288 



LOWER INVERTEBRATES. 



Fig. 319. — Jaw of 

 Llmax fiavus. 



the foot is a corneous or calcareous structure, the operculum, which is employed to 

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

 be either horny or calcareous, and frequently shows a 

 spiral structure. Some of the calcareous opercula of 

 the smaller top shells are in common use as ' eye-stones.' 

 By the older conchologists it was sometimes held that 

 the Cephalophora possessed bivalve shells like the 

 Acephals, the true shell being regarded as one valve, 

 and the operculum as the other. This view has been 

 shown to be erroneous, and now it is usually thought 

 that the operculum corresponds to the byssus of the 

 other group. 

 1... 10 »f 7 <i 7 .■ 1, In most of the forms there is a chamber formed 



r IG. .J18. — Melantho ponderosa partially 



extended, showing the operculum (0) on either side of the body by the free edsre of the 



on the upper surface of the foot. J J o 



mantle and the body itself. Usually the pallial cham- 

 ber on one side (usually the right) is larger than the 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 except a very few forms an odoutopliore, 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 jirocesscs. 

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



by appropriate muscles. 

 When in use it is drawn 

 over some supporting 

 cartilages, and the teeth, 

 acting like a file, rasp 

 away the substance to 

 which the mouth is ap- 

 plied. The action can 

 be partly seen by watch- 

 ing 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 but 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 _3S very important in the arrangement of molluscs, but like all other good 

 things 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; .7, jaw; m^ 

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



