206 INVERTEBKATE ANIMALS. 



neck, whicli can be opened and closed at will. Though thus adapted 

 for breathing 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 very much. Some, such as the 

 common Land-snails (fig. 142), have a well-developed shell within 



Fig. \i(].—Limax SovxrViji, one of the Slugs. (After Woodward.) 



which the animal can completely withdraw itself for protection. 

 Others, such as the common Slugs (fig. 146), have a rudimentary 

 shell which is completely concealed within the mantle. Others are 

 entirely destitute of a .shell. They all agree with the typical Gas- 

 teropods in creeping about on a broad, flattened foot. 



Class III. Pteropoda. 



This class is a very small one, and includes a number of minute 

 oceanic Molluscs, often called " Winged Snails," 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. 147) attached to the sides of the head, and formed 

 by a modification of a portion of the foot. The body is usually pro- 

 tected by a symmetrical glassy shell (fig. 147, D), consisting of two 

 plates united along their edges, or in other cases foi-ming a spiral 

 (fig. 147, A and C). In some, however, there is no shell, and the 

 body is quite naked (fig. 147, B). The head is rudimentary, and 

 bears the mouth, which ia furnished with an odontophore. The 

 heart consists of an auricle and ventricle, and the respiratory organs 

 are extremely rudimentary. The sexes are united in the same indi- 

 vidual in all the Ptempixlu. 



The Pteropoda occur, as already said, in the open ocean, and they 

 are found in all seas from the tropics to within the arctic circle, 

 sometimes in such numbers as to discolour the water for many 

 miles. Minute as they are, they constitute in high latitudes one of 

 the staple articles of diet of the wliale, and they themselves in turn 



