HERMAPHRODITISM IN MOLLUSCA. 35 



hermaphrodite, is an organ possessing a single mesodermic mode 

 of origin, as in all the other Triploblastica. 



V. Origin of Hermaphroditism in Mollusca. 



It has just been shown that, both from the phylogenetic and 

 ontogenetic points of view, the hermaphrodite condition with 

 separate male and female glands is the most specialised, and 

 the undifferentiated hermaphrodite condition (with gland 

 producing spermatozoa and ova at the same spot) the most 

 archaic. Now this last state is that which most nearly 

 approximates to the unisexual condition, since it only differs 

 in the supplementary production of the elements of the other 

 sex, — a phenomenon which is sometimes to be observed as an 

 abnormality in dioecious molluscs (e. g. Anodonta and 

 Ampullaria). 



The question may, then, be asked in the case of the Mollusca, 



1. Whether the hermaphrodite state is not derived from 

 the unisexual; and, in the event of an affirmative reply, 



2. Upon which of the sexes the hermaphrodite condition 

 has become established. 



In the following pages I shall try to show — 



1. That hermaphroditism is not a primitive arrangement 

 in the Molluscan phylum (47), and that it has been derived 

 from the unisexual state. 



2. That it has become superimposed upon the female 

 condition. 



1. Hermaphroditism has been derived from the 

 unisexual condition. — Let us consider separately the classes 

 in which hermaphroditism is found and those in which the 

 dioecious state alone exists. 



i. In the classes in which hermaphroditism exists it is 

 abundantly clear that the forms with separate sexes are the 

 most archaic, especially in the conformation of their repro- 

 ductive apparatus (absence of special duct, accessory gland, 

 and penis) : 



A. Gastropoda Docoglossa and various Rhipidoglossa : the 

 genital gland opens into the kidney. 



