166 J. E. S. MOORE. 



ontogenetic life, and tlie line of their invagination remaining 

 as a faint streak in the adult. 



Whether we deal with the Prosobranchs or the Opistho- 

 branchs in this matter of the modifications which the original 

 external genital apparatus may undergo^ it will thus be seen 

 that in both orders the modification is along the same lines, 

 and results in the conversion of an open groove into one or 

 two closed conduits as the case may be. In both orders the 

 original brood-pouch of the Prosobranchs and the introvert- 

 ible penis of the Opisthobranchs tend to become lost, and to be 

 replaced by permanent external penes, supplied with secondarily 

 acquired internal tubes. Very obviously, therefore, we are 

 here dealing with one of those numerous examples of parallel 

 modifications of an originally similar structure present in 

 two types which have become distinct. The whole apparatus 

 in its complete ancestral prosobranchiate form is too complex 

 and too peculiar in the arrangement of the different parts to 

 have been evolved twice from different things. We know 

 further that the Melanias to which I have referred, as well as 

 the T. rufofilosa of Tanganyika, are indubitably extremely 

 ancient forms, and that the complete structures they possess 

 were once widely distributed among their prosobranchial 

 ancestry, for parts of these structures as we have seen are 

 present in widely different prosobranchiate forms. It would 

 appear, therefore, that we are fully justified in concluding that 

 in all these cases we are dealing with an incompletely repre- 

 sented condition of the reproductive apparatus that was once 

 common to the Prosobranchs and the Opisthobranchs alike. 



Lastly, it will have been seen that this view, drawn from 

 the study of Prosobranch morphology alone, and Professor 

 Pelseneer's view respecting the origin and nature of the Opis- 

 thobranchiata, mutually confirm each other ; for, as I have 

 already pointed out, Pelsenecr has shown from an entirely 

 different line of inquiry — 



1. That the Opisthobranchs arose from the Prosobranchs. 



3. That their characteristic hermaphroditism was secondarily 

 acquired by the females of this group. 



