Evidence is presented to show that the mesogloea of the coelenterates examined, including Aequorea, Aurelia, Cyanea, Chrysaora, Calliactis, and Metridium contains collagen-like material in the form of fibres of varying sizes. Cells are also present in the mesogloea of some species but there is little evidence concerning their functions.

In Aequorea and the Scyphozoa examined the fibres appear to be straight and very transparent in life but on injury and fixation they adopt a convoluted and irregular appearance. Cells are present in Aurelia and form about 0.3 per cent, of the mesogloea by volume but are absent from the other medusae examined. The mesogloea of actinians is of a more densely fibrous character than that of medusae. In Calliactis and Metridium the crossed fibrillar structure present is determined by mechanical forces acting on the tissue.

Reference is made to the physical properties of Calliactis mesogloea, which are described in another paper. The material is tough, almost resembling cartilage in its consistency, and has considerable tensile strength.

The chemical nature of the mesogloea is shown to be primarily that of a protein of which the amino-acid composition is similar to that of vertebrate collagen.

The role of the mesogloea in the life of the animal is that of a base for the muscles, a limiter of the body volume, and a resister of rapid movements, both those caused by the musculature and those enforced by the environment without. Its viscous-elastic properties are well adapted to this role since the elastic component, having a short timescale, can provide a restoring force for rapid deformations.

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