GASTROPODA — PROSOBRANCHIATA. 99 



Class GASTROPODA. 



Head distinct, usually furnished with ej^esand tentacles ; body 

 mostly protected by a spiral or conical univalve shell ; lower 

 surface of animal developing a thickened, expanded, creeping 

 disk or foot. 



The following subclasses conveniently separate the immense 

 number of molluscan types having the gastropod structure : 



Subclass PROSOBRANCHIATA. Scxcs Separate, in diiferent indi- 

 vidual's. Mostly marine animals, provided with a shell and 

 generally, an operculum — at least all operculated mollusks 

 belong to this group. The animal breathes by gills or branchiae. 



Subclass OPISTHOBRANCHIATA. Marine slugs breathing by arbor- 

 escent or fasciculated branchise, whiph are more or less completely 

 exposed on the back and sides, towards the posterior end 

 (opisthen) of the body. A large division of the Opisthobran- 

 chiates is shell-less ; another possesses a spiral, conical or 

 lamellar shell, partially concealing the branchise, and itself more 

 or less concealed by the mantle-lobes. Sexes united. 



Subclass PULMONIFERA. Scxcs United in the same individual. 

 Mostly terrestrial (a portion being fluviatile) mollusks, usually 

 provided with a shell, without operculum ; breathing air by the 

 simplest form of lung, a pouch with external opening, lined with 

 a network of respiratory vessels. 



The pulmonifera are closely related to the plant-eating sea- 

 snails (holostomata), through Cyclostoma,and to the nudibranchs 

 by Oncidium. As a group, they are generally inferior to the 

 sea-snails, on account of the comparative imperfection of their 

 senses, and the union of the functions of both sexes in each 

 individual. 



Subclass PROSOBRANCHIATA. 



The prosobrancliiates are typically marine animals, but there 

 are many exceptions to the rule ; for not only do we find a 

 certain number of genera inhabiting brackish water, but some 

 live in fresh water only, and others again are terrestrial. It is 

 not without some modilication of the breathing organs that such 

 diversity of station exists, and this modification is coexistent 

 with other adaptations. 



Whilst the pulmoniferous mollusks have no operculum, the 

 terrestrial and fluviatile sections of the prosobranchiates are 

 provided with a very efficient one, completely closing the aper- 

 ture of the shell. The canaliculate aperture, the operculum usually 

 too small to fill this aperture, and, frequently, the want of an 

 operculum are characteristic of the major portion of the proso- 



