24 PAUL PELSENEEE. 



The first point capable of immediate demonstration is that 

 there are both hermaphrodites and dioecious oysters. 



A. Hermaphrodite Oysters. 



a. Ostrea edulis, Linne, the hermaphroditism of which 

 was first demonstrated by Davaine (10), and subsequently 

 confirmed by Lacaze-Duthiers, Hock, &c. 



b. Ostrea stentina, Payr. (= plicata, Chemnitz); also 

 recognised as hermaphrodite by Lacaze (11). 



c. Lastly, it has been recently stated that the oyster of the 

 N.W. coast of America is also hermaphrodite ; this, I suppose, 

 is Ostrea lurida. 



In the genital glands of hermaphrodite oysters the sperma- 

 tozoa and the ova arise in the same acini, but at different times, 

 so that the products of the two sexes are not often to be seen 

 in the same individual. This perfect alternation is the most 

 striking and distinctive characteristic of the heramaphroditism 

 of oysters ; it explains how it has been believed, and how at 

 first sight it would still be possible to believe, that the oyster 

 is unisexual. 



The male products are the first to appear. This protandry 

 was discovered by Davaine (13), and confirmed by P. J. van 

 Beneden (14). 



B. Dioecious Oysters. 



a. Ostrea virginica, Lister, from the E. coast of the 

 United States (15). 



b. Ostrea angulata, Lam. (= lamellosa, Broc), the 

 "Portuguese^' oyster, from the Atlantic (16). 



c. Lastly, I have made out the separation of the sexes in a 

 third species, O. cochlear. Poll, from the Mediterranean, 

 which belongs to the same subdivision (Gryphsea) as the 

 preceding form. 



I have not had the opportunity of studying this species alive, 

 but I have had numerous specimens of it of very difi'erent sizes, 

 collected at difi'erent times of the year. 



Of all these there was not a single individual which pre- 



