1908.] ASPIDOBRANCH GASTROPOD MOLLUSCS. 865 



fig. 48, Hy.g., and in the fem.^le in figs. 62 & 63, Hy.g. 

 The duct of the gland opens into the right-hand posterior 

 corner of the mantle-cavity by a fairlj- large pore situated 

 below the posterior end of the swelling formed by the genital 

 complex, and at the base of the vestigial right ctenidium (tig. 49, 

 Hy.g.a.). As a consequence of the great size of the sper- 

 matophore-sac and epididymis in Paranerita, the hypobranchial 

 gland is carried back with it and some of its tubules may lie 

 at the posterior end of the body, apparently among the viscera, 

 but really in the tissue surrounding the sperinatophore-sac 

 (figs. 62 ife 63) or its equivalent in the male. The tubules are 

 lined by a simple columnar glandular epithelium, the cells of 

 which have small nuclei and clear contents. The hypobranchial 

 gland attains relatively enormous dimensions in the Helicinidse, 

 penetrates among the viscera, and forms a considerable part of 

 the mass of the visceral spire. 



The Generative Organs. 



The position of the gonad is the same in both sexes, and the 

 oviduct or sperm-duct formed by the union of the ducts of the 

 numerous ovarian or spermatic follicles passes up on the right- 

 hand of the spire towards the posterior end of the right columellar 

 muscle and then turns inward to pass into the base of the genital 

 complex. It is not necessary to add anything to the account 

 given by Lenssen of the structure and histology of the ovary and 

 testis in Neritina fluviatilis. It holds good for all the Neritidfe 

 that I have studied. 



Lenssen gives a rather summary — but, on the whole, an 

 accurate — description of the male accessory organs of 2^. fluviatilis. 

 It is ojDen to criticism in matters of detail, but these are not of 

 sufiicient importance to justify a discussion of them in this place. 

 There is very little difierence between the male oi'gans of the 

 difi;erent species of Neritidce. Of Xerita I have examined lineata, 

 and melanotraga ; of Paranerita, gagates, variegata, and longi- 

 spina ; and I have verified Lenssen's description of Neritina 

 ■fluviatilis. Fig. 50 is a drawing, founded upon sections and 

 dissections, of the male organs of Paranerita gagates, and those of 

 variegata, longispina, and of the unknown species from Fiji are 

 indistinguishable. The sperm-duct is seen passing forward and 

 upward with a slightly sinuous course close to the external body- 

 wall on the right side. Just behind the posterior end of the 

 right columellar muscle it comes into close relations with the 

 gonadial extension of the ccelom and forms a few coils close against 

 its wall. At this j)oint the sperm-duct turns abruptly to the 

 left and is immediately thrown into most complicated convolutions 

 which are closely applied to one another and form a large ovoid 

 mass — the epididymis. This, as has been explained above, is 

 enclosed in a fine membi-anous bag, and is so large that it 

 projects backwards into the ccelomic cavity and extends across to 



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