32 PAUL PELSENEBR. 



of Anatinacea, or Latched specimens of Cyclascornea smaller 

 than 4 millimetres : at this size both spermatozoa and ova are 

 already to be founds and not infrequently even developing 

 eggs among the gills. But according to the old observations 

 of Davaine (39) and P. J. van Benedeu (40), Ostrea edulis 

 is protandric. 



The alternating activity of the two sexes in the same herma- 

 phrodite MoUuscan individual is a perfectly certain fact, and 

 explains various mistakes like that of Saint-Loup, who believed 

 he had found a unisexual organisation in Aplysia associated 

 with external sexual characteristics (41). 



It must, however, be remarked that this succession of the 

 two sexual conditions may be more or less rapid according to 

 the particular genera under consideration, and that, conse- 

 quently, the alternation is better marked in certain organisms 

 than in others. It seems to me to be more appreciable where 

 the hermaphroditism of the genital gland is most complete 

 (i. e. where the ova and spermatozoa are produced at the same 

 spot, as in Ostrea, Aplysia, &c.) than where there are acini 

 or regions of different sex (Nudibranchs, &c.). 



As for the general fact that the male precedes the female 

 activity, even in the absence of physiological observations it 

 might have been deduced from the morphological constitution 

 of the genital glands in the adult of a great number of herma- 

 phrodite molluscs. In those forms, in fact, which possess acini 

 or regions of each sex, the male part is always nearest the 

 efferent ducts or the genital aperture : the male products will 

 accordingly be the first to reach them. This arrangement 

 exists in P e c t e n, Nudibranchs, Pleurobranchid se, Onchidiop- 

 sis, Entoconcha, Pneumonoderma, Clionopsis, &c. 

 Cyclas alone constitutes an exception, — a fact for which I 

 can offer no explanation. 



For the rest, our knowledge of the physiological evolution of 

 individual hermaphroditism in other groups of the animal 

 kingdoms shows that protandry is general in all forms which 

 have been examined from this point of view, e. g. Sponges, 

 Plathelminthes (Trematodes and Cestodes), hermaphrodite 



