326 MOLLUSCS. 



(d) The ova and spermatozoa pass from (b) towards the head 

 along a common duct, but not at the same time. Moreover 

 their paths are different, for the portion of the duct down which 

 the ova travel is much plaited, while the path which the 

 spermatozoa follow is a less prominent, more granular 

 groove, incompletely separated anatomically from the other. 

 Both paths are glandular, and the glands on the male side 

 are often called prostatic. 



(e) At the base of this common duct,, a distinct vas 

 deferens diverges to the left, twists round the root of the right 

 long horn, and leads into a muscular penis, which can be 

 protruded at the single genital aperture and retracted by a 

 special rriuscle. At the point where the vas deferens enters 

 the penis, a long process or flagdlum is given off. It is like 

 the lash of a whip, and is as long as the common duct. 

 Within it a spermatophore is partly formed, which seems to 

 be completed in the penis. This spermatophore is laden 

 with a large number of spermatozoa, and is transferred by 

 the penis into the genital aperture of another snail. 



(/) Continued from the oviducal side of the common 

 duct, there is a separate ciliated oviduct. This has a shorter 

 course than the vas deferens, and ends in the common 

 genital aperture. Before it reaches this, however, the 

 oviduct is associated with two structures. The first of these 

 is a long process, as long as the common duct beside which 

 it runs, in appearance suggesting the flagellum, but expand- 

 ing at its free end into a globular sac — the receptaculum 

 seminis. It is into this long duct and sac that a sperma- 

 tophore from another snail passes, and is after some days 

 dissolved, liberating hundreds of spermatozoa. By these 

 spermatozoa the ova of this snail are fertilised at some part 

 of their course. The second structure associated with the 

 female duct is a conspicuous mucous gland, formed of two 

 sets of finger-like processes. The mucous secretion of this 

 gland is very abundant during copulation, and as it contains 

 not a little lime, it is possible that it may form the calcareous 

 shells of the eggs. 



{g) Finally, between the entrance of oviduct and penis 

 into the terminal aperture, which is hardly large enough to 

 be called a chamber, there lies a firm cylindrical structure, 

 larger than the penis and with muscular walls. It is the 



