84 ANIMAL FORMS 
masticatory apparatus which consists of a kind of tongue 
with eight to forty thousand minute teeth in our land 
forms (Fig. 51), 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. 
Fie, 51.—A small portion of the radula or In the stomach of some 
tongue-rasp of a snail (Sycotypus). 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. 
85. 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. 48). 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 
