126 THE GASTROPODA 



the Elysiomorpha. Entoconcha, Enteroxenos, and Bathysciadium 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.g] in general, including the Thecosomata ; in 

 the Aplysiomorpha (Fig. 154, i), 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 Carolinia longi- 

 rostris among the Bullomorpha, and among the Pulmonata in all the 

 Auriculidae except Pythia, 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, 

 bifurcates 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 Adaeon 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 Valvata, Oncidiopsis 

 (Fig. 103, f.o, pe), the Basommatophora in general, the Oncidiidae 

 (Fig. 59, o./, o.m), and Vaginula (Fig. 87, o.f). 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, II), 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, etc., is provided with an accessory branch 

 (Fig. 104, KS). 



A third differentiation of the genital ducts is brought about 



