coelom

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.

coelom Principal body cavity in most animals, forming the cavity around the gut in many Annelida, and in Echinodermata and Vertebrata. In Arthropoda and Mollusca, the main body cavity is an expanded part of the blood system (a haemocoel) and the coelom is small.

coelom A fluid-filled (see COELOMIC FLUID) body cavity that originates by the splitting of the mesoderm of triploblastic animals. It separates the muscles of the body wall from the gut, allowing them to move independently, and provides an area for the enlargement of internal organs, thus permitting the gut to be differentiated for various functions. In many animals, the coelom plays an important part in collecting excretions and acts as a storage site for the maturation of gametes. It is well developed in Vertebrata, Echinodermata, and Annelida, but in Arthropoda and Mollusca it is reduced and its role replaced by the haemocoel.