Mesoglea refers to the extracellular matrix found in cnidarians like coral or jellyfish as well as ctenophores that functions as a hydrostatic skeleton.
It is related to but distinct from mesohyl, which generally refers to extracellular material found in sponges.
Other than water, the mesoglea is composed of several substances including fibrous proteins, like collagen and heparan sulfate proteoglycans.
[5] The mesoglea may be thinner than either of the cell layers[6] in smaller coelenterates like a hydra or may make up the bulk of the body in larger jellyfish.
In order to differentiate the use of the word mesenchyme in vertebrate embryology (that is, undifferentiated tissue found in embryonic true [ento-]mesoderm from which are derived all connective tissues, blood vessels, blood cells, the lymphatic system, and the heart) and the use in invertebrate zoology (a more-or-less solid but loosely organized tissue consisting of a gel matrix [the mesoglea, in strict sense] with various cellular and fibrous inclusions, located between epidermis and gastrodermis), some authors prefer to use the term mesoglea (in wider sense) in lieu of mesenchyme when referring to the middle layers of sponges and diploblasts, reserving the term mesenchyme for the embryological sense.