1908.] ASPIDOBRAXCH GASTROPOD MOLLUSCS. 841 



distance posteiior to the anal and ovipositoiy apertures (fig. 6, 

 ■ap.de.). The vaginal canal runs parallel to the ductus enigmaticus 

 as far as the opening of the latter, and then bends abruptly back- 

 wai'd. Its lumen becomes very narrow, and its muscular wall 

 relatively thick ; after a short course backward it opens by a 

 minute pore into a sac (sjo.s.), which corresponds to the copulatory 

 vesicle of Gilson and Lenssen, the receptaculum seminis of Thiele. 

 As may be seen in the diagram, it is the dilated posterior end of 

 the vagina {vag.), and is of small size in ,Septaria borhonica and 

 hougainvillei. But in the tropical NeritincK, in jVerita, and, 

 according to Thiele, in S'eptaria parva it is relatively of enormous 

 size and in\ariably contains a number of spermatophores of 

 complex structure. Hence I shall call it the sjyennatophore-sac, 

 although I have not found a trace of spermatophores in any of the 

 specimens of Sejitaria that I have examined. The lumen of the 

 spermatophore-sac gradually diminishes anteriorly and passes 

 Avithout sensible alteration of structure into the vagina, which 

 opens into the mantle-cavity by the vaginal aperture situated on a 

 prominent papilla some little way in front of the anus. 



Gilson has given a clear account of the process of fertilization 

 in Xeritina Jiuviatilis. Owing to the small size of the spermato- 

 phore-sac it must be somewhat different in Septaria horbonica 

 and hougainvillei. The spermatozoa must be deposited by the 

 male in the vagina and must travel by way of the vaginal canal to 

 the receptaculum seminis, Avhere they are disposed, just as Gilson 

 describes, in a very regular manner, all heads turned towai'ds the 

 centre of the vesicle and all tails directed outwards. At the time 

 of impregnation the spermatozoa must be passed, by contraction 

 of the muscular walls of the I'eceptaculum and its duct, into the 

 fundus of the ootype and thence into the egg-duct, where the ova 

 are fertilized. The function of the ductus enigmaticus can only be 

 guessed at. It ma}^ sei've to admit water into the cavity of the 

 sperm-sac, or contrariwise may serve for the expulsion of fluids 

 accumulating in the spenn-sac. 



Histology of the Genital Ducts. 



From what precedes it will have been gathered that there are 

 live different kinds of glands on the course of the ootype and 

 egg-duct, viz., the vitelline gland, the rose-coloured gland, the 

 mucous gland, the clear gland, and the ootype or shell-gland. 

 Similar glands occur in the same positions in all the Neritid?e I 

 have studied, iwclwAmg Xeritina Jiiwiatilis. The histology of the 

 shell-gland of the last-named species has been correctly described 

 by Lenssen, but he gives a very summary, and, in the case of the 

 vitelline gland at least, a somewhat incorrect account of the 

 remainder. The important thing is that all the glands are of 

 the same fundamental structural j^lan, and only differ from one 

 another in the proportions and staining-properties of the cells and 

 the secretions produced by them. It may therefore be inferred that 



