194 THE BIOLOGY OF THE SEA-SHORE 



specialised of them is that practised by Gasteropods, in which 

 the organ known as a " radula " is employed, aided by the 

 horny margins of the mouth. The radula is a long ribbon- 

 like structure, beset with transverse rows of chitinous teeth, 

 which works over the surface of ai muscular pad lying in 

 the floor of the mouth (see Plate XHI). By means of 

 muscles the radula may be drawn backwards and forwards 

 over the odontophore as over a pulley and so made to 

 rasp the surface of any food substance with which it may 

 be brought into contact. New material is being con- 

 tinually added to the posterior end of the radula to 

 take the place of that lost by wear and tear. The 

 same apparatus, which is nearly always present, serves 

 both for plant and flesh eating Gasteropods. It displays, 

 however, very considerable specific individuality as regards 

 the size and arrangement of the teeth and other points, which 

 are evidently correlated with differences in the quality of 

 the food. These adaptive modifications are well illustrated 

 by the sea-slugs. The Doridae, which subsist chiefly on 

 sponges, Bryozoa and compound Ascidians, have no jaws, 

 but a broad radula, and the whole apparatus is clearly suited 

 to eating a stationary object which does not need to be 

 captured and offers no projections to be bitten off, but does 

 require to be shredded before being digested. Other 

 forms have jaws with edges that cut like shears ; Tritonia 

 feeds upon the spiculate Alcyoniuni by cutting out portions 

 with its horny jaws and triturating them with the radula, 

 which is broad. In many forms feeding is accomplished by 

 rasping the surface of seaweeds and sucking up their juice, 

 the function of the radula not being strictly masticatory. 

 An exceptional mode of feeding is practised by Melihe which 

 has no radula at all and only feeble jaws. In compensation, 

 the mouth is surrounded by a large funnel with cirri which is 

 swept over the surface of stones to capture small crustaceans. 

 The margins of the funnel then contract and force the 

 contents down the oesophagus into the stomach where the 

 hard exo-skeletons are broken up by a ring of stomach- 

 plates (Eliot, 1 910). 



