GASTROPODA—PROSOBRANCHIATA. 99 
Cuass GASTROPODA. 
Head distinct, usually furnished with eyes and 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. Sexes separate, in different indi- 
viduals. 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 branchiz. 
Subclass OPISTHOBRANCHIATA. Marine slugs breathing by arbor- 
escent or fasciculated branchiz, which 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 branchie, and itself more 
or less concealed by the mantle-lobes. Sexes united. 
Subclass PULMONIFERA. Sexes 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. 
Susctass PROSOBRANCHIATA. 
The prosobranchiates 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 modification 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- 
