THE ODONTOPHORA. 425 



But they differ from the eyes of Vertebrata, and resemble 

 those of other invertebrated animals, in that the structures 

 which answer to the rods and cones are situated on that face 

 of the retina which is turned toward the light, while the fibres 

 of the optic nerve traverse the pigment layer to reach them. 



The reproductive organs of the Odontophora present very 

 great diversities of structure. They may be either dioecious 

 or monoecious, and each type of reproductive organs may pre- 

 sent various degrees of complexity. Of the dioecious repro- 

 ductive organs there are two chief forms : the one in which 

 the duct of the ovarium or testis is continuous with the gland; 

 and the other in which the duct opens into a sac, into which 

 the ova or spermatozoa are set free by the dehiscence of the 

 follicles in which they are developed. The latter arrange- 

 ment is met with in the Cephalopoda ; the former appears 

 to prevail among all the other dioecious Odontophora. 



In these, the racemose generative gland is usually situ- 

 ated close to the liver. In the female, the oviduct ordinarily 

 presents a uterine dilatation toward its termination, which is 

 generally situated in the pallial cavity on the right side of 

 the body. In some rare cases (Paludina, Neritina), a dila- 

 tation or a special vesicular appendage of the uterus may 

 serve as a vesicula seminalis and in Paludina, according 

 to Leydig, an albumen-gland opens into it. 



A penis is not always present. When it exists, it is a 

 muscular process of the mesosoma, to which the semen may 

 be led from the opening of the vas deferens by a groove ; or 

 it may be traversed by the vas deferens which opens near, or 

 at, its apex. 



In all the monoecious Odontophcra which have as yet been 

 thoroughly examined, there is a generative gland termed the 

 ovotestis, in which both spermatozoa and ova are produced. 

 Only in the anomalous genus Rhodope (Kolliker) are the 

 spermatozoa and ova formed in distinct cseca ; in all the rest, 

 each caecum is hermaphrodite, the spermatozoa and the ova 

 being usually developed in different parts of the caecum. The 

 duct of the ovotestis may remain single to its termination at 

 the genital aperture, or become only incompletely divided 

 into two semicanals (Pteropoda, Pleurephyttidia, Uml>reUa^ 

 Aplysid) \ or it may become, at first partially, and then com- 

 pletely, divided into an oviduct and a vas deferens (Nudi- 

 branchiata, Pleurobranchia, Pulmonatd). 



In the former case there is but one genital aperture. The 

 common duct usually receives the secretion of a uterine gland 



