HEEMAPHEODITISM IN MOLLUSCA. 29 



hand, as has been shown above, there is a very great diversity 

 in the structure of the hermaphrodite gland. 



Indeed, it was Leuckart who was the first to recognise that 

 among the Euthyneurous (or hermaphrodite) Gastropods the 

 genital gland is not always formed in the same manner, and 

 that different types of structure are recognisable in it. The 

 facts known to us to-day show that such is the case not only 

 for the Gastropoda Euthyneura, but also for all the remaining 

 groups of hermaphrodite molluscs — the Amphiueura, Gas- 

 tropoda Streptoneura, and Lamellibranchia. 



Four different principal types can be distinguished in the 

 conformation of the hermaphrodite gland of molluscs, but 

 certain forms are transitional between one type and another. 



1. The undifferentiated gland, i. e. with acini com- 

 pletely hermaphrodite. — As development shows (vide 

 infra), the most simple condition of an hermaphrodite genital 

 gland is that in which the gonadial wall is still undifferentiated, 

 and produces ova and spermatozoa side by side. This condi- 

 tion is exhibited in — 



(1) Valvata. 



(2) A great number of Tectibranchs (e.g. Bulla, 



Aplysia, Umbrella). 



(3) Almost the entire group of Pulmonata. 



(4) Ostrea edulis and stentina. 



A commencement of specialisation is observable in Neo- 

 meniidse, where each genital gland generally gives rise to male 

 products towards its lateral face, and to ova towards its axial 

 face. 



Amphibola among Pulmonates (vide supra) also fur- 

 nishes a transition to the following condition. 



2. Gland with separate male and female acini (but 

 as yet without separation into different male and female 

 regions). — After the undifferentiated condition comes that in 

 which the gonadial surface shows a clearly marked specialisa- 

 tion into distinct male and female acini, — the acini, however, 

 not forming regions of different sex. 



