recap

30.2 recap

The body plans of animals are variations on patterns of symmetry, body cavity structure, segmentation, appendages, and nervous systems.

learning outcomes

You should be able to:

  • Explain how differences in body symmetry, body cavities, nervous system organization, segmentation, and appendages among animals are functionally related to movement and feeding.

Question 1

Why is bilateral symmetry strongly associated with cephalization, the concentration of sensory organs in an anterior head?

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.

Question 2

Explain several ways in which body cavities, segmentation, and centralized nervous systems improve control over movement.

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.

Many of the modifications to the body plans of animals affect their ways of finding, capturing, and processing food. Evolutionary changes in symmetry, body cavities, appendages, segmentation, and sensory systems have played key roles in enabling animals to obtain food from their environment as well as helping them avoid becoming food for other animals.