182 COMPARATIVE ANATOMY CHAP. 



certain distance its lower surface. Immediately in front of the tongue 

 there is always a depression in the ventral pharyngeal wall, forming 

 a sort of pocket. The radula, at the posterior extremity of the 

 tongue, sinks into a narrow more or less long tube, the radular sheath, 

 which is an outgrowth of the pharyngeal cavity running downward 

 and backward. The radula, always lying upon the anterior or 

 ventral wall of this sheath, which is anteriorly thickened to form 

 the tongue, extends to the base of the sheath, which is the place 

 of its formation. 



The tongue with the radula on it is movable, and in most cases its movements can 

 be compared with those of the cat's tongue when licking, but are usually slower. 

 This action helps to rasp the food which has been seized, and often also broken up, by 

 the jaws. The tongue can either move inside the pharyngeal or buccal cavities, or 

 can be extended to the oral aperture or even protruded more or less far beyond it. 



In or under the fleshy tongue, a lingual cartilage is very commonly found, 

 consisting of two or four or even more pieces. This cartilage forms a support for 



the radula, and affords firm points of attachment 

 for certain muscles belonging to the lingual 

 apparatus. 



The musculature of the pharynx, which can 

 be separated into bundles or strands, and is often 

 very complicated, consists first of the muscles 

 which form the wall of the pharynx, and which, 

 being principally developed ventrally and laterally 

 round the radula, determine the special licking 

 movements of the tongue ; secondly, of muscles 



wufch rao the whole ?"7" or the 1' ole , f the 



lingual apparatus, evagmating or protruding them. 

 The second group consists, speaking generally, of protractors and retractors, attached 

 at the one end to the pharynx and at the other to the body w r all after running 

 through the cephalic or body cavity. Pressure of blood may also take some part in 

 the protrusion of the pharynx. 



The tongue and radula further often serve for seizing prey (e.g. in the carnivorous 

 Heteropoda, in Testacclla, etc.). 



The radula is of great importance in classification. Further details concerning 

 it must be sought in special works and in text-books of conchology. The points to 

 be specially noticed are (1) the size and form of the whole radula, (2) the number of 

 longitudinal and transverse rows of teeth, and (3) the form of the teeth in each of 

 these rows. As a rule the transverse rows resemble one another, but exceptional 

 rows differently constituted from those immediately preceding or following them 

 recur at intervals. 



Three kinds of teeth have been, as a rule, distinguished. First, there is usually 

 a single median longitudinal row of central or rachial teeth. On each side of this 

 row are several rows of more or less similar lateral teeth or pleurae. Finally, at the 

 lateral edges of the radula, there are single or very numerous longitudinal rows of 

 marginal teeth or uncini. 



Dental formulae are used for the radular teeth, in the same way as for the teeth 

 of mammals ; in these the number of central, lateral, and marginal teeth in a 

 transverse row are given. 



The reader will find the dental formula of some of the Molluscs in the Systematic 

 Review. 



