Thus the disposition of the endoderm-cavities is roughly comparable to the gastrovascular system of a medusa.
(2) The body contains but a single internal cavity, the coelenteron or gastrovascular space, which may be greatly ramified, but is not shut off into cavities distinct from the central digestive space.
The archenteron becomes the gastrovascular system or coelenteron.
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.
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.
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.
The gastrovascular system shows every degree of complexity from a very primitive to a highly elaborate type of structure.
6) and consists of a short, hollow rod, the wall of which is composed of the two bodylayers, ectoderm and endoderm, enclosing a cavity continuous with that of the gastrovascular system.