34 PAUL PELSENEEE. 



On the other hand, in a form where the male and female 

 acini are sharply separated in the adult state, viz. Pelta 

 (= Runcina), I find that in the young individual ova and 

 spermatozoa arise side by side on the walls of a common 

 pouch. 



I conclude from this that the undifferentiated condition of 

 the hermaphrodite gland is the most primitive, and the morpho- 

 logical observations recorded above (II) lead also to the same 

 conclusion. The Nudibranchs and Pleurobranchidae (with 

 acini either male or female) are more specialised than 

 Umbrella (with hermaphrodite acini); Onchidiopsis (with 

 acini either male or female) is more specialised than Valvata 

 (with hermaphrodite acini); Pneumonoderma (with acini 

 either male or female) is more specialised than Aplysia (with 

 hermaphrodite acini), &c. ; lastly, those Lamellibranchs with 

 male and female glands altogether separated (Anatinacea) are 

 the most specialised (cf. the closure of the mantle, the reduc- 

 tion of the foot, the complexity of the gill, involving the loss 

 of the external layer of the external branchial lamella). 



In direct opposition to the theory of the sexuality of the 

 embryonic layers, we find that the hermaphrodite gland has a 

 single origin in the mesoderm (which is itself endodermic), 

 not only in the Opisthobranchs and Pulmonates hitherto 

 studied — where the gland is not yet divided into regions of 

 different sex, — but also in Cyclas (44), where two distinct 

 male and female regions exist (fig. 12, v and xiv). The same 

 mesodermic origin is, moreover, to be observed in the case of 

 both the male and female glands of those forms with separate 

 sexes, such as Chiton, Paludina, and the Cephalopods. 

 These results are also in complete accord with what one 

 finds in other groups of hermaphrodite Invertebrates, e. g. 

 Oligochseta (45), Sagitta, Turbellaria, Plathelminthes, 

 Hirudinea (46). 



Lastly, the fact that the wall of the genital gland is in con- 

 tinuity with the coelomic epithelium (mesoderm) in Nuculidae 

 (Lamellibranchs), Neomeniidse (Amphineura), and Cephalo- 

 poda^ shows that the genital gland, whether male, female, or 



