The Coelentera may thus be briefly defined as Metazoa which exhibit two embryonic cell-layers only, - the ectoderm and endoderm, - their body-cavities being referable to a single cavity or coelenteron in the endoderm.
As a result of this change of form the gastric cavity or coelenteron becomes of compressed lenticular form, and the endoderm lining it can be distinguished as an upper or exumbral layer and a lower or subumbral layer.
The cathammal areas may remain very small, mere wedge-shaped partitions dividing up the coelenteron into a four-lobed stomach, the lobes of which communicate at the periphery of the body by a spacious ring-canal.
By coalescence of the endoderm-layers, the coelenteron may be reduced to vessels, usually eight in number, opening into a ring-sinus surrounding the pore.
The next event is a great growth in thickness of the gelatinous mesogloea, especially on the exumbral side; as a result the flattened coelenteron is still further compressed so that in certain spots its cavity is obliterated, and its exumbral and subumbral layers of endoderm come into contact and undergo concrescence.
In Actinia, as in all Anthozoan zooids, the coelenteron is not a simple cavity, as in a Hydroid, but is divided by a number of radial folds or curtains of soft tissue into a corresponding number of radial chambers.
In different parts of the coelenteron the endoderm may be of three principal types - (i) digestive endoderm, the primitive type, with cells of large size and considerably vacuolated, found in the hydranth; some of these cells may become special glandular cells, without flagella or contractile processes; (2) circulatory endoderm, without vacuoles and without basal contractile processes, found in the hydrorhiza and hydrocaulus; (3) supporting endoderm (fig.
More usually each cathamma is a wide triangular area, reducing the peripheral portion of the coelenteron to the four narrow radial canals and the ring-canal above described.
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.