MOLLUSCA. 277 



right side of the body. Over six thousand species belong 

 here, some (snails) having a well-developed spiral shell, 

 while the slugs are apparently shell-less; but in these slugs 

 one can frequently find a rudimentary 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 large, and adapted for digging in the sand, in 

 which these animals live. There is no distinct head, but 

 the mouth is provided with a lingual ribbon. In the 

 anterior part of the mantle-cavity are a pair of bunches of 

 long threads of unknown function; possibly they are sen- 

 sory, possibly respiratory, in nature. All of the tooth-shells 

 are marine. 



CLASS IV. CEPHALOPODA (see p. 264). 

 CLASS V. ACEPHALA (see p. 257). 



