164 J. E. S. MOORE. 



conclude that the fully developed apparatus was present in the 

 remote ancestors of all these forms. Hitherto little or no atten- 

 tion has been paid to the spermatic grooves which have 

 long been known to exist in the males of many forms, but 

 directly their relation to the fully developed female apparatus 

 which I have described becomes apparent their morphological 

 importance becomes immense, for it will doubtless have been 

 seen how closely the fully developed apparatus in these 

 female Prosobranchs corresponds to the spermatic grooves and 

 invaginated penes of the Opisthobranchs ; and indeed, when 

 we look further into the matter, the comparison becomes so 

 striking and so complete that there can be little doubt of the 

 existence in this feature of an actual bond of morphological 

 unity between the two. I may, however, preface the remarks 

 which I have to offer on this subject by stating that it has 

 already been shown, as a result of Professor Pelseneer's elabo- 

 rate and painstaking investigations^ concerning the morpho- 

 logy of the Opisthobranchs, that the characteristic hermaphro- 

 ditism of these organisms appears to have been secondarily 

 acquired, and to have arisen in the female by the evolution 

 of a functional male gland. 



If we bear in mind this result of a profound research while 

 comparing the genital ducts in the Opisthobranchs with those 

 of the Prosobranchs which I have just described, much of the 

 initial prejudice that would be likely to exist against such a 

 comparison will disappear, and it will be more readily seen 

 that in their simplest forms the external genitalia in both 

 orders are exceedingly similar. 



In Aplysia, Pelta (PI. 16, fig. 5), and several other forms 

 the hermaphrodite genital aperture, as is well known, leads 

 into a forwardly extended groove (PI. 16, fig. 5, g. v.), which 

 terminates anteriorly in a pit beneath the eye, and this pit 

 communicates with the cavity of an internal sac, the walls of 

 which can function as an introvertible penis when required. 

 Except in the addition of the muscles which introvert this sac, 



' ' Arcli. de Biol.,' t. xiv, 1895. See also ' Quart. Journ. Micr. Sci.,' vol. 

 37, p. 19, 1895. 



