There are, however, variations in regard to the relation of the blastopore to the mouth and to the anus which are probably modifications of the original process described above.
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.
Round the blastopore hollow outgrowths, variable in number, arise by the evagination of the entire body-wall, both ectoderm and endoderm.
The general features of this process and of the relation of the blastopore to mouth and anus have been explained in treating of the development of Mollusca generally.
In this development the blastopore is not elongated; it persists as the anus.
In the anthopolyp the blastopore is carried inwards by an in-pushing of the body-wall of the region of the peristome, so that the adult mouth is an opening leading into a short ectodermal oesophagus or stomodaeum, at the bottom of which is the blastopore.
A, pilidium larva anus by the closure in of nemertine; B, Asplanchnapus the middle of such a slitschematized; C, a ploimal rotifer; like blastopore opening D, trochosphaera female (schematized into a sack-like stomach from Semper); E, veliger larva of mol as is seen in the larvae lusc; F, trochophore larva of annelid.
The little mass of hypoblast or enteric cell-mass now enlarges, but remains connected with the cicatrix of the blastopore or orifice of invagination by a stalk, the rectal peduncle.
The enteron arises as a space in the endoderm, and an opacity - the primitive streak - appears at the hind end of the blastopore (fig.
After a time the planula fixes itself by the anterior pole, with the blastopore uppermost.