What do coelomates have that distinguishes them from other types of animals?

Coelomates are characterized by the presence of a true coelom, which is a fluid-filled body cavity that is entirely lined with mesodermal tissue. This anatomical feature allows for the development of complex organ systems and provides a space for organs to grow and move independently from the outer body wall. The coelom also enables the formation of a more sophisticated circulatory system, as the cavity can function in the distribution of nutrients and waste products.

In contrast to other types of body plans, such as acoelomates, which lack a body cavity altogether, or pseudocoelomates, which have a body cavity that is only partially lined with mesoderm, coelomates exhibit this distinct and functional body structure. The presence of a true coelom facilitates greater complexity in the organism's internal architecture and physiological processes, supporting more advanced life forms as seen in many animal phyla.