1883.] NATURAL SCIENCES OF PHILADELPHIA. 227 



odontophore, in order to show the direction of the teeth on the 

 radula (r). Tiie arrow (c) in the same diagram shows the direction 

 in which the radula moves wlien the animal is rasping the food. 



As regards the disappearance of the worn-out and useless teeth, 

 Semper says : " There are only two ways possible, since the view 

 that each tooth continually grows is not to be considered at all. 

 Once we thought, as did Troschel, Claparede and others, that the 

 radula gradually moved forward, and that the forward teeth that 

 were worn out were thus gradually replaced ; or there must be a 

 periodical shedding of the radula. This latter view seems to me 

 the most natural."^ 



Above it was shown that "the epithelium of the radula had no 

 connection whatever with the formation of the radula. On the 

 other hand it was observed that the radula as well as the teeth, a, 

 b, c, d, etc. (PL X, fig. 5), with the exception of the caps, grew 

 from behind, that is, from the cells of the matrix 1-5 (PL X, 

 fig. 5). 



From this we see that the radula grows at the posterior end of 

 the odontophore and must gradually be shoved forward, and that 

 the teeth that are used up at the mouth are gradually being replaced 

 from beliind. The view of a renewal of the radula by a periodical 

 shedding, as Semper thought most probable, is consequently 

 excluded. In many sagittal sections it is easy to see the anterior 

 part of the radula breaking awa^^ at the point, a; (PL X, fig. 2). 

 At this point separate teeth and parts of the radula could be seen, 

 and they would have been cast out at the mouth. 



Trinchese ^ gives in his paper on Spurilla Neapulitana a short 

 notice on the development of the radula in this species. He 

 speaks of from five to seven cells which go to form the teeth, and 

 also the cells forming the layer which I have called the epithe- 



1 "Hier sind nur zsveiFallo moglich, da die Annahme, dass jeder Zabn 

 fortwiihrend wachse, nicht welter zvi beriicksichtigen ist. Einmal konnte 

 man nun annehmen, dass, wie es audi Troschel, Claparede u. A. thun, 

 die Reibniembran alhiiiihlig vorriicke uiid dadurcb sowolil die vordern 

 untauglicheu Zabne ersetzt wiirden, als audi eine Grosseuzunahrae der 

 Zaline ermoglicht sei, oder man miisste eine von Zeit zu Zeit stattfindende 

 Hautung annehmen ; die letzten Annahme sdieint mir die natiirlichste." 

 Zum fein. Bau d. Molluskenzunge, jj. 277. 



* Anat. e fisiol. della Spurilla Neapolitana. Estrat. d. Serie III, Tomo 

 IX, d. Mem. dell' Acad, delle Scienze dell' Institute di Bologna, 2 Febbriao 

 1878. 



