202 INVERTEBRATE ANIMALS. 
forms which are found swimming at the surface of the open 
sea, instead of creeping about at the bottom of the sea. In. 
order to adapt them for this mode of life, the foot, instead of 
forming a creeping disc, is modified to form a compressed fin . 
(f). The Heferopoda are to be regarded as the most highly 
organised of all the Gasteropodu, at the same time that they are 
not the most typical members of the class. Some of them can 
retire completely within their shells, but others have large 
bodies, and the shell is either small or entirely absent. In 
Carinaria, which may be taken asa good example of the group, 
there is a little limpet-shaped shell protecting the gills (4) and 
heart. The animal swims, back downwards, by means of a 
vertically-flattened ventral fin (/), on one side of which is a 
Fig. 106.—Heteropoda. Carinaria cymbium. '~ Proboscis and mouth; 
z Tentacles ; 6 Gills; s Shell;_/ Foot ; d Disc (after Woodward). 
little sucking disc (d), by which the animal can adhere at plea- 
sure to floating seaweed. Carizaria is found in the Mediter- 
ranean and other warm seas, and is so transparent that the 
course of the intestine can be seen along its whole length. 
The last group of the class is that of the “air-breathing” 
Gasteropods, so well known as land-snails, pond-snails, and 
slugs (fig. 107). All the members of this group are formed 
to breathe air directly, instead of through the medium of 
water, and they, therefore, never possess gills or branchie. 
In place of these they have a pulmonary chamber or lung, 
formed by a folding of the mantle, and having air admitted to 
it by a round hole on the right side of the neck, which can be 
opened and closed at will. Though thus adapted for breath- 
