MOLLUSKS 



101 



FIG. 57. A small portion of the radula or 

 tongue-rasp of a snail (Sycotypus). 



masticatory apparatus which consists of a kind of tongue 

 with eight to forty thousand minute teeth in our land 

 forms (Fig. 57), while in certain marine snails they are 

 beyond computation. With the licking motion of the 

 tongue this rasp tears the food into shreds before it is 

 swallowed, and in the whelks or borers it serves to wear a 

 circular hole through the shells of other mollusks, which 



are thus killed and devoured. 

 This latter process is facili- 

 tated by the secretion of the 

 salivary glands, which has a 

 softening effect upon the 

 shell. Ordinarily the saliva 

 of snails exercises some di- 

 gestive action. 



In the stomach of some 

 snails are teeth or horny 

 ridges which also are instrumental in crushing the food, 

 and in numerous minor respects peculiarities exist in differ- 

 ent species according to the nature of the food ; but in its 

 general features the digestive tract is similar to that of 

 the clams. 



The processes of circulation and excretion are also car- 

 ried on by means of systems which show a certain resem- 

 blance to those of the clams. As might be expected, certain 

 differences exist, sometimes very great, but they are of too 

 technical a nature to concern us further. 



96. Sense-organs of lamellibranchs and gasteropods. 

 The eyes of mollusks differ widely in their structure and 

 the position they occupy in the body. In our common 

 land snails two pairs of tentacles are borne on the head, 

 the lower acting as feelers, while each of the upper ones 

 bears on its extremity the eye, appearing as a minute black 

 dot (Fig. 54). In this same position the eyes of many 

 marine snails occur, but there are numerous species in 

 which there are other accessory eyes. In many of the 



