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Average of a little bristle : the coelom surrounds the central Typhlosolis

With coelom (Greek. Κοίλωμα koiloma "depression", "cavity" of κοῖλος koilos "hollow", eingedeutscht also coelom ) refers to the secondary body cavity of many animals , such as the chordates or annelids . It is a fluid-filled cavity that is surrounded by a mesodermal epithelium and, as a fluid cushion, takes on the function of a hydroskeleton in many protostomes . The coelom is formed a little later in the course of ontogenesis by folding from the primitive intestine ( archenteron ) or from special mesodermal cells derived from the endoderm and often completely displaces the primary body cavity ( blastocoel ) that has arisen from the furrow cavity .

In many animal groups the coelom is reduced and only partially preserved as a remainder. In the molluscs it still exists in the form of the pericardium and the gonadal cavity , in the arthropods it has changed into a mixocoel , with clear remnants also being preserved (e.g. as a covering of the metanephridia , the so-called nephrocoel ).

In mammals, the coelom develops into the chest cavity , the abdominal cavity and the pericardial cavity . The coelomepithelium forms the tunica serosa ( pleura , peritoneum ).


A distinction is made between the acoelomate, pseudocoelomate, coelomate and mixocoelomate organization.

Acoelomate organization

There is no single extracellular cavity. Between the basal matrix of the epidermis and that of the intestine and the internal organs, the body is only filled with connective tissue (mesenchyma, “parenchyma”) and connective tissue muscles. Mesenchymatic tissue completely fills the space between the derivatives of ectoderm and endoderm, at best intercellular gaps form a schizozoel. The primary body cavity is limited to the extracellular matrix between the mesodermal cells. The digestive tract, which is ontogenetically derived from the gastric space, is the only continuous body cavity.

Pseudocoelomate organization

The mesodermal cell associations do not completely take up the space between ectodermal and entodermal formations, the primary body cavity remains even as an adult and can even become a shape-determining body cavity ( pseudocoel ). The primary body cavity is a fluid-filled cavity between the body wall and the intestine. There is a cavity between the epidermis and the intestine, but it is not lined with its own epithelium.

Coelomate organization

The primary body cavity is narrowed by the secondary body cavity to the space between the coelothelia or between the coelothelia and the body or intestinal wall. The coelom is bounded by a real epithelium. It more or less completely displaces the primary body cavity, the remainder of which is only present in blood vessels.

Mixocoelomate organization

In the embryo, mesodermal strips and Coelom sacs are still applied, but subsequently the Coelom walls are broken down to form various organ systems (e.g. muscles). This leads to a fusion of the primary and secondary body cavity, the resulting cavity is called Mixocoel (Hämocoel). This is made possible by the chitin cuticle, which makes the assumed hydroskeletal function of the coelomic spaces superfluous. As a result of the mixocoel formation, an open blood vessel system with a uniform fluid, the hemolymph, developed.


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Web links

Wiktionary: Coelom  - explanations of meanings, word origins, synonyms, translations