Facts and Arguments for Darwin | Page 5

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
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.
CHAPTER 3.
MORPHOLOGY OF CRUSTACEA--NAUPLIUS-LARVAE.
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 larvae of Shrimps ('Archiv fur Naturgeschichte' 1860 1
page 8), and I must admit that this discovery gave me the first decided
turn in Darwin's favour.
(FIGURE 2. Tanais dubius (?) Kr. female, magnified 25 times, showing
the orifice of entrance (x) into the cavity overarched by the carapace, in
which an appendage of the second pair of maxillae (f) plays. On four
feet (i, k, l, m) are the rudiments of the lamellae which subsequently
form the brood-cavity.)
The similar number of segments* 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.
(* Like Claus I do not regard the eyes of the Crustacea as limbs, and
therefore admit no ocular segment; on the other hand I count in the
median piece of the tail, to which the character of a segment is often
denied. In opposition to its interpretation as a segment of the body, only
the want of limbs can be cited; in its favour we have the relation of the
intestine, which usually opens in this piece, and sometimes even
traverses its whole length, as in Microdeutopus and some other
Amphipoda. In Microdeutopus, as Spence Bate has already pointed out,
one is even led to regard small processes of this tubular caudal piece as
rudimentary members. Bell also ('British Stalk-eyed Crustacea' page
20), states that he observed limbs of the last segment in Palaemon
serratus in the form of small moveable points.

The attempt has often been made to divide the body of the higher
Crustacea into small sections composed of equal numbers of segments,
these sections consisting of 3, 5 or 7 segments. None of these attempts
has ever met with general acceptance; my own investigations lead me
to a conception which nearly approaches Van Beneden's. I assume four
sections of 5 segments each--the primitive body, the fore-body, the
hind-body, and the middle-body. The primitive body includes the
segments which the naupliiform larva brings with it out of the egg; it is
afterwards divided, by the younger sections which become developed
in its middle, into the head and tail. To this primitive body belong the
two pairs of antennae, the mandibles and the caudal feet ("posterior pair
of pleopoda," Sp. B.). Even in the mature animal the fact that these
terminal sections belong to one another is sometimes betrayed by the
resemblance of their appendages, especially that of the outer branch of
the caudal feet, with the outer branch (the