THE MOLLUSCAN RADULA. 129 



washed in water and returned to the platinum spoon and 

 again heated : it was then infusible and Avas presumably 

 silica. In a quantitative analysis the amount of silica was 

 found to be about one third of the total ash. In one case, in 

 the month ot April, 500 radulae were incinerated with a 

 Biinsen and the ash afterwards ignited in a Herapath with a 

 dissimilar result from the preceding ; no fusion of the ash 

 occurred, and in the analysis which followed no silica was 

 found. On the other hand, the presence of a large quantity 

 of phosphoric acid was indicated : the ash, which weighed 

 O'OlOl grm., yielded 0'0036 grni. of P0O5, or no less than 

 35"6 per cent. 



Development. 



The history of the study of the development of the radula 

 has been so excellently set forth by Eossler that on this point 

 I cannot do better than refer to his paper (15). 



It has already been stated that there is a certain amount of 

 disagreement among those who have studied the growth of 

 the radula. The radula passes backwards into the radular 

 sac, which is the seat of its renewal ; it exhibits a continuous 

 forward movement due to growth from behind taking place 

 simultaneously with wear of the front end which is in use. 

 The radular sac is an evagination of the wall of the foregut ; 

 it consists of a basal epithelium and a roofing epithelium ; 

 these two epithelia pass into each other by way of a heap 

 of cells at the posterior end of the sac. In front of this cell 

 aggregate, immediately behind and below the last formed 

 tooth, are a set of cells, the odontoblasts, which may be few 

 and large or more numerous and small. The roofing epi- 

 thelium extends between the young teeth at the posterior end 

 of the sac, fitting closely into the spaces between them ; it, 

 in many cases, secretes a cuticular substance in the anterior 

 portion of the sac which likewise fits into the spaces between 

 the teeth, so that the teeth lie in pits of this secretion. All 



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