It seems to be a necessity for all crabs which remain for a long time out of the water (but why is of no consequence to us here), that air shall penetrate from behind into the branchial cavity. Now these crabs, which have become more or less estranged from the water, belong to the most different families—the Raninidæ (_Ranina_), Eriphinæ (_Eriphia gonagra_), Grapsoidæ (_Aratus, Sesarma,_ etc.), Ocypodidæ (_Gelasimus, Ocypoda_), etc., and the separation of these families must doubtless be referred to a much earlier period than the habit of leaving the water displayed by some of their members. The arrangements connected with aerial respiration, therefore, could not be inherited from a common ancestor, and could scarcely be accordant in their construction. If there were any such accordance not referable to accidental resemblance among them, it would have to be laid in the scale as evidence against the correctness of Darwin’s views. I shall show hereafter how in this case the result, far from presenting such contradictions, was rather in the most complete harmony with what might be predicted from Darwin’s theory. Fig. 1. Melita exilii n. sp., male, enlarged. The large branchial lamellæ are seen projecting between the legs. Fig. 1. _Melita exilii_ n. sp., male, enlarged. The large branchial lamellæ are seen projecting between the legs. A second example.—We are already acquainted with four species of _Melita_ (_M. valida, setipes, anisochir,_ and _Fresnelii_), and I can add a fifth (Fig. 1), in which the second pair of feet bears upon one side a small hand of the usual structure, and on the other an enormous clasp-forceps. This want of symmetry is something so unusual among the Amphipoda, and the structure of the clasp-forceps differs so much from what is seen elsewhere in this order, and agrees so closely in the five species, that one must unhesitatingly regard them as having sprung from common ancestors belonging to them alone among known species. But one of these species, _M. Fresnelii,_ discovered by Savigny, in Egypt, is said to want the secondary flagellum of the anterior antennae, which occurs in the others. From the trustworthiness of all Savigny’s works there can scarcely be a doubt as to the correctness of this statement. Now, if the presence or absence of the secondary flagellum possessed the significance of a distinctive generic character, which is usually ascribed to it, or if there were other important differences between _Melita Fresnelii_ and the other species above-mentioned, which would make it seem natural to separate _M. Fresnelii_ as a distinct genus, and to leave the others united with the rest of the species of _ Melita_—that is to say, in the sense of the Darwinian theory, if we assume that all the other _Melitæ_ possessed common ancestors, which were not at the same time the ancestors of _M. Fresnelii_—this would stand in contradiction to the conclusion, derived from the structure of the clasp-forceps, that _M.