CHAPTER XXII 



THE FOOD OF ANIMALS— OMNIVOROUS MOLLUSCS, 

 INSECTS, AND CRUSTACEANS 



MOLLUSCS (Mollusca) 



Of the five great groups here included, Cephalopoda (Cuttle- 

 Fishes, &c.), Gastropoda (Snails and Slugs), Scaphopoda (Tusk- 

 Shells), Lamellibranchia (Bivalves), and Protomolluscs (Mail- 

 Shells, &c.), the first includes eminently carnivorous forms. 



SNAILS AND SLUGS (Gastropoda) 



The majority of the marine members of this group are car- 

 nivorous, though many are vegetarians, while the opposite is the 

 case with land forms. The rasping organ which they in common 

 with the Cephalopods possess, and which has been already fully 

 described (p. 95), is capable of dealing with almost any kind of 

 food, and it is not therefore surprising to find that some species 

 are omnivorous. This is especially true for certain Land -Slugs, 

 such as the Grey Field -Slug {Limax agrestis) and the Black 

 Slug {Arion ater), which prefer vegetable matter, especially 

 fruit, but also devour all sorts of animal substances, and appear 

 to be partial to earth-worms. No interesting specializations, how- 

 ever, to the omnivorous habit have been so far observed, and 

 here, as in almost all other cases, accurate and extended obser- 

 vation on the way of life and its relation to structure are badly 

 needed. 



TUSK-SHELLS (Scaphopoda) 



The curious Tusk-Shells (see vol. i, p. 339) which constitute 

 this class are apparently omnivorous. The knobbed filaments 

 they possess, and which can be protruded from the mouth of 

 the shell, are said to be used for the capture of small organisms 

 living in the surrounding sand, especially minute flinty -shelled 



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