All the Gasteropods and Pteropods have a shell-bearing larval form like that first described, with the exception of a few forms, such as Limax and perhaps some other Pulmonata, in which the shell-gland closes up and gives rise to an internal shell.
The subsequent metamorphosis in the different groups is very various, but in all cases it is accompanied by the disappearance of the velum, though in some cases remnants of the velum may persist as the subtentacular lobes (Lymnæus, Lankester) or the lip tentacles (Tergipes, Nordmann). In prosobranchiate Gasteropods the larval shell is gradually added to, and frequently replaced by, a permanent shell, though the free-swimming veligerous larva may have a long existence. In many of the Opisthobranchiata the larval shell is lost in the adult and in others reduced. Lankester, who has especially worked at the early stages of this group, has shewn that the larvæ are in almost every respect identical with those of prosobranchiate Gasteropods. They are all provided with a subnautiloid shell, an operculated foot, etc. The metamorphosis has unfortunately been satisfactorily observed in but few instances. In Heteropods and Pteropods the embryonic shell is in many cases lost in the adult.
The following sections contain a special account of the development in the various groups of Gasteropoda and Pteropoda which will complete the necessarily sketchy account of the preceding pages.
Gasteropoda. To illustrate the development of the Gasteropoda I have given a detailed description of two types, viz. Nassa mutabilis and Paludina vivipara.
Fig. 104. Segmentation of Nassa mutabilis.
(From Bobretzky.)
A. Upper half divided into two segments. B. One of these has fused with the large lower segment. C. Four small and one large segment, one of the former fusing with the large segment. D. Each of the four segments has given rise to a fresh small segment. E. Small segments have increased to thirty-six.
Nassa mutabilis. This form, the development of which has been very thoroughly worked out by Bobretzky (No. [242]), will serve as an example of a marine Gasteropod with a large food-yolk. The segmentation has already been described, p. [102]. It will be convenient to take up the development at a late stage of the segmentation. The embryo is then formed of a cap of small cells which may be spoken of as the blastoderm resting upon four large yolk cells of which one is considerably larger than the others ([fig. 104] A). The small and the large cells are separated by a segmentation cavity. The general features at this stage are shewn in [fig. 105] A, representing a longitudinal section through the largest yolk cell and a smaller yolk cell opposite to it. The blastoderm is for the most part one cell thick, but it will be noticed that, at the edge of the blastoderm adjoining the largest yolk cell, there are placed two cells underneath the edge of the blastoderm (me). These cells are the commencement of the mesoblast. In the later stages of development the blastoderm continues to grow over the yolk cells, and as it grows the three smaller yolk cells travel round the side of the largest yolk cell with it. As they do so they give rise to a layer of protoplasmic cells ([fig. 105], hy) which form a thickened layer at the edge of the blastoderm and therefore round the lips of the blastopore. These cells form the hypoblast. The whole of the protoplasmic matter of the yolk cells is employed in the formation of the hypoblast. The rest of them remains as a mass of yolk. A longitudinal section of the embryo at a slightly later stage, when the blastopore has become quite narrowed, is represented in [fig. 105] C. The greater part of the dorsal surface is not represented.