The gelatinous noncellular layer between the outer and inner cell layers in the body wall of ctenophores and cnidarians. It may be thin, as in Hydra, or tough and fibrous, as in the larger jellyfish and sea anemones. It often contains cells that have migrated from the two body layers but these do not form tissues and organs and the mesoglea is not homologous with the mesoderm of triploblastic animals.