192 



GASTEROPODA. 



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 (Lymnseus, 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 ex- 

 istence. 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 larvae 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 develop- 

 ment 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. 



Nassa mutabilis. This form, the development of which has been 

 very thoroughly worked out by Bobretzky (No. 242), will serve as an 

 A B 



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. 



