The shell-gland arises as an epiblastic thickening on the posterior and dorsal side. In this thickening a deep invagination ([fig. 101], shs.) is soon formed, in which a chitinous plug may become developed (Paludina, Cymbulia? etc.), and in abnormal larvæ such a chitinous plug is generally formed.
The foot is a simple prominence of epiblast on the ventral surface, in the cavity of which there are usually a number of mesoblast cells ([fig. 101], f). The larval form just described has been named by Lankester the trochosphere larva.
Before considering the further external changes which the larva undergoes, it will be well to complete the history of the invaginated hypoblast.
Fig. 102. Embryo of A Heteropod. (From Gegenbaur; after Fol.)
o. mouth; v. velum; g. archenteron; p. foot; c. body cavity; s. shell-gland.
The hypoblast has after its invagination either the form of a sack ([fig. 102]) or of a solid mass ([fig. 101]). Whether the mouth be the blastopore or no, the permanent œsophagus is formed of epiblast cells, so that the œsophagus and buccal cavity are always lined by epiblast. When the blastopore remains permanently open the outer part of the œsophagus grows as a prominent ridge round the opening.
The mesenteric sack itself becomes differentiated into a stomach adjoining the œsophagus, a liver opening immediately behind this, and an intestine. The cells forming the hepatic diverticula and sometimes also those of the stomach may during larval life secrete in their interior peculiar albuminous products, similar to ordinary food-yolk.
The proctodæum, except when it is the blastopore, arises later than the mouth. It is frequently developed from a pair of projecting epiblast cells symmetrically placed in the median ventral line behind the foot. It eventually forms a very shallow invagination meeting the intestine. Its opening is the anus. The anus, though at first always symmetrical and ventral, subsequently, on the formation of the pallial cavity, opens into this usually on the right and dorsal side.