THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 191 



nerve which springs from the small osphradial ganglion. 

 Externally the ridge is covered with ciliated and glandular 

 cells, the relations of which and the characters of the osphradial 

 nerve are shown in section in fig. 8. There is nothing peculiar 

 about the gills except their great length. 



The Reproductive Apparatus. — In Typhobia the sexes 

 are distinct and the female viviparous, the whole reproductive 

 apparatus being simple but somewhat peculiar. The ovaries 

 and testes occupy the upper surface of the last two whorls of 

 the spire, and in the female the eggs, with their bright green 

 yolk, pass directly into the simple oviduct (fig. 47, 1). From 

 this they reach the lower expansion of the oviduct (fig. 47, 2) ; 

 and in this sac, which functions as a brood chamber or uterus, 

 they go through the greater part of their development. The 

 walls of the sac are very thin, and while the animal is alive the 

 bright green yolk of the eggs is distinctly seen through the 

 delicate serai-transparent shell, so that the sexes can be distin- 

 guished at a glance. The sac opens near the rectum, at the 

 junction of the mantle and the body wall (fig. 3, 3). The 

 mollusc breeds during the months of June and July. 



The testis (fig. 54, 6) opens by several small collecting 

 channels into the simple vas deferens (fig. 54, 4). This tube 

 becomes somewhat but not much convoluted on its way, and 

 ultimately expands into the curious enlargement represented 

 in fig. 54, 2. On opening this it was seen in every case to bear 

 about six singular parallel folds (fig. 49, 6). Beyond this ex- 

 pansion there is a curious finger-like outgrowth, extending from 

 the duct into the mantle wall ; this process contains a muscular 

 mass, which has all the appearance of, and probably is, an 

 introvertible penis (figs. 45 and 46, ^). The lower extremity 

 of the male genital duct opens by an elongate slit (fig. 49, 5). 

 The possession of a penis in the mantle wall is a most curious 

 fact, which would seem to indicate that that organ is analogous 

 to, though very likely not homologous with, the penis of the 

 Ampullarise and other pulmonale Prosobranchs. In Typhobia 

 the male genital gland is extremely interesting from a cyto- 

 logical point of view, as this genus is one of those Prosobranchs 



