Zoology 10th Edition by Stephen Miller,John Harley

Exercise 9

Animal bodies are organized around a point or an axis. Radially symmetrical animals are arranged around an oral-aboral axis. Bilaterally symmetrical animals are arranged around a longitudinal axis. A particular organization is often associated with a particular way of life. Protists and animals have bodies composed of cells. Animal bodies are often organized into tissue layers. Animals may be diploblastic, triploblastic acoelomate, triploblastic pseudocoelomate, or triploblastic coelomate. Body cavities have advantages that promote increased body size and complexity.
Evolution is often portrayed as producing greater complexity and increased size. Can you think of an example where evolution resulted in reduced complexity or smaller size?