126 The Under-Water World 



importance, many being of value as food, 

 ornament and building material. A few 

 play an important part in the religious 

 life of certain oriental races. 



Molluscs are divided into a number of 

 groups which include such different forms 

 as the snails, limpets, whelks, oysters, 

 slugs and octopods. Those bearing 

 external shells are divided into the 

 univalves and the bivalves, the shell in 

 the former being made in one piece, as 

 in the snail, and the latter in two united 

 by a hinge as in the oyster. 



In the univalve molluscs the animal is 

 provided with a large fleshy " foot " by 

 means of which it can crawl and climb, 

 and a long, ribbon-shaped tongue bear- 

 ing sharp-edged :{ teeth " numbering 

 sometimes several thousands. The ner- 

 vous system is well developed but not 

 concentrated as in the higher molluscs 

 the Octopods and Cuttle-fish to form 

 a brain-like mass. 



