Outermost cellular layer.

[Cubo-/Scyphomedusae] the outer tissue-layer of the adult cnidarian body-plan, the inner layer being termed gastrodermis. The terms ectoderm and endoderm have also been widely used for these layers but with some danger of inconsistency. In many other animal phyla the embryonic ectodermal and endodermal tissues can be shown to give rise to the corresponding tissue layers in the adult, to which the terms ectoderm and endoderm can be validly applied. In cnidarians the sexually-produced embryo forms a planula, similarly having inner and outer layers which are correctly termed ectoderm and endoderm. But the planula metamorphoses into a polyp with loss of identity of the original tissue layers. Thus, during both metamorphoses, the integrity of the original layers may be lost, making the homology of the newly produced layers unclear and the terms ectoderm and endoderm misleading. The terms epidermis and gastrodermis are neutral in that they refer just to position of tissue layers in the (adult) animal and not to their embryonic origin. See also mesoglea.