coelom
coelom (s lm) [key], 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.
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