores have nervous systems, but some other animals (such as sponges and placozoans) do not. Therefore either nervous systems evolved once in the ancestor of all animals and were subsequently lost in sponges and placozoans, or nervous systems evolved separately in ctenophores and in other animals. Second, analysis of whole genomes showed that the genes involved in nervous systems have been independently duplicated and specialized in ctenophores, cnidarians, and bilaterians, thereby suggesting that the nerve nets of ctenophores and cnidarians, and the centralized nervous systems of bilaterians, each evolved independently.

2. The group of organisms called “animals” represents a specific monophyletic group of multicellular organisms on the tree of life. Sponges retain many of the ancestral features of animal relatives (such as choanoflagellates), and split from the other animals before the evolution of complex organ systems. Placozoans, by contrast, may have evolved from ancestors with distinct organ systems, but they appear to have lost these systems and become secondarily simplified.

3. Placement of glass microscope slides (or other smooth substrates for placozoan attachment) in warm tropical waters often results in colonization by placozoans. The glass slides can be suspended in water in survey areas, then later retrieved and examined for the presence of placozoans.

1. 

2. Refer to figure above. The number of changes is shown on the tree for each branch; each arrow shows one change.

3. Amino acid positions 5 and 6, both of which exhibit the same state in ctenophores and the outgroup, but a derived state in the remaining animals, support the ctenophores as the sister group of the remaining animals.

4. Each of these groups in marked on the tree above (answer to Question 1).

1. Based on these data, the chemical disturbance of CsCl induced reverse development as compared with the control population. Increasing concentrations of CsCl resulted in stronger reverse development, as evidenced by the higher percentages of animals in polyp and reducing medusa stages in response to increasing concentrations of CsCl. Reverse development allows for some cnidarians to survive periods that are not suitable for their survival and reproduction as medusae.

2. The medusa form is motile and sexual, while the polyp form is sessile and asexual. Disadvantages associated with reverse development include decreased outbreeding opportunities, decreased dispersal, and investment in reverse development that could otherwise be devoted to maintenance or reproduction as a sexual medusa. These negative trade-offs are tolerated because reverse development allows survival in an environment that is not suitable for medusae.

3. Yes, you would expect the two populations to look different. Polyps promote survival and maintenance of local populations because they can reproduce without a mate, but they limit the ability to disperse. Medusae may enhance dispersal probability and/or population expansion and increase genetic diversity, but medusae are more energetically expensive, and developing and maintaining reproductive structures can be costly. So when conditions are good (high food abundance), it is favorable to be a sexual, motile medusa, and when conditions are poor (low food abundance), it is favorable to be an asexual, sessile polyp.