coelomate

A coelomate is an animal whose main body cavity is a true coelom— a fluid-filled chamber completely lined by mesoderm-derived tissue (the peritoneum). The coelom forms during embryonic development either by schizocoely (the coelom arises from splitting of the mesodermal mass) or by enterocoely (pouches from the gut form the coelom). This distinguishes coelomates from acoelomates, which lack a body cavity, and pseudocoelomates, whose cavity is not entirely lined by mesoderm.

In coelomates, the coelomic space provides a protective and flexible cushion for internal organs, allows the development and organization of complex organ systems, and often functions as a hydrostatic skeleton that facilitates movement. The coelomic lining (the mesothelium) also contributes to lubrication and supports organ movement within the cavity. The presence of the coelom can enable greater diversification of body plans and organ arrangements.

Representative groups include annelids (such as earthworms), mollusks (snails, clams, octopuses), arthropods (insects, spiders, crustaceans), echinoderms (sea stars, sea urchins), and chordates (vertebrates and their relatives). In some lineages, the coelom is reduced or modified in the adult stage—for example, certain arthropods develop a hemocoel that functions as part of their circulatory system—though their embryonic development demonstrates the formation of a true coelomic cavity.

See also: acoelomate, pseudocoelomate.