The mesoglea is mostly water. Other than water, the mesoglea is composed of several substances including fibrous proteins like collagen and heparan sulphate proteoglycans. The mesoglea is mostly acellular, but in both cnidaria and ctenophora the mesoglea contains muscle bundles and nerve fibres. Other nerve and muscle cells lie just under the epithelial layers. The mesoglea also contains wandering amoebocytes that play a role in phagocytosing debris and bacteria. These cells also fight infections by producing antibacterial chemicals.
The mesoglea may be thinner than either of the cell layers in smaller coelenterates like a hydra or may make up the bulk of the body in larger jellyfish. The mesoglea serves as an internal skeleton, supporting the body. Its elastic properties help restore the shape after it is deformed by the contraction of muscles. However, without the buoyancy of water to support it, the mesoglea is not stiff enough to bear the weight of the body and coelenterates collapse when they are taken out of water.
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