The alimentary canal has, typically, the form of a bent tube with a ventral concavity, constituted (when an anus is present) of three sections, viz. an œsophagus, a stomach, and a rectum. The œsophagus and sometimes the rectum are epiblastic in origin, while the stomach always and the rectum usually are derived from the hypoblast[139].
Fig. 218. Diagram of an embryo of Pleurobranchidium. (From Lankester.)
f. foot; ot. otocyst; m. mouth; v. velum; ng. nerve ganglion; ry. residual yolk spheres; shs. shell-gland; i. intestine.
To the above characters may be added a glass-like transparency; and the presence of a widish space possibly filled with gelatinous tissue, and often traversed by contractile cells, between the alimentary tract and the body wall.
Fig. 219. Larvæ of Cephalophorous Mollusca in the veliger stage. (From Gegenbaur.)
A. and B. Earlier and later stage of Gasteropod. C. Pteropod (Cymbulia). v. velum; c. shell; p. foot; op. operculum; t. tentacle.
Considering the very profound differences which exist between many of these larvæ, it may seem that the characters just enumerated are hardly sufficient to justify my grouping them together. It is, however, to be borne in mind that my grounds for doing so depend quite as much upon the fact that they constitute a series without any great breaks in it, as upon the existence of characters common to the whole of them. It is also worth noting that most of the characters which have been enumerated as common to the whole of these larvæ are not such secondary characters as (in accordance with the considerations used above) might be expected to arise from the fact of their being subjected to nearly similar conditions of life. Their transparency is, no doubt, such a secondary character, and it is not impossible that the existence of ciliated bands may be so also; but it is quite possible that if, as I suppose, these larvæ reproduce the characters of some ancestral form, this form may have existed at a time when all marine animals were free-swimming, and that it may, therefore, have been provided with at least one ciliated band.