124 THE GASTROPODA 



Chenopodidae, Cassididae, Doliidae, Trotonidae, Naticidae, Cyp- 

 raeidae, Calyptraeidae, Xenophoridae, Strombidae (Fig. 75, s.gr) ; and 

 in some Stenoglossa, viz. the Muricidae, Magtius, Voluta, Lyria, the 

 Harpidae, Terebra, and in all the Heteropoda. In all other forms, 

 that is to say, in a certain number of Taenioglossa and almost all 

 the Stenoglossa, the spermiduct is closed in for the whole of its 

 extent and the penis is hollow (Fig. 44, h). Thus the male orifice is 

 secondarily removed to the extremity of the penis, and consequently 

 is at a considerable distance from the primitive position of the 

 genital aperture, a position which is retained by the female aper- 

 ture. A penis exists in the Neritacea among the Rhipidoglossa, 

 and in all the Pectinibranchia, with the exception of those families 

 of the Taenioglossa enumerated above. When it does not exist 

 copulation cannot take place, and the ova are fertilised in the sea 

 by contact with the spermatozoa emitted by the male. The penis 

 exists only in a rudimentary form in sedentary species, but in all 

 others it is a well-developed, non-invaginable excrescence, situated 

 on the right side of the anterior part of the body, except in cases 

 of situs inversus, when it is on the left. All the aerial Streptoneura 

 are necessarily provided with a penis, since in them copulation is 

 indispensable. But the penis is not homologous throughout the 

 group; it is developed at different parts of the body, at the point 

 where the spermiduct ends. Thus the Neritacea have a cephalic penis, 

 as has also Paludina, whose penis is a part of the right tentacle. In 

 the Ampullariidae and Cyclostomatidae the penis is developed from 

 the mantle, but in all other forms it is exclusively of pedal origin. 

 Sometimes it is provided with an external whip-like appendage or 

 flagellum : such is the case in many Taenioglossa, viz. nearly all 

 the Littorinidae except Cremnoconchus, in Dolium, and especially 

 in Hydrobitt, ftithynin, the Naticidae, the Lamellariidae, and the 

 Heteropoda. 



The genital ducts are rarely provided with well-differentiated 

 accessory organs in the dioecious Gastropoda. In certain cases there 

 is a glandular tract in the oviduct, which is sometimes specialised 

 to form an albuminiparous gland (Ampullaria, Pahidina, the 

 Naticidae, Lamellariidae and Calyptraeidae, Triton and Castidaria). 

 There is a copulatory pouch or receptaculum seminis in the 

 Neritacea, Paludinidae, Cyclostomatidae, and Heteropoda. In some 

 Neritacea viz. Neritim, Nerita, Naviwlla, etc. the receptaculum 

 has its own external opening distinct from the oviducal aperture ; 

 this probably corresponds with the right kidney of other Rhipido- 

 glossa, which in this case has been lost by the male sex (Thiele). 

 In some freshwater Taenioglossa, e.g. Tanganyika (Fig. 78) and 

 Melania episcopalis (Fig. 109), the oviduct is continued into a ciliated 

 groove which leads to an incubatory pouch situated in the head ; 

 this pouch has been homologised with the penis by Moore. The 



