1909.] Hermaphroditism in Mollusc Crepidula fornicata. 479 



however, I have adopted the simpler plan of calling all forms with a penis, 

 males ; it being understood that nothing more than the presence of the 

 external male character is implied. 



The youngest forms, however, are doubtless exclusively males as regards 

 function, and the oldest forms probably exclusively females. 



Examination of the gonad, then, at different periods in the life-history of 

 Crepidula fornicata, makes clear that at first it produces ripe sperms only, 

 but becoming with advancing age more and more egg-producing, until finally 

 it is probably entirely egg-producing. There is, therefore, no doubt that all 

 the individuals of this species are born as males, and change in the course of 

 - their life-history into females. 



It is interesting to note that all stages of the gonad may occur among the 

 individuals of a single chain. A comparison of the primary with the 

 secondary sexual characters made at any phase in the life-history of an 

 individual shows that the development of the former is always in advance of 

 the latter ; indeed, the primary sexual characters forecast the secondary sexual 

 characters.* 



It has been shown that the right antero-lateral borders of the shells of all 

 the individuals in a chain are very close together ; since the penis of the 

 male and the external aperture of the uterus of the female are also on the 

 right side anteriorly, it follows that any male in a chain could transfer sperms 

 to any female ; but no such transference has yet been observed. 



Crepidula fornicata appears to be the only one of the many species of the 

 genus which has taken to the habit of forming chains of more than two 

 individuals ;f in several other species, however, namely, C. adunca, C. plana, 

 C. convexa, a male is often found mounted on a female (5). Crepidula 

 fornicata is also the only species of the genus yet described as hermaphro- 

 dite, but probably other species are hermaphrodite, as will be shown below. 



In Crepidula fornicata it would seem, therefore, that chain-formation and 

 hermaphroditism are in some way causally connected. Knowing, as we do, 

 that most of the genus Crepidula are sedentary in habit, and that 

 sedentariness is associated throughout the whole animal kingdom with 

 hermaphroditism ; knowing, further, that a closely allied species, Crepidula 

 plana, shows at least signs of, if^not complete hermaphroditism (see below), it 

 would seem that chain-formation is an adaptive phenomenon, which has 

 arisen along with, and favoured the acquisition of, protandric hermaphro- 

 ditism. 



* In view of the conception of a sexual formative substance (4, p. 85), this phenomenon 

 is not without significance, 

 t See footnote, page 482. 



