Diagram of the structure of a medusa; the ectoderm is left clear, the endoderm is dotted, the mesogloea is shaded black; a-b, principal axis (see Hydrozoa); to the left of this line the section is supposed to pass through an inter-radius (I.R.); to the right through a radius (R).
Between the ectoderm and endoderm a gelatinous supporting layer, termed the mesogloea, makes its appearance.
En, Endoderm lining the enteric cavity.
In the course of development, however, cells from the ectoderm and endoderm may migrate into it.
E.l, Endoderm lamella.
It has usually been regarded as representing both endoderm and mesoderm, and the groove which usually leads to its formation has been compared to the abnormally elongated blastopore of a typical gastrula.
The cavity communicating with the blastopore and lined by the endoderm is the archenteron.
The endodermal spadix (sp) of the sporosac represents the endoderm of the manubrium; the ectodermal lining of the sporosac (ex.) represents the ex-umbral ectoderm of the medusa; and the intervening layers, together with the sub-umbral cavity, have disappeared.
In this stage the body is composed of two layers, ectoderm (d) externally, and endoderm (c) internally, surrounding a central cavity, the archenteron (b), which communicates with the exterior by a pore (a), the blastopore.
As in all Arthropoda, it is composed of three divisions, a fore-gut or stomodaeum, ectodermal in origin and lined by an inturning of the chitinous cuticle, a mid-gut formed by endoderm and without a cuticular lining, and a hind-gut or proctodaeum, which, like the fore-gut, is ectodermal and is lined by cuticle.