The first of these, according to the nomenclature of the mid-gut from the stomodaeum and proctodaeum may be of Heymons (see fig.
After the formation of the gastrula by epibole the larva becomes enclosed by an ectodermic test covering the whole of the original surface of the body, including the shell-gland, and leaving only a small opening at the posterior end in which the stomodaeum and proctodaeum are formed.
In the four couples of mesenteries which are attached to the sides of the elongated stomodaeum the muscle-banners of each couple are turned towards one another, but in the sulcar and sulcular couples, known as the directive: FIG.
Gastrulation takes place by epiboly, and the stomodaeum (oral invagination - mastax pharynx) takes place in two stages of the region of the closed blastopore.
One end of the blastopore becomes nearly closed, and an ingrowth of ectoderm takes place around it to form the stomodaeum or fore-gut and mouth.
It has been sought to prove that the interior of the hypostome is lined by ectoderm, so as to form a stomodaeum or ectodermal oesophagus similar FIG.
B, Trochosphere of an Opisthobranch (Pleurobranchidium) showing - shgr, the shell-gland or primitive shell-sac; v, the cilia of the velum; ph, the commencing stomodaeum or oral invagination; ot, the left otocyst; pg, red-coloured pigment spot.
The elongation of the mouth and stomodaeum confer a bilateral symmetry on the body of the zooid, which is extended to other organs of the body.
The fourth pair, having its muscle-banners on the sulcar faces, is developed at the opposite extremity of the stomodaeum in close connexion with the sulculus.
In Actinia and its allies, and most generally, though not invariably, in Anthozoa,the stomodaeum is not circular, but is compressed from side to side so as to be oval or slit-like in transverse section.