126 THE GASTROPODA 
the Elysiomorpha. ntoconcha, Enteroxenos, and bathyscoadium are 
the only genera in which the male and female acini are quite 
distinct. In its most primitive condition the genital duct is 
hermaphrodite, that is to say, it is a spermoviduct throughout its 
length, and is therefore called monaulic. It generally is provided 
with an internal double longitudinal fold. The hermaphrodite 
aperture is situated on the right side, near the opening of the 
pallial cavity, and is connected by a ciliated seminal groove with 
the more anteriorly situated penis. This condition is found in the 
Bullomorpha (Fig. 98, s.7) in general, including the Thecosomata ; in 
the Aplysiomorpha (Fig. 154, 7), including the Gynmosomata (Fig. 
84, IV, XI); and in the Pulmonata Pythia (Fig. 171). The edges of 
this seminal groove unite to form a complete tube in Cavolinia longr- 
rostris among the Bullomorpha, and among the Pulmonata in all the 
Auriculidae except Pytlia, and as a consequence the primitive 
genital aperture serves only for the emission of the female pro- 
ducts, the male products passing through a spermiduct closed 
throughout its extent. In subsequent stages of evolution of the 
genital duct the spermiduct takes its origin from the hermaphro- 
dite duct above the external opening: this latter duct, therefore, 
bifureates or becomes ‘‘diaulic,” the female branch of the duct 
opening by the primitive hermaphrodite orifice. | This condition is 
characteristic of Valvata and Oncidiopsis (Fig. 103), of Actaeon and 
Lobiger among the Bullomorpha, of the Pleurobranchidae and the 
Nudibranchia except the Doridomorpha and most of the Elysio- 
morpha, and of the Pulmonata. At the point of bifurcation the 
male and female sections of the duct are separated by a narrow 
slit, which only allows the spermatozoa to pass. In this case 
therefore, as in the dioecious Gastropoda, the female orifice 
remains in the same place as the primitive genital aperture, and 
the male orifice is carried far forward, to the extremity of the 
penis. The two external orifices, male and female, are thus at some 
distance from one another, as may be seen in Valvuta, Oncidiopsis 
(Fig. 103, f.0, pe), the Basommatophora in general, the Oncidiidae 
(Fig. 59, of, om), and Vaginula (Fig. 87, of). But the female 
aperture itself may be secondarily shifted from its original position, 
and come so near to the penial aperture as to be contiguous to it, 
a condition found in the Pleurobranchidae and the Nudibranchs in 
general; or the two apertures may reunite in a common cloaca, as 
in the Stylommatophora (Fig. 177, Il), Siphonaria, and Amphibola. 
In these various cases the female duct, like the hermaphrodite duct 
of the monaulic forms, bears a bursa copulatrix or receptaculum 
seminis, which in certain stylommatophorous Pulmonates, such as 
Helix aspersa, Clausilia, ete., is provided with an accessory branch 
(Fig. 104, Li.s). 
A third differentiation of the genital ducts is brought about 
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