Mesentery (zoology)

A mesentery is a membrane inside the body cavity of an animal. The term identifies different structures in different phyla: in vertebrates it is a double fold of the peritoneum enclosing the intestines; in other organisms it forms complete or incomplete partitions of the body cavity, whether that is the coelom or, as in the Anthozoa, the gastrovascular cavity.

The word "mesentery" is derived from the Greek mesos, "in the middle" and enteron, an "intestine".

In vertebrates, a mesentery is a membrane consisting of a double fold of peritoneum that encloses the intestines and their associated organs and connect them with the dorsal wall of the abdominal cavity. In invertebrates, a mesentery is a support or partition in a body cavity serving a similar function to the mesenteries of vertebrates.

In bilaterally symmetrical organisms there is often a major mesentery separating the two halves of the coelom. In segmented organisms such as earthworms, there are a pair of coelomic cavities in each segment. The mesothelium of the body wall is extended round the central gut to form a mesentery, a longitudinal partition in the sagittal plane. Above the gut is the dorsal mesentery and below the gut, the ventral mesentery. The transverse partition between the separate segments is known as a septum.

In the phylum Cnidaria and the class Anthozoa, the mesenteries are sheet-like partitions that extend from the body wall of the animal into its gastrovascular cavity. They are composed of a layer of mesogloea sandwiched between two layers of gastrodermis. They can either be "complete", joining the gastrodermis of the body wall with that of the pharynx, or "incomplete", extending only part way into the cavity (the terms "perfect" and "imperfect" are sometimes used instead). They stretch from the pedal disc to the oral disc, and there are two perforations or ostia near the oral disc. Mesenteries are usually in pairs. The free edge of incomplete mesenteries are thickened to form glandular, ciliated bands called mesenterial filaments. The lower ends of the mesenterial filaments are elongated into acontia which are armed with cnidocytes (stinging cells) and can protrude through the body wall or mouth. The gonads are situated on the mesenteries alongside the mesenterial filaments.

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