26 PAUL PELSENEER. 



not confined to a particular region (fig. 9) as in Onchidiopsis, 

 for example, although the female acini open into the male 

 acini as in the last-named genus. I have never, however, 

 noticed ova and spermatozoa in the same cul-de-sac, as Lacaze- 

 Duthiers found to be constantly the case in C. norvegicum 

 (18). 



(3) Entovalva. 



The structural details of the hermaphrodite genital gland 

 are not known. There exists a single gland, not differentiated 

 into sexual regions, on each side (19). Probably, therefore, it 

 is constituted like that of Ostrea and Cardium. 



(4) Pecten. 



The greater number of species of this genus hitherto exa- 

 mined have been found to be hermaphrodite, viz. P. glaber 

 (20), P. Jacobseus (20a), P. maximus (21), P. opercularis 

 (22), P. irradians and magellanicus (23). 



I can myself vouch for the hermaphroditism of P. glaber 

 and maximus. Moreover I have observed that P. flexuosus, 

 Poli, from the Mediterranean, also has the two sexes united in 

 the same individual (fig. 10). 



On the other hand, among all the species examined, I have 

 only met with two dioecious forms, viz. P. inflexus, Poli, from 

 the Mediterranean, and P. varius, Linne, from the Atlantic, 

 in which latter form this fact was already known (Humbert, 

 23a). 



The hermaphroditism of Pecten is recognised with unusual 

 facility. The anterior part of the visceral mass may be readily 

 seen from the outside to be whiter than the posterior part; 

 the whiter part is the male region. Examination of fresh 

 material, or a section, shows at once that the genital products 

 of different sexes are formed in different regions of the herma- 

 phrodite gland (fig. 10). At the same time, as is already 

 known in the case of certain species, there is only one common 

 genital orifice on each side (opening into the nephridium), and 

 only a single genital duct, which ramifies in the various parts 



