coelom, fluid-filled body cavity, found in animals, which is lined by cells derived from mesoderm tissue in the embryo, and which provides for free, lubricated motion of the viscera. In animals of the phyla Annelida, Mollusca, and Arthropoda, the mesoderm forms as a mass of tissue from special embryonic cells between an outer layer, the ectoderm, and an inner layer, the endoderm. The coelom then forms as a result of the splitting and hollowing out of the mesodermal mass. In animals of the phyla Echinodermata and Chordata, the mesoderm arises as the lining of folds developing from the endoderm, and the spaces within these folds form the coelom. The structure of the embryonic coelom is relatively simple; in an adult other organs push into the coelomic cavity, and it is also subdivided into compartments, e.g., the pericardial cavity, in which the heart develops. The origin of the coelom is uncertain. The acoelomate theory holds that it evolved from an acoelomate ancestor; the enterocoel that it evolved from gastric pouches of cnidarian ancestors. Recent research, particularly with flatworms and with small worms recently discovered in marine fauna, supports the enterocoel theory.