The gastrovascular system shows every degree of complexity from a very primitive to a highly elaborate type of structure.
The ectoderm covers the whole external surface of the animal, while the endoderm lines the coelenteron or gastrovascular space; the two layers meet each other, and become continuous, at the edge of the mouth.
As a general rule the mouth is the only aperture of the gastrovascular system; in a few cases, however, excretory pores are found on the ring-canal, but there is never any anal opening.
Thus the disposition of the endoderm-cavities is roughly comparable to the gastrovascular system of a medusa.
Oesophagus, stomach, radial canals, ring-canal and tentacle-canals, constitute together the gastrovascular system and are lined throughout by endoderm, which forms also a flat sheet of cells connecting the radial canals and ring canal together like a web; this is the so-called endoderm-lamella (e.l.), a most important feature of medusan morphology, the nature of which will be apparent when the development is described.
The archenteron becomes the gastrovascular system or coelenteron.