GASTROPODA. 



379 



together (most ISTudibranchs), or the two ducts join again to open 

 into a common cloaca (Stylommatophora, SipJionaria) (Fig. 298). 



There is a receptaculum seminis, which opens into the herma- 

 phrodite duct in the monaulic forms, and into the ovidu(?^/( vaginal 

 part) in the diaulic forms ; in the Dorids and Elysids this structure 

 not only opens into the oviduct but communicates directly with 

 the surface of the body by a tube which may be called the vagina. 

 The vagina opens close to the oviduct. This is the triaulic arrange- 

 ment, and recalls that of the Trematoda. 



The penis, which is always invaginable, is cephalic in Pulmonates, 

 pedal in most Opisthobranchs. 



The accessory glands are numerous in the hermaphrodite forms 

 (Fig. 298). In addition to the receptaculum seminis already men- 

 tioned, there is generally an albuminous gland in connection with the 

 hermaphrodite duct, and a 

 mucous gland (multifid 

 vesicles of Helix) opening 

 into the vaginal part of the 

 oviduct. In some Helicidae 

 a peculiar sac the dart-sac 

 which produces in its in- 

 terior a dart-like calcareous 

 rod, opens into the vagina 

 between the two groups of 

 multifid vesicles. This rod 

 the so-called love-dart is 

 attached to a papilla at the 

 base of the sac ; it is pro- 

 truded during copulation, 

 and seems to play the part 

 of a stimulating organ. It 

 is usually broken off during 

 use, and often remains in the 

 body of the other snail. 



The vas deferens often 

 presents a prostatic gland, 

 which in some forms is in 

 close connection with the 

 evaginable penial part, and is called the flagellum. 



The hermaphrodite forms appear as a rule to be protandrous, and 

 rarely, if ever, self -fertilizing. It is stated that specimens of Avion 



FIG. 298. Sexual organs of the Roman Snail (Helix 

 pomatia), illustrating the diaulic arrangement. 

 Zd hermaphrodite gland ; Zg its duct ; Ed albumen 

 gland ; Od oviduct and seminal groove ; Vd 

 vas deferens ; P penis ; Fl flagellum ; Rs recep- 

 taculum seminis ; D mucous glands ; L love-dart 

 in dart-sac ; Go genital opening (after Baasen). 



