NOTES ON SOME PROBLEMS OF ADAPTATION. 



247 



owing to the effects of erosion and its consequences (Arey and 

 Crozier, 1919). 



The total number of ctenidia is at first 64-66, and increases in a 

 fairly regular manner as growth proceeds. The average maxi- 

 mum number is about 98. The data are given in Table I. 



TABLE I. 



DISTRIBUTION OF TOTAL NUMBERS OF CTENIDIA IN RELATION TO LENGTH OP 

 FOURTH SHELL-VALVE; ALL INDIVIDUALS. 



2. Since the number of gills is determined in part through 

 growth, it is of interest to learn if the extent of the gill series is 

 influenced by sex. Fig. I makes it plain that such influence, if 

 real, must be slight. It is possible that there is on the average a 

 slightly higher number of ctenidia in the males, when the compari- 

 son is made of males and females equal in size; on the basis of 

 age this slight difference is increased, for the females are the 

 larger, at the same age (unpublished data). It is doubtful if this 



