MOLLUSCA PROPER, 203 
ing air directly, many of the members of this group can only 
live in damp or moist places, while others habitually live in 
fresh water. The common pond-snails are examples of these 
last. The condition of the shell varies much. Some, such as 
the common land-snails, have a well-developed shell within 
which the animal can completely withdraw itself for protec- 
Fig. 107.--Limax Sowerbyi, one of-the slugs (after Woodward). 
tion. Others, such as the common slugs (fig. 107), have a 
rudimentary shell which is completely concealed within the 
mantle. Others are entirely destitute of a shell. They all 
agree with the typical Gasteropods in creeping about on a 
broad, flattened foot. 
CLASS III. PrEROPODA.—This class is a very small one, and 
includes a number of minute oceanzc Molluscs, which are found 
swimming near the surface in the open ocean, far from land, 
and often in enormous numbers. The organs of locomotion 
are two wing-like fins (fig. 108) attached to the sides of the head, 
Fig. 108.—Pteropoda. a@ Cleodora pyramidata: b Cuvieria columella. 
(After Woodward). 
and formed by a modification of a portion of the foot. The 
body is usually protected by a symmetrical glassy shell (fig. 108), 
consisting of two plates united along their edges, or in other 
cases forming a spiral. In some, however, there is no shell, 
