vii MOLLUSCATHE ALIMENTARY CANAL 183 



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

 mondi to 39,596 in Helix Ghietsbrcghti. 



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 

 number of teeth (carnivorous Prosobranchia} ; (2) absence of a protrusible proboscis, 

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

 often large, teeth (ffeteropoda, carnivorous Pulmonata and Cephalopoda). 



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

 it may be half (Daudebardid) 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 occupy the anterior end of the 

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



In very rare cases (apart from the Lamellibranchia] the radula completely 

 atrophies ; this is the case in parasitic Gastropoda (Stilifer, Eulima, Thyca, Ento- 

 concha], in the ComUiophilidce (Coralliophila, Leptoconchus, Magilus, Rhizochilus) , 

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

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

 single tooth of the radula is retained. 



Even in certain carnivorous Prosobranchia 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 Terebra). 



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 Opisthobranchia they appear as 

 cuticular formations secreted by several 

 transverse rows of large epithelial cells 

 the odontoblasts (Fig. 156) ; the basal 

 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 FlG 156 ._ Longltudil , al sec tion 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 Pulmonate (after Rossler), diagram. 1, 2, 

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

 ,1.1 c . , , , , . , , , , f formative cells of the basal membrane ; 6. 7, 



tudmal row of teeth there is at the base of ^ of ^ ^^ . g> ^ membrane ; 



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 Chiton idee, Prosobranchia, 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, Scaphopoda, Opisthobranchia, and Cephalo- 

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

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

 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 



