THE GASTROPODA 123 



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, Falvata, Mar- 

 senina, Oncidiopsis, Odostomia, Entosiphon, EntoconcJia, 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, Paludinti, Littorina, various species 

 of Crepidula, Pleurotoma, etc. : 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 Cerithiuni, Quoy and 

 Gaimard), in the radular teeth (certain Buccinidae, Troschel), in 

 the absence of the pedal sucker in the female PterotracJiea, of ten- 

 tacles in the female of some Firoloida, 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 Pleurotomaria, 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, 



