Dividing the Body

Although a fluid-filled coelom provided an efficient hydrostatic skeleton for burrowing, precise control of body movements was probably difficult for the earliest coelomates. The force of muscle contraction in one area was carried throughout the body by the fluid in the undivided coelom. This limitation was removed when a series of partitions (septa) evolved in the ancestors of annelids and arthropods. When septa divided the coelom into a series of compartments, components of the circulatory, nervous, and excretory systems were repeated in each segment. This repetition of body segments is called segmentation (also called metamerism).

The advent of metamerism was significant because it made possible the evolution of much greater complexity in structure and function. Metamerism not only increased burrowing efficiency, it also made possible the independent movement of separate segments. Fine control of movements allowed, in turn, the evolution of a more sophisticated nervous system. Moreover, repetition of body parts gave the organisms a built-in redundancy that provided a safety factor: if one segment should fail, others could still function. Thus an injury to one part would not necessarily be fatal.

The evolutionary potential of the metameric body plan is amply demonstrated by the large and diverse phyla Annelida, Arthropoda and Chordata, which represent three separate evolutionary origins of metamerism.