850 PROF. G. C. BOURNE ON THE [ISToV, 17 



and appearance are illustrated in fig. 30 {Neritina Jluviatilis), 

 fi^. 31 {JVerita melanotraga), and fig. 32 {Paranerita gagates). It 

 differs somewhat in size and shape in the other species I ha.ve 

 examined, but I have never seen it bifurcated as described by 

 Bouvier, nor of the shape figured by him for X. cariosa. 



In all the species I have examined it is a solid muscular out- 

 growth of the base of the right tentacle, inneivated, so far as I am 

 able to determine from sections, by a slender branch of the right 

 tentacular nerve. Its external side, that is the side nearest the 

 tentacle, is deeply grooved ; the lower lip of the groove is swollen 

 and prominent and clothed with a layer of long cylindrical 

 epithelial cells, among which are numerous sense-cells. Elsewhere 

 the surface of the organ is covered by a moderately high ciliated 

 cylindrical epithelium, the cilia being specially well developed 

 along the lower surface, but the groove itself is lined by a simple 

 non-ciliated cylindiical epithelium. The hinder end of the 

 groove opens almost behind and above the base of the right 

 tentacle, and there is no sign of any furrow or ciliated tract 

 leading from the groove to the aperture of the male duct. The 

 male opening lies close to the base of the right eye-stalk, but 

 rather in advance of it, and it is not easy to understand how any 

 connection can be established between it and the penial groove. 

 Communication on the ventral side of the tentacle seems to be 

 hindered by the presence of the epipodial ridge. It is, however, 

 difiicult to judge from the examination of contracted specimens. 

 The organ is probably very extensile, and its shape seems adapted 

 for grasping and conveying the complicated spermatophores of 

 Nerita and Paranerita to the vaginal opening of the female ; 

 but I must confess that I have been unable to find any proof of 

 this. I have found no trace of sj^ermatophores either in the male 

 ducts or in the groove of the penis in any species, and I have 

 failed to observe the act of copulation in the numerous Neritina 

 Huviatilis that I have kept in aquaria. 



The relative positions of heart, rectum, kidney, and ctenidium, 

 as seen in an external vieAv of the entire animal, are shown in 

 fig. 29 and need no further comment. 



In the marine species {Nerita) there is a well-developed 

 opercular gland opening by a median aperture just in front of 

 what is morphologically the anterior edge of the operculum, 

 between it and the posterior edge of the mantle-flap. (In a 

 conti"acted specimen, such as that drawn in fig. 29, the anterior 

 edge of the opercular lobe appears to be posterior.) This gland is 

 very large in Nerita plicata, melanotraga, and peloronta ; smaller 

 but still conspicuous in Ihieata and i^lexa. In freshwater species 

 {Paranerita and Neritina) it can scarcely be said to exist, being 

 represented only by a glandular area, sometime^ prodviced 

 internally into a few crypts, in the angle between the mantle 

 and the opercular lobe. The gland in Nerita is clearly an 

 invagination of this glandular area, and there is a gradual and 

 interesting transition from the extei'nal epithelium to the highly 



