coelomates

Coelomates, also called eucoelomates, are animals that possess a true coelom, a fluid-filled body cavity completely lined by mesoderm-derived tissue. The coelomic cavity houses and supports internal organs and provides space for their development and movement. The lining, the mesothelium, is separate from the gut and from the outer body wall.

During embryonic development, the coelom forms by two main processes. In many protostomes (such as annelids, mollusks, and some arthropods), it develops by schizocoely, where the mesoderm splits to create the cavity. In deuterostomes (including echinoderms and chordates), the coelom forms by enterocoely, as pouches from the archenteron enlarge and pinch off.

Coeloms enable greater body complexity by allowing the growth and organization of organs independent of the body wall. They can function as a hydrostatic skeleton, aid in the organization of circulatory, excretory, and reproductive systems, and provide a protective space for internal structures. In many groups, the coelom is associated with a closed circulatory system, though in several lineages it is reduced or modified into a hemocoel.

Examples of coelomates include the annelids, mollusks, arthropods, echinoderms, and chordates. Coelomates are contrasted with acoelomates, which lack a body cavity, and pseudocoelomates, in which the body cavity is not completely lined by mesoderm. The coelom's presence is a hallmark of many, but not all, complex animal body plans.