M. palmata.

2. From the presence or absence of the secondary flagellum: that M.
palmata, etc. and M. exilii, etc. would branch off together from a stem
which branches off from 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 Leucothoe,
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
(Coxae Sp. B.) of the caudal feet (uropoda Westw.), etc., to be a true
Amphithoe* possesses it (* 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').)--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 branchiae--and that from all this, at the present day when the