150 JOURNAL OF CONCHOLOGY, VOL. 16, NO. 5, JANUARY, I92I. 



In all these instances, the radula has probably been replaced by 

 something in the nature of a feeding-tube furnished with suctional 

 properties. This is inserted into the slime or tissues of the host, and 

 draws nourishment therefrom ; gruel or jelly needs no teeth to bite 

 it, and teeth accordingly have aborted. Doubtless to similar causes 

 is due the absence of radula in Coralliophila, which lives exclusively 

 on corals, and in Magilus, which becomes imbedded in Meandrifia^ 

 and communicates with the outer world by a long shelly tube. 



Other cases occur in which the radula is clearly on its way to 

 profound modification or total disappearance. The whole genus 

 Columbella presents a median tooth which is a mere thin plate, 

 destitute of cusps, while the laterals are singularly aberrant. This 

 degradation must date very far back. Liomesus ebiirneus M. Sars has 

 a somewhat similar median tooth, while the laterals are unicuspid. 

 While the other species of the genus, FolHa, have normal radula ; 

 F. pagodus Rve. has only three shapeless masses for median and 

 laterals, the true form of which is seen at the ' nascent end ' of the 

 radula, but becomes obscured at the forward part. (In this con- 

 nexion it would be worth while to look carefully for vestigial ' nascent 

 ends ' in the ' Gymnoglossa,' which might furnish evidence of their 

 true relationship). In these last-mentioned cases there is no evidence, 

 whether of habitat or food, to determine the reason of the degrada- 

 tion. Two cases have occurred to me. Morula spectnwi (Rve.) and 

 Drupa digitata (Lam.) in which the radula was not degraded, but 

 profoundly modified as compared with all the other known species 

 of the group. -^ It appeared eventually that both species live only on 

 coral, and, without knowing why coral should produce this curious 

 effect, one may be quite sure that there is some definite cause under- 

 lying the radula-change in the other cases cited. 



If, the conditions of life, the habits, not at one stage of life only, 

 and the food staples of the mollusca are more carefully studied, 

 valuable information bearing on the development and modification 

 of the radula cannot fail to be acquired. 



I Cambridge Natural History, vol. iii, p. 22?, f. 124 : Proc. Malac. Soc, xii, 1917, p. loi. 



