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In page Polyp (zoology):

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The body of the polyp may be roughly compared in a structure to a sac, the wall of which is composed of two layers of cells. The outer layer is known technically as the ectoderm, with the inner layer as the endoderm (or gastroderm). Between ectoderm and endoderm is a supporting layer of structureless gelatinous substance termed mesoglea, secreted by the cell layers of the body wall.[1] The mesoglea can be thinner than the endoderm or ectoderm or comprise the bulk of the body as in larger jellyfish. The mesoglea can contain skeletal elements derived from cells migrated from ectoderm.[citation needed]