STUDIES ON SEX IN CREPIDULA 49 



the adult life of the ammal, is based upon insufficient evidence. 

 The observations on Crepidula certainly do not bear this out. 

 On the contrary they go to show that the male and female germ 

 cells are recognizable as such very soon after the swimming 

 larva takes up the creeping mode of life. When a method has 

 been perfected for rearing the larvae through the free-swimming 

 period, it is hoped that the still earlier history of the gonad may 

 be investigated. 



SUMMARY 



Crepidula plana is a protandric hermaphrodite in which the 

 male and female phases are completely separated from each 

 other. 



The assumption of the male condition does not always occur 

 at the same stage in the life history, with respect to the age or 

 size of the animal; and there is reason to believe that the male 

 phase is sometimes entirely omitted. 



The growth of the animal during the first part of its life, i.e. 

 during the period in which male development may occur, is 

 very variable and depends in part upon: 1) the amount of move- 

 ment of the animal; 2) the amount of space available for the 

 extension of the mantle; 3) the season of the year. 



Primordial male and primordial female cells are both present 

 in the gonad at all periods of life from the post-larval up to the 

 adult female phase and are visibly different from each other. 



Crepidula plana possesses, in common with other Proso- 

 branchs, atypical as well as true spermatozoa. The former are 

 of the 'apyrene' variety. They develop from cells which cannot 

 at first be distinguished from spermatogonia. After the cells 

 which are to form apyrene spermatozoa are differentiated there 

 are no maturation or other divisions. 



During the change from the male to the female condition part 

 of the testicular cells complete their development and are passed 

 into the seminal vesicle as adult spermatozoa. The rest are 

 absorbed into the wall of the testis where they are dissolved; or 

 else they degenerate in the lumen of the gonad. The gonad 

 undergoes a temporary reduction in size; it again becomes large 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 23, NO. 1 



