162 



GASTROPODA. 



The two lobes of the velum are very large and give the larva a 

 'characteristic appearance (Fig. 72, v). They remain undivided, are 

 -very broad and are beset with long thick cilia ; the fissure between 

 .them, however, usually carries short and delicate cilia, the bilobed 

 (character of the velum being in this way emphasised. The shell has 

 lost its flat and (later) cup-like shape and, in the free-swimming larva, 

 has already become coiled. The foot (/) develops an operculum on 

 its dorsal surface. We thus find, in the Opisthobranchia, the same 

 general condition already met with in the Prosobranchia and the 

 Heteropoda. Although most Opisthobranchs, as adults, are entirely 



nice, 



d.-' 



FIG. 72. Veliger larva of an Opisthobranch. a, anus ; ad, anal gland (? probably an 

 excretory organ, like n) ; d, alimentary canal ; di, diverticulum of the stomach ; f, 

 foot ; m, mouth ; mu, muscle (retractor of the velum) ; op, operculum ; ot, otocyst ; 

 s, shell ; v, velum. 



devoid of a shell, or else, as in the majority of the shell-bearing forms, 

 have a highly specialised, often greatly reduced, or even internal 

 shell, the larva possesses a coiled, often nautiloid shell into which 

 the body can be withdrawn and which can be closed by an operculum 

 (M. SAES). The shell is usually thrown off and the operculum al- 

 most invariably has the same fate. When, however, the adult 

 possesses a shell, we may fairly safely assume that this has been 

 derived from the larval shell. In only a few Opisthobranchs is the 

 shell of the adult so large that the whole body of the animal can be 



