The evolution of body cavities within the kingdom Animalia has a very interesting history. In fact, the increasing complexity of animal form and function during the evolution of the group can be directly linked to the evolution of ever-more-sophisticated body cavities.

The most primitive animal phyla possess only a single body cavity, which typically has either digestive or circulatory functions, or both. There is no secondary body cavity, or coelom, and consequently these phyla are referred to as the acoelomates.

The cross sections visually represent the structural differences between the acoelomate, pseudocoelomate, and the coelomate. Redrawn from Holt et al., 1998.

Most animal phyla, however, have evolved a second body cavity of one form or another. The pseudocoelomates, which include a number of worm-like phyla, are characterized by a secondary body cavity known as the pseudocoelom. The pseudocoelom has some but not all of the characteristics of...