1908.] ASPIDOBRAXCH GASTROPOD MOLLUSCS. 871 



and the two sides of the ootype are formed by a thick mass of 

 the diaracteristic glandular tissue, but the two ends of its fissure- 

 like cavity remain very thin. 



To return again to the lower level depicted in fig. 67. Some- 

 what fui'thei' doAvn than this the right-hand portion of the wall 

 of the ootype disappears, and the bottom of its cavity ends in a 

 large thin-walled sac, which is really the lowest portion of the sac 

 shown on the right-hand of figs. 65, 67, &, 68. This is clearly 

 comparable to the sperm -sac of Farcmerita and Sej^taria, but it is 

 not bilobed and its structure and further relations in Nerita are 

 different, and can best be explained by reference to the diagram 

 fig. 65. The sac turns upwards and is closely applied to the left 

 face of the rectum. Its outer or abrectal wall is deeply pigmented, 

 rendering it a very conspicuous object in dissections, and is 

 thrown into a number of folds running on the whole longitudinally. 

 The pigment is deposited in the bodies of the veiy distinct band of 

 columnar ciliated cells which forms the outer wall of the sac, and 

 may be described as a plicated ciliated band I'ather than a groove. 

 This band of ciliated cells, at a lower level than is shown in fig. 67, 

 passes round the left and lower side of the sac and is continued 

 into the ciliated epithelium of the egg-duct, which again, as 

 described above, is continuous with the ciliated epithelium of the 

 oviduct. The rest of the walls of this pigmented sac are extremely 

 thin and non-ciliated. It is a remarkable feature that the adrectal 

 part of the cavity of the sac and also its lower part adjoining the 

 base of the uterine gland is broken up by a number of trabeculee,. 

 which in arrangement greatly resemble those of the glandular 

 part of the kidney, but they are not in this case glandular. Some- 

 of these trabecules are shown in fig. 67. They are much more 

 developed in X. melanotraga than in iV^^. 2>licata, and they have 

 been noted by Thiele in ^V. pica. At the top of the pigmented 

 sac the abrectal ciliated band becomes a distinct groove, and this 

 separates otF as a duct, at first thin-walled, and lined by the same 

 pigmented ciliated epithelium as the groove. The duct soon 

 diminishes notably in diameter ; its pigmented ciliated epithelium 

 gives ]3lace to a non-pigmented epithelium composed of much 

 smaller cells but still ciliated, and at the same time it acquires a 

 thick external muscular coat. This duct (fig. 65, vag.c.) pursues a 

 very slightly sinuous course joosteriorly (it is somewhat longer and 

 more convoluted in N. melanotraga), and while its lumen continues 

 to diminish, its muscular coat increases greatly in thickness- 

 (fig. 68, vag.c). It is evidently, I think, the homologue of the 

 vaginal canal of Paranerita and Septaria. It opens into a dila- 

 tation, with less thick but muscular walls, lined by an epithelium 

 which is continuous throughout all the structures I have yet to 

 describe and whose characters I will specify presently. From the 

 lower end of the dilatation a diverticidum is given off, sometimes, 

 as shown in the diagram fig. 65, in the form of a narrow stalk 

 with a bulbous termination, sometimes a simple blind tube of 

 subequal diameter throughout. It contains spermatozoa and is- 



