CHAPTER XVII 

 THE FOOD OF ANIMALS— PLANT-EATING MOLLUSCS 



The class of Cephalopoda, including Cuttle- Fishes and their 

 allies, contains no vegetarian forms, and since the Tusk-shells 

 (Scaphopoda) and bivalve molluscs (Lamellibranchia) affect a 

 mixed diet, they will be noticed in a later chapter. The great 

 class of Gastropoda, however, including Snails and Slugs in the 

 broadest sense of the words, embraces a large number of forms 

 which live exclusively upon vegetable food, Periwinkle, Limpet, 

 and Garden-Snail being familiar examples. As previously noted 

 (see p. 96) the shells of such forms are distinguished by the fact 

 that their opening or mouth has a continuous outline and is not 



Fig. 435. — Single Rows of Teeth from radulse of Plant-eating Sea-Snails (greatly enlarged) 

 A, Trochus. ji, Periwinkle {Littorina) 



interrupted by a notch for the reception of a breathing tube. 

 There are also a number of characteristic features about the soft 

 parts of vegetable-feeding gastropods. The mouth is not placed 

 at the end of a proboscis capable of being either protruded or 

 drawn back into the body, as is the case in many carnivorous 

 species, but either opens directly on the rounded front end of 

 the head, or upon a short blunt snout. The rasping organ 

 (odontophore) projecting from the floor of the mouth is provided 

 with a horny ribbon (radula) that is often of great length, and 

 instead of being studded by comparatively few and specialized 

 teeth (see p. 96) is provided with a very large number of simple 

 bluntish ones (fig. 435). We also find additional hard parts in 

 the form of one or more horny jaws projecting from the roof 



