The typical larva of a Mollusc, as has been more especially pointed out by Lankester, is essentially similar to the larva of a number of invertebrate types, and especially the Chætopoda, with the addition of certain special organs characteristic of the Mollusca.
It has a bent alimentary tract, with a mouth on the ventral surface and a terminal or ventral anus. The alimentary tract is divided into three regions: œsophagus, stomach, and intestine. There is a variously developed præoral lobe with a ring of cilia—the velum, and a peri-anal lobe, often with a patch of cilia (Paludina, etc.). In all these characters it is essentially similar to a Chætopod larva. The two characteristic molluscan organs are (1) a foot between the mouth and anus, and (2) an invagination of the epiblast on the dorsal side at the hinder end of the body, which is connected with the formation of the shell.
The larvæ of most Gasteropoda, Pteropoda, and Lamellibranchiata present no features which call for special remark; but the larvæ of the Gymnosomata amongst the Pteropoda, and of the Scaphopoda, Polyplacophora and Cephalopoda present interesting peculiarities.
The larvæ of the Gymnosomata are peculiar in the presence of three transverse ciliated rings, situated behind the velum ([Fig. 109]). These rings might be regarded as indications of a rudimentary segmentation; but, as already indicated, this view is not satisfactory. There is every reason for thinking that these rings have been specially acquired by these larvæ.
At first sight the larvæ of the Gymnosomata might be supposed to resemble those of the Scaphopoda, which are also provided with transverse ciliated rings; but, as shewn above, the rings of the Scaphopoda are merely parts of the extended velar ring.
Thus, the ciliated rings of the two larvæ—so similar in appearance—are in reality structures of entirely different values, being in the one case parts of the velum, and in the other special developments of cilia behind the velum.
The great peculiarity of the early larva of the Scaphopoda is the enormous development of the præoral lobe, which gives room for the development of the ciliated rings. In the presence of a central tuft of cilia, at the anterior extremity, the larva of the Scaphopoda resembles that of the Lamellibranchiata, etc.
The larva of the Polyplacophora resembles that of Lamellibranchiata in its anterior flagellum, and that of the Scaphopoda in the large development of the præoral lobe; but is quite peculiar amongst Mollusca in the transverse segmentation of the mantle area.
The embryo of the Cephalopoda agrees very closely with that of normal Odontophora in the formation of the mantle and (?) of the shell-gland, but is quite exceptional (1) in the almost invariable presence of a more or less developed external yolk-sack, (2) in the absence of a velum, (3) in the absence of a median foot, and in the presence of the arms.
The presence of a yolk-sack may most conveniently be spoken of in connection with the foot, and we may therefore pass on to the question of the velum.