124 THE GASTROPODA 
Chenopodidae, Cassididae, Doliidae, Trotonidae, Naticidae, Cyp- 
raeidae, Calyptraeidae, Xenophoridae, Strombidae (Fig. 75, s.gr) ; and 
in some Stenoglossa, viz. the Muricidae, Magilus, 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, 1). 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 Hydrobia, Bithynia, 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, Paludina, » the 
Naticidae, Lamellariidae and Calyptraeidae, Triton and Cassidaria). 
There is a copulatory pouch or receptaculum seminis in the 
Neritacea, Paludinidae, Cyclostomatidae, and Heteropoda. In some 
Neritacea—viz. Neritina, Nerita, Navicella, ete.—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. Tanganyicia (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 
