THE GASTROPODA 12 
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the respiratory cavity at the anterior extremity of the osphradium, 
and in it also the optic nerve traverses the retina. 
6. Reproductive Organs—The Streptoneura are dioecious, with 
the exception of the genera Bathysciadium, Cocculina, Valvata, Mar- 
senina, Oncidiopsis, Odostomia, Entosiphon, Entoconcha, and Enteroxenos. 
All the Euthyneura are monoecious. 
In the dioecious Gastropoda sexual dimorphism is generally 
very slightly marked. The males are externally recognisable only 
by the penis, when this organ exists; their shape, however, is more 
elongate than that of the females, and their greatness is often smaller, 
as, for instance, in Rhipidoglossa, Paludinu, Littorina, various species 
of Crepidula, Pleurotoma, ete.: the most typical case in this respect 
is that of Lacuna pallidula (Fig. 7), in which the females are on 
the average ten times as heavy as the males. In addition, sexual 
differences are sometimes found in the aperture of the shell (Littorina 
obtusata), in the operculum (some species of Cerithium, Quoy and 
Gaimard), in the radular teeth (certain Buccinidae, Troschel), in 
the absence of the pedal sucker in the female Pterotrachea, of ten- 
tacles in the female of some /iroloida, and of the slit in the mantle 
in the male Vermetus. 
The gonad is always unpaired, even in the most archaic Aspido- 
branchia. It is generally placed on the dorsal side and at the 
summit of the visceral mass. It has the form of a racemose gland, 
made up of a great number of acini, and it may be compact or 
arborescent, with ramifications extending over and into the liver 
mass. In Aspidobranchia such as Plewrotomaria, the Trochidae, and 
Fissurellidae the gonad opens into the reno-pericardial duct, in the 
same manner as in some protobranchiate Lamellibranchs, viz. 
Solenomya. In all other Aspidobranchs, except the Neritacea, the 
gonad discharges into the kidney. In the Neritacea and Pectini- 
branchia the reproductive apparatus always possesses its own proper 
orifice, and there is a genital duct of greater or less length, which, 
however, is incompletely closed in various Melaniidae, Cerithiidae, 
Turritellidae, and Vermetidae. This duct opens into the pallial 
cavity to the right of the intestine in both sexes in the Ampul- 
lariidae, and in such forms as have not acquired a penis, that is to 
say, besides the four families mentioned above, in the Capulidae, 
Hipponycidae, and Solariidae. In all forms the male duct or 
spermiduct differs from the female duct or oviduct in the fact that 
it terminates in a copulatory organ (Fig. 99, XI). In its less special- 
ised form the spermiduct is continued into a seminal furrow or groove 
(Fig. 85, v.d’) which extends from the primitive genital orifice to 
the extremity of the penis, and is capable of being closed for part 
of its course, remaining open only in the neighbourhood of or on 
the penis. This condition is found in a large number of Taenio- 
glossa, viz. Ampullaria, the Littorinidae, Modulidae, Struthiolariidae, 
