MOLLUSCS. 



201 



luscs (fig. 39) are without such protection in the adult, 

 although shells are present in the young. In the nudi- 

 branchs there are commonly developed gills upon the dor- 

 sal surface, and in the living condition these forms are, 

 from their bright colors, among the most attractive of 

 molluscs. Here, too, are forms (Pteropods) especially de- 

 veloped for a life on the surface of the ocean, the foot 

 being modified into a pair of wing-like structures. 



ORDER II. PULMONATA. 



The great majority of the land- and 

 fresh-water snails and slugs belong 

 here. In them gills have disap- 

 peared, and the mantle-cavity has 

 been modified into an organ (lung) 

 for breathing air, the opening to 

 which is to be seen on the right 

 side of the body. Over six thousand 

 species belong here, some (snails) 

 having a well-developed spiral shell, 

 while the slugs (fig. 40) are appar- 

 ently shell-less; but in these slugs 

 one can frequently find a rudimen- 

 tary shell imbedded in the mantle. 



CLASS III. SCAPHOPODA 



(TOOTH-SHELLS) . 



In these the mantle edges are fused 

 below, forming a tube, and as a result 

 there is formed a tubular shell, open 

 at both ends, in shape something like the tusk of an 

 elephant. The foot is relatively large, and is adapted for 

 digging in the sand in which these animals live. There 

 is no distinct head, but the mouth is provided with a 



FIG. 40. Slug (Limax 

 campestris). s, respir- 

 atory opening. From 

 Ludwig's Leunis. 



