FIGURE 29.4
Body plans of the bilateria. The various organ systems of the animal develop from the three germ layers that form in the embryo. (a) Acoelomates lack a body cavity between the gut and outer body wall. (b) Pseudocoelomates have a body cavity only partially lined by mesodermally derived tissue. (c) Coelomates have a true coelom, a body cavity completely lined by mesodermally derived tissue.