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coelom (sēˈləm), 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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The mesodermally lined body cavity of most animals above the flatworms and nonsegmented roundworms. Its manner of origin provides one basis for classifying the major higher groups.

Annelids, arthropods, and mollusks have a coelom which develops from solid mesodermal bands. Within the trochophore larva of annelids, a single pole cell proliferates two strips of mesoblast lying on either side of the ventral midline. These bands subdivide transversely into bilateral solid blocks, the somites. Each somite then splits internally to form a hollow vesicle, the cavity of which is the coelom. The mollusks also form bands of mesoderm from a single pole cell, but these bands do not segment. They split internally to form single right and left coelomic sacs, but the cavities are soon reduced and the surrounding mesoblast disperses as separate cells, many of which become muscle. The only remnants of the coelom in the adult are the pericardial cavity and the cavities of the gonads and their ducts. In arthropods paired bands of mesoblast may proliferate from a posterior growth center or may separate inward from a blastoderm, a superficial layer of cells, on the ventral surface of the egg. These bands divide into linear series of somites which then hollow out. Their cavities represent the coelom.

Echinoderms and chordates constitute a second major group, characterized by the origin of the coelom from outpocketings of the primitive gut wall. In echinoderms one pair of bilateral pouches evaginates and separates from the archenteron or primitive digestive cavity. Each pouch constricts into three portions, not homologous to the metameres of other animals.

The protochordates of the groups Hemichordata and Cephalochordata have three coelomic pouches formed by separate evaginations of the archenteral roof. In hemichordates the head cavity remains single as the cavity of the proboscis and has a pore to the exterior on each side. The second pouches form cavities within the collar and also acquire external pores. The third pair is contained within the trunk and forms the major perivisceral cavity.

In cephalochordates the head cavity divides into lateral halves. The left side communicates, by a pore, to an ectodermal pit called the wheel organ. The second pair of pouches forms the pair of mesoblastic somites, and the third pouches subdivide transversely to give rise to the remainder of the linear series of somites. The upper or myotomic portion of each somite remains metameric and forms the segmental muscles. As it enlarges, the coelomic space is displaced ventrally and expands above and below the gut to form the perivisceral cavities and mesenteries, as described for annelids.

In vertebrates the mesoderm arises as a solid sheet from surface cells that have been involuted through the blastopore. Lateral to the notochord, beginning at about the level of the ear, the mesoderm subdivides into three parts: (1) the somites; (2) the nephrotomic cord, temporarily segmented in lower vertebrates, which will form excretory organs and ducts; and (3) the unsegmented lateral plate. The coelom arises as a split within the lateral plate. See Animal kingdom, Gastrulation

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



(also body cavity), the space between the body wall and the internal organs in higher multicellular animals. The coelom is enclosed by its own epithelial linings, which are of mesodermal origin. It contains coelomic fluid and usually opens to the exterior through special ducts.

The most important function of the coelom is to act as a support, inasmuch as the muscles of the body wall are able to contract only with the internal support of the coelomic fluid. This fluid plays the role of a “hydrostatic skeleton,” being incompressible but capable of readily changing shape. The coelom maintains the biochemical equilibrium of the body’s saline, ionic, aqueous, gaseous, and thermal regimes. In addition, it performs various secondary functions in the processes of digestion, respiration, excretion, and reproduction.

Higher animals possessing a coelom are united in the group Coelomata, which includes the higher protostomes (Echiurida, Mollusca, Sipunculida, and Annelides), Tentacula, Pogonophora, Chaetognatha, and deuterostomes (Hemichordata, Echinodermata, and Chordata, including Vertebrata).

In mollusks and vertebrates, including humans, a small part of the coelom around the heart forms the pericardium. In echinoderms part of the coelom is converted into the ambulacral system.

Several theories have been proposed to explain the origin of the coelom. According to the enterocoeliac theory, the coelom develops from the peripheral pouches of the gut of coelenterates. Advocates of the gonocoeliac theory consider the coelom the developed cavity of the sex glands. According to the nephrocoeliac theory, the coelom is homologous to the expanded ducts of the protonephridia. The best substantiated is the schizocoeliac theory, which hypothesizes that the coelom is formed as a result of the growth and development of inter-tissue areas of the primary body cavity.


Ivanov, A. V. “O proiskhozhdenii tseloma.” Zoologicheskii zhurnal, 1976, vol. 55, no. 6.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


The mesodermally lined body cavity of most animals higher on the evolutionary scale than flatworms and nonsegmented roundworms.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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