LOWER IN VKll TELiltA TES. 



Fig. 310. — Jaw of 

 Limax jiavus. 



the foot is :i conit'ous or calcareous structure, the operculum, which is eiiqiloyed to 

 close tlie ajierture of the shell when tlie animal retracts itself. The operculum may- 

 be either liorny or calcareous, and frequently shows a 

 spiral structure. Some of the calcareous opercula of 

 t he smaller top shells are in common use as ' eye-stones.' 

 \l\ the older conchologists it was sometimes held that 

 till Cephalophora possessed bivalve shells like the 

 Acephals, the true shell being regarded as one valve, 

 and tlie operculum as the other. This view has been 

 shown to be erroneous, and now it is usually thouglit 

 that the ojierculum correspionds to the byssus of the 

 other grouji. 



In most of the forms there is a chanilier formed 



.:. ;ils. — Mchvttho ponderosa \)iu%\n\\y 



exteniied, showing the operculum (o) on either side of the bodv by the free edye of the 



on the upper surface of the foot. , , , i • ,^ tt ,i , ,'.-,, 



mantle and the body itself. Usually the palhal cham- 

 ber on one side (usually the right) is larger than tlie other, and contains the gills when 

 these organs are present. It also contains the outlet of the alimentary canal. 



The mouth is situated on the under side of the head, and is armed with variously 

 arranged jaws or plates of a hard chitinous or calcareous nature. Besides these, there 

 is found in all e.xcept a very few forms an odontojihore, or, as it is 

 occasionally called, a tongue or lingual ribbon. This consists 'of a 

 ribbon-like band of chitin, attached at one end and free at the other, 

 and bearing on its upper surface numbers of hard, tooth-like processes. 

 The odontophore is attached to the floor of the mouth, and is moved 



by appropriate muscles. 

 When in use it is drawn 

 over some su]i]iorting 

 cartilages, and the teeth, 

 acting like a lile, rasp 

 away the substance to 

 which the mouth is ap- 

 lied. The action can 

 1 10 partly seen by watch- 

 iiig pond snails feeding 

 upon the green slime 

 which frequently collects 

 on the sides of an aqua- 

 rium. The size of the 

 odontophore varies 

 greatly, as does also the 

 number of teeth. In some it is very long, in others it is more oval. The teeth them- 

 selves are arranged in transverse series, there being in some species about two hundred 

 in a single transverse row, while in others there are Vint three. By use this ribbon is 

 constantly wearing away at the tip, but the loss is compensated by a continuous growth 

 at the other end. 



Within recent years the characters derived from the lingual ribbon have been 

 regarded as very important in the arrangement of molluscs, but like all other good 

 thiii<TS this means of classification has been carried to an extreme ; forms which in 



Fig. 320. — Diagram of the mouth and lingual ribbon of a gasteropod; j, j 

 mouth; a, ojsophagus; r, lingual ribbon; s, support of ribbon. 



