DICECIA — APHALLIA AND EXOPHALLIA. 
So2 
Tliese genital ducts may be special, or the nepliridial ducts may be 
utilized, the genital products falling into the kidneys direct or by 
way of the pericardium and thence conveyed to the exterior. 
The Dicecia for convenience of study may be classified as Ajdiallia, 
Exophallia, and Cryptophallia. 
The Apiiallia (a, not ; (/xiXAos, penis) are characterized by the 
absence of any organ by which copulation can be accomplished, the 
reproductive organs simply consisting of a paired ovary or testis, and 
the ducts leading therefrom to the exterior, no accessory organs are 
developed, the Si)ermatozoa being simply shed by the male into the 
circumambient water and some making their way to the ova or to the 
oviducal tract of the female ; this organization is possessed by all 
the Dioecious Pelecypoda, the Ghitonidar, and many of the more 
archaic Diotocardiate Gastropods. There is little sexual dimorphism 
displayed, but the female of Unio tumidus is said to develop a broader 
.shell than that of the male. 
The Exophallia (e^o, external; (fiuXXo'i, penis) embrace those 
dioecious species in which the 
males have acquired an ex- 
ternal copulatory organ, pro- 
jecting freely from the right side 
of the neck and arising as a 
muscular outgrowth from the 
body-wall ; it may be as a 
dependence of the head as in 
I'ir/jHtnt, etc., of the mantle as 
in .1 mpullarhi, or of the foot as 
in the generality of species. 
It is strongly erectile and 
extensile by concentrated blood 
l)ressure, and though not 
eversible and retractile as in 
the Gryptoi)hallia, it can often 
be bent in a sigmoid form and 
hidden beneath the mantle 
folds, the spermatozoa being conveyed thereto, in the more primi- 
tive forms, in which the male organ is a solid and thick appendage, 
liy an open ciliated duct ; in more highly organize<l forms this duct 
becomes closed and included within the body, and constitutes the 
Fig. GI4. Fig. 645. 
Fig. 644. — Male reproductive organs of Cycle- 
stoina clegans x .3, illustrating the 
organization of the Diccciate E.xophallia. 
p. penis ; v.d. vas deferens ; t. testis ; pr, 
prostate. 
Fig. C45. — Female reproductive organs of 
Cyclostoma elegans (Mull.), X 3 (the indistinct 
and only partially separated spermatheca is not 
shown). 
ov. ovary ; 7ti. sacculated or glandular oviduct ; 
V. vagina ; <?. female orifice. 
