CHAPTER XIV. 



MOLLUSCS. 

 The MOLLUSCA include three large classes : — 



[A separate small class includes the Elephant's Tooth 

 Shells {Bentaltum), and some authorities admit some other 

 small classes.] 



General Characteristics of Mollusca. 



Most Molluscs live in the sea, from the shore to the great 

 depths, but there are many fresh-water Gasteropods and 

 bivalves, and the terrestrial snails and slugs are legion. 



The bivalves feed on microscopic animals and organic 

 ddbris ; the Gasteropods are carnivorous or vegetarian ; the 

 Cephalopods are voracious flesh-eaters. The members of 

 the last two classes have a rasping ribbon (radula) in their 

 mouths. 



In contrast to the ringed Arthropods and Annelids, 

 the Molluscs are unsegmented and without serial appen- 

 dages. 



The bilateral symmetry of the larva is usually retained in 

 the sluggish bivalves and in free-swimmers, e.g., cuttlefishes, 

 but most Gasteropods are markedly lop-sided. 



A single or double fold of skin, called the mantle, lines 



