Chapter Summary
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30.1 Some Animal Characteristics Evolved More Than Once
Animals share a set of derived traits not found in other groups of organisms. These traits include similarities in the sequences of many of their genes, the structure of their cell junctions, and the components of their extracellular matrix.
Several features common to most living animals, including multicellularity, motility, and nervous systems, evolved independently in different lineages. In particular, the decentralized nerve nets of ctenophores and cnidarians evolved independently of the central nervous systems of bilaterians.
Patterns of embryonic development provide clues to the evolutionary relationships among animals. Diploblastic animals, which include the ctenophores, placozoans, and cnidarians, develop two embryonic cell layers. The monoblastic sponges lack embryonic cell layer differentiation, whereas the triploblastic animals develop three cell layers. Review Figure 30.1
Differences in their patterns of early development characterize two major triploblastic clades, the protostomes and the deuterostomes.
Sponges are structurally simple animals that lack well-
differentiated cell layers and true organs. They have skeletons made up of silicaceous or calcareous spicules. They create water currents and capture food with flagellated feeding cells called choanocytes. Review Figure 30.2
30.2 Animals Diverged with Distinct Body Plans
Animal body plans can be described in terms of symmetry, body cavity structure, segmentation, types of appendages, and nervous system development.
A few animals have no symmetry, but most animals have either radial symmetry or bilateral symmetry. Review Figure 30.5
Most bilaterally symmetrical animals exhibit cephalization: the concentration of sensory organs and nervous tissues in an anterior head.
On the basis of their body cavity structure, animals can be described as acoelomates, pseudocoelomates, or coelomates. Review Figure 30.6, Activity 30.1
Segmentation, which takes many forms, improves control of movement, as do appendages. The development of a nervous system is important for the coordination of muscular movement and the processing of sensory information.
30.3 Animals Use Diverse Forms of Movement to Feed
Motile animals can move to find food; sessile animals stay in one place but may expend energy to move the environment and the food it contains to them
Filter feeders strain small organisms and organic molecules from their environment.
Herbivores consume plants, usually without killing them.
Predators have morphological features such as sharp teeth, beaks, and claws that enable them to capture and subdue animal prey.
Parasites live in or on other organisms and obtain nutrition from those host individuals.
Detritivores consume dead organic matter and return the nutrients it contains to the ecosystem.
30.4 Animal Life Cycles Involve Trade-
The stages of an animal’s life cycle may be specialized for different activities. An immature stage whose morphology is dramatically different from that of the adult stage is called a larva.
Most animal life cycles have at least one dispersal stage. Many sessile marine animals can be grouped by the presence of one of two distinct larval dispersal stages: trochophore or nauplius. Review Figure 30.11
A characteristic of an animal or a life cycle stage may improve the animal’s performance in one activity but reduce its performance in another, a situation known as a trade-
off. Parasites have complex life cycles that may involve one or more hosts and several larval stages. Review Figure 30.12
In some groups of animals, asexual reproduction without fission leads to the formation of colonies composed of many genetically homogeneous, physiologically integrated individuals.
30.5 The Root of the Animal Tree Provides Clues to Early Animal Diversification
Ctenophores are radially symmetrical and have two cell layers separated by an inert extracellular matrix called mesoglea. Their weakly differentiated organ systems are independently derived from those in eumetazoans. Review Figure 30.16
Eumetazoans have well-
developed organ systems and include all animals except ctenophores, sponges, and placozoans. The triploblastic protostomes and deuterostomes belong to a large monophyletic group called bilaterians. Review Figure 30.1, Investigating Life: Reconstructing Animal Phylogeny from Protein-Coding Genes, Activity 30.2 Placozoans are asymmetrical as adults. They have only a few cell types and lack true organs, although some features of their simplicity may be secondarily derived.
The life cycle of most cnidarians has two distinct stages: a sessile polyp stage and a motile medusa stage that reproduces sexually. A fertilized egg develops into a free-
swimming planula larva, which settles to the bottom and develops into a polyp. Review Figures 30.19, 30.23, Animation 30.1 See Activity 30.3
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