GENERATIVE ORGANS OF MOLLUSCA. 385 



generative orifice; a ciliated groove conducts the semen, which 

 passes out from the generative orifice, to the copulatory organ. 



Among the Pteropoda,Pneumoderinon has the penis merely repre- 

 sented by a papilla placed within the generative orifice ; while in the 

 Thecosomata there is an eversible organ below the opening of the 

 vagina. 



§ 297. 



In many hermaphrodite Gastropoda the germ-glands may be 

 seen to alternate in function, so that at one time they represent a 

 male, and at another a female organ. This fact is an indication of 

 that separation of the sexes which is completed in most of the 

 Prosob ranchiata . 



Notwithstanding the sexes being separate, we find the lowest 

 conditions in the Prosobranchiata, for in many the apparatus is merely 

 represented by the germ-glands. They repeat, therefore, the same 

 characters as those which we met with in the Lamellibranchiata. 

 There are no efferent ducts in Haliotis aud Patella. As in many of 

 the Lamellibranchiata, the germ-gland appears to pass out its secre- 

 tion through the excretory organ. In Fissurella this arrangement 

 is still more definite in character, owing to the connection between 

 the efferent duct and the excretory organ. 



The generative organs of the two sexes have very much the same 

 general characters, so that it is often only possible to distinguish 

 them microscopically by the presence of the copulatory organ in 

 the male. The male and female germ-glands are, like the herma- 

 phrodite glands of many hermaphrodite Gastropoda, hidden in, or 

 placed close to, the liver. 



In the female organs an oviduct, which is ordinarily coiled, arises 

 from the ovary, and turns towards the hind-gut, where it widens 

 out and forms a uterus. A short vagina is given off from this, which 

 passes to the generative orifice, which is placed close to the anus. 

 Accessory organs are rare in the dioecious Gastropoda. Where they 

 are best known they consist of an elongated spermatheca (recepta- 

 culum), which opens into the end of the saccular uterus, and which 

 has the duct of an albuminiparous gland connected with it(Paludina). 

 In the Heteropoda the seminal receptacle only is present, and it is 

 either attached to the end of the uterus (Atlanta), or united to the 

 vagina in front of this organ (Pterotrachea) . 



In the male organs, the duct (vas deferens) is either quite simple 

 or is provided, on its way to the penis, with a swelling, which 

 functions as a seminal vesicle. The end of the vas deferens opens 

 on the surface, and on the right side, of the body. There is no 

 copulatory organ in Haliotis, Patella, or Trochus. Where it is present it 

 is formed by a process of the dermo-muscular tube, and is a massive 

 broad body, which is frequently curved at its tip ; it is placed on tho 

 right side of the body, or on the head, at the base of the right 

 tentacle, or at times (Heteropoda) close to the anus. A ciliated 

 semi-canal, which often extends for some distance on the surface of 



2 c 



