40 LAND AND FRESH-WATER SHELLS. 



taining a more or less rounded structure (germinal vesicle), which 

 in its turn encloses a smaller, more refractile body (germinal spot], 

 It is a primitive undifferentiated cell pure and simple ; the 

 germinal vesicle represents the nucleus, the germinal spot the 

 nucleolus. We shall have to refer to this ovum more in par- 

 ticular directly, when we come to consider the chief steps in the 

 development of the snail. 



The ducts of the follicles of the ovotestis unite to form a duct 

 common to them all, lined with non-ciliated squamous or columnar 

 epithelium the hermaphrodite duct which leaves the gland, 

 courses through the albuminiparous gland, and at last, aftei 

 receiving the duct of that gland, leaves it to form the common 

 generative canal. The common generative canal is composed of 

 the vas deferens and oviduct united together. The lumen of the 

 vas deferens is in communication with that of the oviduct by 

 numerous slits in the dividing membrane, and around the vas 

 deferens a somewhat fluffy-looking mass will be seen on a minute 

 examination the so-called prostate. This common generative 

 canal, however, at last divides into a longer independent portion 

 the vas deferens proper which belongs to the male organs of 

 generation, and into a shorter independent portion the oviduct 

 which constitutes part of the female generative organs. The 

 common generative canal is lined with a ciliated epithelium, and 

 its walls are glandular, the independent portion of the oviduct is 

 ciliated likewise, but in the vas deferens proper the cilia are 

 absent. The albumen or albuminiparous gland is a greyish-white 

 or yellowish tongue-shaped organ, situated between the stomach 

 and the left lobe of the " Mitteldarmdriise ". It is a racemose 

 gland, the ducts of the acini of which open into a common duct 

 the duct of the albuminiparous gland which has its epithelium 

 ciliated, and which opens into the hermaphrodite duct just before 

 it leaves the gland as the common generative canal. There may 

 be two accessory albumen glands, as in Limnaa stagnates, or one, 

 as in Limn tea peregra. 



The vas deferens proper runs between the female generative 

 organs and the buccal mass, lies on the back of the crop, and 

 terminates in the penis sac. It gives off a long coiled diverticu- 

 lum the flagellum previous to its termination. Sometimes 



