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THE AMERICAN NATURALIST 



[Vol. LIII 



not met by growth from interambulacrum V, produces a rela- 

 tively considerable distortion of the old lunule. Moreover, the 

 coalescence of the lunule-walls may be carried to a length which 

 seems greater than it need be, as a a in Fig. 3. For these rea- 

 sons the idea of a specific regulatory character in the re-forma- 

 tion of the lunule seems unnecessary. 



It would be of interest to observe the effect, upon the course 

 of regeneration, of an injury deep enough to remove the area 

 of a lunule down to its proximal border; I have seen no cases 

 of this kind, nor any in which the interambulacral lunule had 

 been affected. 



Morgan^ was unable to find in the literature any mention of 

 regeneration among echini. The present observations may con- 

 sequently help to fill a gap at that point, since I am not aware 

 that any one has previously commented upon the matter, al- 

 though the conditions described are perhaps well known to other 

 students of echinoderms. For M. pentapora cases have indeed 

 been noted in which a lunule, unclosed, extended to the periphery 

 of the disc; but these instances have been referred either to 

 accidental injury or to delayed growth, and "repair" phenomena 

 seem not to have been observed. In M. sexiesperforata regenera- 

 tion (of a sort) does undoubtedly occur, giving evidence of a 

 respectable degree of plasticity in a stony structure where such 

 might not be expected. 



W. J. Crozier 



