Sense Organs of Molluscs. 87 



this lid (or operculuni) can be seen in the whelk and 

 it is in shape similar to the outline of a section aero s 

 the opening of the last whorl of the shell. 



The mouths of some shells are channeled at their 

 front end (the end farthest from the coiled part), and 



FIG. 52. 



Diagram of the Anatomy of a Whelk, the shell being removed, c, stomach"; 

 e, end of the intestine ; g, gills ; d, auricle ; //, ventricle of the htart ; 

 /, nerve-ganglia of the mouth ; b, salivary gland. 



sometimes at their posterior end; these channels are 

 for siphon-like tubes, and as a general rule such 

 molluscs as possess these siphons are carnivorous, 

 while those with unchanneled or entire edges are 

 herbivorous. 



Some univalves, like the common snail and slug, 

 live on dry land ; in such forms gills would be useless, 

 and hence they are absent, and a part of the mantle 

 cavity is set apart for air-breathing, and the lining of 

 this region is full of dilated blood-vessels. The 

 mouth of this air-chamber is small, arid can be seen 



