96 



NATURAL HISTORY. 



a borer, but it prefers stone or clay to wood for its excavations. We have 

 several > ( >ecies of piddocks on our coast, but from their burrowing habits 

 they are only occasionally eaten, and do not form an article of diet as 

 they do in Europe. 



GASTEROPODS. 



There is quite a contrast between the gasteropods and the acephals. 

 I., ili.- present group the shell consists of but a single valve (except in 

 ti„. chitons), and is usually, but not always, coiled in a spiral, — a feature 



which destroys to 

 a greater or less 

 extent the sym- 

 metry so charac- 

 teristic of other 

 molluscs. There 

 is also a well-de- 

 veloped head sup- 

 porting the sense- 

 organs, and the 

 mouth with a very 

 few exceptions is 

 provided with a 

 'lingual ribbon' — 

 a peculiar appara- 

 tus for comminuting food — which is lacking among the acephals, but 

 which reappears again in the squids and cuttle-fishes. 



This lingua] ribbon is really an animal file with teeth of quartz, and 

 coiled lip just inside the mouth. It is constantly growing at one end, 

 to make up for the constant wearing away by use at the other. When in 

 use, the ribbon and its supporting cartilages are advanced and pressed 

 againsl the object, and then the ribbon is rasped over the surface, at 

 each stroke taking off: small particles of it. The pattern and number 

 of teeth on one of these ribbons varies with the species, and hence 

 characters derived from these organs are used to a considerable extent 

 in systematic work. 



The chitons are the only gasteropods with more than one shell; here 

 there arc several parts, one ranged after another, giving the body a banded 

 appearance. The numbers of these parts never exceed eight. From a 

 popular standpoint the chitons possess almost no attractive features, but 

 to the naturalist they are among the most interesting of all molluscs, on 



Fig. 88. — Chitons. 





