STRUCTURE OF THE SNAIL. 323 



turpentine, it smells strawberries from afar. We know, 

 moreover, where this sense of smell is " located," for a dis- 

 horned snail has none. The tips of both pairs of horns 

 bear sensory cells connected with ganglionic tissue and 

 nerve-fibres within. 



Other sensory cells, probably of use in tasting, lie on the 

 lips ; and there are many others, which may be called tactile, 

 on the sides of the foot, and on various parts of the body. 

 In short, the snail is diffusely sensitive. 



Alimentary System. — The snail feeds for the most part on 

 the leaves of plants. It files these by means of the radula 

 or toothed ribbon which lies in the mouth, and grasps the 

 debris with its lips. 



The radula is a long strip of membrane, bearing several 

 longitudinal rows of little chitinoid teeth. It rests on a 

 cartilaginous pad on the floor of the mouth-cavity, and is 

 moved (backwards and forwards, and up and down) in a 

 curve, by protractor and retractor muscles. The whole 

 apparatus, including radula-teeth, membrane, and pad, is 

 called the odontophore. The work of rasping wears the radula 

 anteriorly, but it is continually being added to posteriorly 

 within a radula sac which projects like a knob from the floor 

 of the oral or buccal cavity. Its action on leaves may be 

 compared very roughly to that of a file, but its movements 

 within the mouth also produce a kind of suction which 

 draws food particles inwards. In this suction the mus- 

 cular lips and the abundant cilia in the mouth-cavity 

 assist. 



Altogether apart from the radula, lying on the upper 

 surface of the buccal chamber, sometimes visible when the 

 snail opens its mouth, is a hard, crescent-shaped jaw-plate. 

 Against this the radula seems to work. 



The ducts of two large salivary glands open on the dorsal 

 surface of the buccal cavity, and there are numerous distinct 

 glandular cells close to the entrance of the two ducts. The 

 salivary glands are large lobed structures, and extend 

 far backward on the crop. They consist of hundreds of 

 glandular cells or unicellular glands, which secrete a clear 

 fluid stuff. This travels up the ducts, and is forced, in part 

 at least by muscular compression, into the buccal cavity. 

 While some say that this fluid converts starch into sugar 



