REPRODUCTIVE SYSTEM — DIOiCIA. 
351 
(Umce of large glycogen-bearing plasma shells by which they are so 
densely enveloped, this feature being most strikingly displayed by 
their accumulation among and around the constituent follicles of the 
germ glands ; their presence being clearly concerned in the provi- 
sioning and maturation of the ova and spermatozoa, as the store 
around the germ glands and sexual organs generally becomes 
exhausted during the reproductive season. 
Although reproductive power is usually associated with the attain- 
ment of full gTOwth, it is well established that sexual maturity, with 
power to perpetuate their kind, is often fully acquired by individuals 
whose shells are far from being completely grown and therefore quite 
destitute of the usual adult characters. 
The male organs fiequently mature somewhat in advance of those 
of the female, constituting Proter.yndry (tt/dwtos, first ; arqp, male) 
or more rarely, as in AgrioUmdx kvvis, may exhibit Proterogyny 
-po>ro<;, first ; yvyT), female), the female organs first accpiring func- 
tional perfection. In many of the Stylommatophora, however. Prof. 
Babor has verified that the genital organs almost invariably undergo 
a series of successive metamorphoses, in which the animals are first 
functionally unisexual and only subsequently become hermaphrodite ; 
this condition, however, is not always the termination of their sexual 
development, as the same animal may again become unisexual, by 
the atrophy of the sexual organs first developed and the retention of 
those acquired later. 
In accordance with their sexual characteristics, the mollusca fall 
naturally into two great groups, Dioecia and Monmcia. 
The Dkecia (Si'w, two ; otKo?, house) or rnisexual species comprise 
the great majority of the mollusca, and include all the Chitons and 
Cephalopods, and nearly all Pelecypods and Streptoneura. 
This group is especially composed of the simply organized or 
primitive species and of certain of the more active and highly 
specialized groups, the sexual organs being often so simple and 
undifferentiated, and so similar in the two sexes as to be undis- 
tinguishable except by microscopic examination, often consisting 
merely of a genital gland or glands, composed of ramified aggregations 
of cceca of simple structure, and a duct or ducts leading therefrom 
and opening within the exhalent chamber in the Pelecypoda or 
directly to the exterior immediately behind and near to the anus 
in the Aphalliate Gastropoda. 
