VII MOLLUSCA 275 



pods and Siphonopods and capable of being protruded at the mouth and 

 used for rasping off food material. The surface of the tongue is covered 

 with a ribbon of conchiolin, known as the radula, the surface of which 

 carries numerous tooth-like spines arranged in transverse, rows. The 

 character, grouping, and numbers of the teeth in each row differ in 

 different groups of the molluscs in question, and in the case of the 

 Gasteropoda the radular formula — an arrangement of figures expressing 

 the number and grouping of the teeth — is made use of in defining the 

 characteristics of the various subdivisions of the group. These radulae 

 when dissected out and freed from adhering soft tissues with the aid of 

 caustic potash make interesting microscopic preparations. The teeth at 

 the front end of- the radula gradually become worn down and ineffective 

 through use and special arrangement exists to compensate for this. At 

 its hinder end the radula is continued along the floor of the radular sac — 

 a blindly ending extension backwards of the floor of the pharynx. At 

 the hind end of this active formation of cuticle is constantly going on 

 by which the radula is there added to. As this new formation goes 

 on the radula is gradually paid out by the radular sac, travelling 

 forwards over the surface of the tongue at a rate just sufficient to make 

 up for the wearing away in front. In the Pelecypoda — in correlation 

 with their feeding on minute floating particles — the radula has completely 

 disappeared. The food particles are drawn into the mantle-cavity in 

 the inhalent current caused by the ciliary movements of the gills, and 

 they are then collected and driven into the mouth by special organs 

 known by the somewhat misleading name labial palps (Fig. iii, C, l.ji). 

 The large crescentic mouth is bounded by a dorsal and a ventral lip which 

 in the middle hardly projects at all but which towards each end increases 

 in depth till it forms a large triangular flap. The dorsal and ventral 

 flap on each side are closely apposed to one another : on their apposed 

 surfaces are richly ciliated channels which lead towards the mouth open- 

 ing, and in these a current of water, laden with food particles, passes 

 inwards to the mouth. In the most archaic group of Pelecypoda — 

 the Protobranchiata (so called from their retaining in an unmodified 

 form the primitive feathery ctenidium) — the two apposed labial palps on 

 each side have become lengthened out to form a tube which can be 

 protruded far beyond the limits of the mantle-cavity and worked over 

 the surface of the sand, drawing in food-particles after the fashion of a 

 minute vacuum-cleaner. 



The alimentary canal of the mollusc forms a long tube which winds 

 about and is more or less distinctly dilated to form a stomach into which 

 there open the ducts of a pair of large digestive glands — the so-called 



