MOLLUSCA PROPER. 185 



highly organized of all the Gasteropoda, 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 as a good example of the 

 group, there is a little limpet-shaped shell protecting the 

 gills (b) and heart. The animal swims, back downward, by 

 means of a vertically-flattened ventral fin (/), on one side of 

 which is a little sucking-disk (<#), by which the animal can ad- 

 here at pleasure to floating sea-weed. Carinaria is found in the 

 Mediterranean 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. 87). 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 branchias. 



FIG. SI.Limax Sowerbyi, one of the slugs (after Woodward). 



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- 

 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- 

 tion. Others, such as the common Slugs (Fig. 87), 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 HI. PTEEOPODA (Gr. pteron, wing ; podes, feet). 



