[Illustration: FIG. 46.--Developed larva of _Cyclas cornea_ L.: _br_, rudimentary branchiae; _by_, byssus; _f_, foot; _m.e_, mantle edge; _sh_, shell. (After Ziegler, highly magnified.)] [Illustration: FIG. 47.--=A=, Advanced veliger of _Dreissensia_: _f_, foot; _m_, mouth; _sh_, shell; _v_, _v_, velum. (After Korschelt and Heider, much enlarged.) =B=, Veliger of a Pteropod (_Tiedemannia_): _op_, operculum; _sh_, shell; _v_, velum. (After Krohn, much enlarged.)] In this so-called _veliger_ stage, the velum serves, in the first place, to cause rotation of the larva within the egg-capsules, and, after hatching, as an organ of locomotion. As a rule, the velum disappears entirely in the adult mollusc after the free-swimming stage is over, but in the common _Limnaea stagnalis_ it persists, losing its cilia, as the very prominent circum-oral lobes. Simultaneously with the development of the velum, and in some cases earlier, appear the rudiments of the shell-gland and of the foot, the latter being situated on the ventral side, between the mouth and anus, the former on the dorsal side, behind the velum, and above the surface of the eventual visceral sac. Thus the prime characteristics of the veliger stage, subsequent to the appearance of the velum itself, are the development of the visceral sac and shell-gland on the upper, and of the foot on the under side. According to Lankester the primitive shell-gland does not, as a rule, directly give rise to the shell of the adult mollusc, but becomes filled up by a horny substance, and eventually disappears; the permanent shell then forms over the surface of the visceral hump from the original centre of the shell-gland. It is only in _Chiton_, and possibly in _Limax_, that the primitive shell-sac is retained and developed into the final shell-forming area, which is much wider, and extends to the edges of the mantle. Within the velar area first appear the rudiments of the tentacles and eyes; when these become developed the velum atrophies and disappears. Several of these veligers when captured in the open sea have been mistaken for perfect forms, and have been described as such. Thus the larva of _Dolium_ has been described as _Macgillivrayia_, that of a _Purpura_ as _Chelotropis_ and _Sinusigera_, that of _Aporrhais pes pelecani_ as _Chiropteron_, that of _Marsenia conspicua_ as _Brownia_, _Echinospira_, and _Calcarella_. _Cephalopoda._--The embryonic development of the Cephalopoda is entirely distinct from that of all other Mollusca. The segmentation of the vitellus is partial, and the embryo is furnished with a vitelline sac, which is very large in the majority of cases (Fig. 48). There is no free-swimming stage, but the embryo emerges from the egg fully developed. =Differences of Sex.=--In the Mollusca there are two main types of sexual difference: (i) sexes separate (_dioecious_ type), (ii) sexes united in the same individual (_hermaphrodite_ type).