Fresnelii_ and the four other species above-mentioned possessed common ancestors, which were not also the ancestors of the remaining species of _Melita._ It would follow— From the structure of the clasp-forceps: From the presence or absence of the secondary flagellum. _M. palmata, etc., M. exilii, etc., M. Fresnelli._ _M. palmata, etc., M. exilii, etc., M. Fresnelii._ As, in the first case, among the Crabs, a typical agreement of arrangements produced independently of each other would have been a very suspicious circumstance for Darwin’s theory, so also, in the second, would any difference more profound than that of very nearly allied species. Now it seems to me that the secondary flagellum can by no means furnish a reason for doubting the close relationship of _M. Fresnelii_ to _M. exilii,_ etc., which is indicated by the peculiar structure of the unpaired clasp-forceps. In the first place we must consider the possibility that the secondary flagellum, which is not always easy to detect, may only have been overlooked by Savigny, as indeed Spence Bate supposes to have been the case. If it is really deficient it must be remarked that I have found it in species of the genera _ Leucothoë, Cyrtophium_ and _Amphilochus,_ in which genera it was missed by Savigny, Dana and Spence Bate—that a species proved by the form of the Epimera (_coxæ_ Sp. B.) of the caudal feet (_uropoda_ Westw.), etc., to be a true _Amphithoë_[1] possesses it—that in many species of _Cerapus_ it is reduced to a scarcely perceptible rudiment—nay, that it is sometimes present in youth and disappears (although perhaps not without leaving some trace) at maturity, as was found by Spence Bate to be the case in _ Acanthonotus Owenii_ and _Atylus carinatus,_ and I can affirm with regard to an _Atylus_ of these seas, remarkable for its plumose branchiæ—and that from all this, at the present day when the increasing number of known Amphipoda and the splitting of them into numerous genera thereby induced, compels us to descend to very minute distinctive characters, we must nevertheless hesitate before employing the secondary flagellum as a generic character. The case of _Melita Fresnelii_ therefore cannot excite any doubts as to Darwin’s theory. [1] I accept this and all the other genera of Amphipoda here mentioned, with the limits given to them by Spence Bate (‘Catalogue of Amphipodous Crustacea’). CHAPTER III. MORPHOLOGY OF CRUSTACEA—NAUPLIUS-LARVÆ.