XII 



BACKBONELESS ANIMALS 



261 



cuttlefish) are active and carnivorous, with light shells 

 or none ; in the dark depths many are blind or with 

 rudimentary eyes, but food seems to be so abundant 

 that there is almost no need to struggle for it. 



As to diet, there are three kinds of eaters carnivores, 

 such as the active swimmers we have mentioned, besides 

 the whelks and many other burglars who bore through 



FIG. 80. THE COMMON OCTOPUS. 

 (From Chambers's Encijclop. ; after Brehm.) 



their neighbours' shells, and the Testacella slugs ; vege- 

 tarians, like the periwinkle, the snail, and most slugs ; 

 and thirdly, almost all the bivalves, which feed on micro- 

 scopic plants and animals, and on organic debris wafted 

 to the mouth by the lashing of the cilia on the gills and 

 lips. In this connection it is important to notice that 

 all molluscs except bivalves have in their mouths a 

 rasping ribbon or toothed tongue (radula, odontophore) by 

 which they grate, file, or bore with marked effect. Of 

 parasites there are few, but one Gasteropod, Entoconcha 



