4, LAND AND FRESH-WATER MOLLUSKS. 
water mollusks belong to the three classes, 
Conchifera, Gasteropoda, and Pulmonifera. 
The ordinary bivalves (Conchifera)—as . the 
oyster, mussel, and cockle—are all aquatic 
animals, and by far the greater number of them 
live in the sea. They breathe by means of 
two pairs of gills in the form of plates or lamine, 
and hence are also called Lamellibranchiata ; 
they are, moreover, destitute of any distinct’ 
head, and are inclosed in a shell, composed of 
two pieces or valves. 
The snail-like animals have a distinct head, 
provided with eyes, horns or tentacles, and a 
mouth armed with cutting jaws; the under side 
of the body forms a single muscular foot, on 
which the animal creeps. The body is usually 
protected by a shell composed of one piece. 
Among these. belly-footed animals some live in 
the water, and breathe by means of gills,—these 
are the true Gasteropoda; others live on the 
land, as the snail and slug, and have the respi- 
ratory organ in the form of a lung,—these are 
comprised in the class Pulmonifera. Some, 
indeed, inhabit our fresh waters ; but, unlike the 
true fresh-water Gasteropods, they are compelled 
to come to the surface to breathe the air, and are 
not capable of suffering a long immersion in the 
water. 
To pursue the scheme of classification. Hach 
