1898.] NATURAL SCIENCES OF PHILADELPHIA. 441 



environment. But in this case, as in that of the irregular shells 

 mentioned above, I have found no evidence that these modifications 

 have become in the least degree heritable. 



III. Marked as is the environmental dimorphism in C. plana, the 

 sexual dimorphism is even greater. The average body volume of a 

 mature male of this species is about 55 cc, while the volume of an 

 adult female is about I cc. ; that is, the average female is almost 15 

 times as large as the average male. In all species of Crepidula the 

 males are smaller than the females, though the difference in size is 

 greatest in C. plana. The following table gives the actual and rel- 

 ative sizes of males and females of the different species : — 



c . Actual vol. of bodv. 



S l 3ecies Male. Female 



C. plana, . . .046 cc. ;667 cc. 



. C. adunca, . .025 cc. .208 cc. 



C. convexa, . .01 cc. .05 cc. 



C.fomicata, . 1.25 cc. 1.6 cc. 1. 1.34 



These averages are derived from the measurement of at least 20 ma- 

 ture individuals of each sex. 



In the case of the males as in that of the dwarfs, the smaller size 

 of the body is due to the smaller number of cells present rather than 

 to the smaller size of the cells. Careful measurements of cells of 

 the intestine, stomach, liver, kidney, muscles of foot, epithelium of 

 gill chamber, and epithelium of gill filaments show that the cell size 

 remains the same in the male as in the female. Whatever the ulti- 

 mate cause of the smaller size of the males may be, it operates in 

 this case as in that of the dwarfs by causing a cessation of cell growth 

 and division. 



In all these species the males are almost invariably found mounted 

 upon the shells of the females, and in plana, adunca and convexa 

 they are able to move about more or less freely, but the full-sized 

 males of fomicata are as immovably fixed to one spot as are the fe- 

 males. In such cases sexual union could take place only between 

 individuals attached near to each other. On muddy bottoms C.for- 

 nicata has the habit of piling together, one individual on top of 

 another, until there may be as many as ten ortwelve individuals in 

 a single chain ; such chains are often found in which there is not a 

 single male, and yet I have never found an unfertilized female. 

 Again, perfectly isolated females with large numbers of fertilized 



29 



