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. 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.
If the absence of contradictions among the inferences deduced from them for a narrow and consequently easily surveyed department must prepossess us in favour of Darwin’s views, it must be welcomed as a positive triumph of his theory if far-reaching conclusions founded upon it should subsequently be confirmed by facts, the existence of which science, in its previous state, by no means allowed us to suspect. From many results of this kind upon which I could report, I select as examples, two, which were of particular importance to me, and relate to discoveries the great significance of which in the morphology and classification of the Crustacea will not be denied even by the opponents of Darwin.
Considerations upon the developmental history of the Crustacea had led me to the conclusion that, if the higher and lower Crustacea were at all derivable from common progenitors, the former also must once have passed through Nauplius-like conditions. Soon afterwards I discovered Naupliiform larvæ of Shrimps (‘Archiv für Naturgeschichte’ 1860, i, p. 8), and I must admit that this discovery gave me the first decided turn in Darwin’s favour.
The similar number of segments[[1]] occurring in the Crabs and Macrura, Amphipoda and Isopoda, in which the last seven segments are always different from the preceding ones in the appendages with which they are furnished, could only be regarded as an inheritance from the same ancestors. And if at the present day the majority of the Crabs and Macrura, and indeed the Stalk-eyed Crustacea in general, pass through Zoëa-like developmental states, and the same mode of transformation was to be ascribed to their ancestors, the same thing must also apply, if not to the immediate ancestors of the Amphipoda and Isopoda, at least to the common progenitors of these and the Stalk-eyed Crustacea. Any such assumption as this was, however, very hazardous, so long as not a single fact properly relating to the Edriophthalma could be adduced in its support, as the structure of this very coherent group seemed to be almost irreconcilable with many peculiarities of the Zoëa. Thus, in my eyes, this point long constituted one of the chief difficulties in the application of the Darwinian views to the Crustacea, and I could scarcely venture to hope that I might yet find traces of this passage through the Zoëa-form among the Amphipoda or Isopoda, and thus obtain a positive proof of the correctness of this conclusion. At this point Van Beneden’s statement that a cheliferous Isopod (Tanais Dulongii), belonging, according to Milne-Edwards, to the same family as the common Asellus aquaticus, possesses a carapace like the Decapoda, directed my attention to these animals, and a careful examination proved that these Isopods have preserved, more truly than any other adult Crustacea, many of the most essential peculiarities of the Zoëae, especially their mode of respiration. Whilst in all other Oniscoida the abdominal feet serve for respiration, these in our cheliferous Isopod (Fig. 2) are solely motory organs, into which no blood-corpuscle ever enters, and the chief seat of respiration is, as in the Zoëae, in the lateral parts of the carapace, which are abundantly traversed by currents of blood, and beneath which a constant stream of water passes, maintained, as in Zoëae and the adult Decapoda, by an appendage of the second pair of maxillæ, which is wanting in all other Edriophthalma.
