Facts and Arguments for Darwin | Page 5

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 so-called scale) of the second pair of antennae. Like the antennae, the caudal feet may also become the bearers of high sensorial apparatus, as is shown by the ear of Mysis.
The sequence of the sections of the body in order of time seems originally to have been, that first the fore-body, then the hind-body, and finally the middle-body was formed. The fore-body appears, in the adult animal, to be entirely or partially amalgamated with the head; its appendages (siagonopoda Westw.) are all or in part serviceable for the reception of food, and generally sharply distinguished