MOLLUSCA—THE ALIMENTARY GAKAL 



183 



The total number of radular teeth varies very greatly, from 16 in Eolis Drum- 

 mondi to 39,596 in Helix GJiiesbreghti. 



As a rule, the teeth are most numerous and finest in herbivorous animals. In 

 carnivorous Molluscs we have two extremes : (1) great development of the proboscis, 

 with weak development of the pharynx and radula, and a comparatively small 

 nimiber of teeth (carnivorous Prosohranchia) ; (2) absence of a protrusible proboscis, 

 with great development of the pharyngeal apparatus and the radula, and numerous, 

 often large, teeth {Heteropoda, carnivorous Pulmonata and Cephalopoda). 



The muscular pharynx is most developed in carnivorous Pulmonates. In these 

 it may be half {Daudcbardia) or even more than half as long {Testacella) as the 

 whole body, and may occupy a very large part of the body cavity. It is protruded 

 in such a way that the tongue with the radula occujjy the anterior end of the 

 evaginated pharynx (Fig. 54, A, p. 44). 



In very rare cases (apart from the Lamellihranchia) the radula completely 

 atro})hies ; this is the case in parasitic Gastropoda {Stilifer, Eulima, Thyca, Ento- 

 concha), in the Coralliophilidce {CoralliopMla, Leptoconchus, Magihis, Rhizochilus), 

 among the Nudibranchia in Tethys and Melihe, among the Amphineura in Neomenia, 

 and certain species of the genera Dondersia and Proneomenia. In Cluetoderma, a 

 single tooth of the radula is retained. 



Even in certain carnivorous Prosohranchia which are furnished with a proboscis, 

 the above-mentioned reduction of the whole pharyngeal apparatus goes so far that 

 the radula disappears (certain species of Tcrchra). 



Formation of the Radula. 



The teeth of the anterior transverse rows of the radula become worn out by use, 

 and are continually being replaced by new teeth which are pushed forward. The 

 formation of new transverse rows of teeth 

 is constantly taking place at the posterior 

 blind end of the radular sheath. In Pul- 

 monata and Ojnsthohranchia they appear as 

 cuticular formations secreted by several 

 transverse rows of large epithelial cells — • 



the odontoblasts (Fig. 156) ; the basal V^9^0Z-a -/'■ ■•jN'W^i^MiMWS 

 membrane which carries the teeth is secreted 

 by the anterior row or rows, the teeth them- 

 selves by the posterior rows. 



Each group of odontoblasts which has j,,^_ ise.^Longitudinal section through 

 formed a tooth is not replaced by another, the posterior end of the radular sheath of 

 but continues to produce new teeth behind a Pulmonata (after Rossler), diagram. 1, 2, 

 those already formed, so that for each longi- 3, 4, Formative cells of the radular teeth ; 5, 



formative cells of the basal membrane ; (5, 

 teeth of the radula ; S, basal membrane. 



tudinal row of teeth there is at the base of 



the radular sheath a group of odontoblasts 



which has produced all the teeth belonging to that row. A layer of "enamel" is 



deposited on the teeth so formed by the epithelial roof of the radular sheath. 



In the Chitonidcc, Prosohranchia, and Cephalopoda, the odontoblasts are very 

 numerous narrow cells, which form, at the base of the sheath, a cushion divided into 

 as many parts as there are teeth in a transverse row of the radula. 



The radular sheath in the Pulmonata, Sccqohopoda, Opisthohranchia, and Cejihalo- 

 poda is short, and is contained in the ventral and posterior nmscular wall of the 

 pharynx, very seldom projecting posteriorly beyond it ; but in many Prosohranchia 

 it is long and narrow, and reaches back into the cephalic cavity or even right into 

 the body cavity. This latter is especially the case in the Diotocardia ; in the 



