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ARTHROPODA

Chfetopods (e.fj., Hesione) and in the Arthropod Peripa tus (Fig. 8). The conversion of the Arthropod’s limb into a jaw, as a rule, is effected by the development of an endite near its base into a hard, chitinized, and often pipSp2pl toothed gnathobase (see Figs. 9 and 10, en'). It is not true that all the biting processes of the Arthropod limb are thus produced—for instance, the jaws of Peripatus are formed by the axis or corm itself, whilst the poison-jaws of Chilopods, as also their maxillje, appear to be formed rather by the apex or terminal region of the ramus of the limb • but the opposingjaws( = hemignaths) of Crustacea, Arachnida, and Hexapoda are gnathobases, and not the axis or corm. The endopodite (corresponding to the fifth endite of the limb of Apus, see Fig. 9) becomes in Crustacea the “walking leg” of the midregion of the body; it becomes the palp or jointed Fio. 8. - -Three somite-appendages or process of anterior segments, parapodia of Peripatus. A, a walk- » j .i, A ing leg; pi to p*, the characteristic ^ Second ramus, the exopo“pads”; /, the foot; cfl, cl?, the elite,” often is also retained two claws. B, an oral papilla, one

„, of the second pair of post-oral 1H tlie lOrm 01 a palp OF appendages. C, one of the first Ieeier T^-.11 Aryna oa tVi p fiomTP post-oral pair of appendages 0r mandibles; ett, eft, the greatly shows, there are four of these apxfemiage^are represented with'the “antenna-like” palps Or filaneural or ventral surface upper- ments On the first thoracic most. Original. t i « ..,, limb. A common modification of the chief ramus of the Arthropod parapodium is the chela or nipper formed by the elongation of the penultimate joint of the ramus, so that the last joint works on it—as, for instance, in the lobster’s claw. Such chelate rami or limb-branches are independently developed in Crustacea and in Arachnida, and are carried by somites of the body which do not correspond in position in the two groups. The range of modification of which the rami or limb-branches of the limbs of Arthropoda are capable is very large, and in allied orders or even families or genera we often find what is certainly the palp of the same appendage (as determined by numerical position of the segments)—in one case antenniform, in another chelate, in another pediform, and in another
 * J- ^-PU-S, db tnt figure

Fig. 9.—The second thoracic (fifth post-oral) appendage of the left side of Apus cancrifonnis, placed with its ventral or neural surface uppermost to compare with Figs. 7 and 8. 1, 2, The two segments of the axis; en the gnathobase; en.2 to e»6, the five following “endites” ; fl, the flabellum or anterior exite; Or, the bract or posterior exite. (After Lankester 0 J Mic. Sci. vol. xxi. 1881.) reduced to a mere stump or absent altogether. Very probably the power which the appendage of a given segment has of assuming the perfected form and proportions previously attained by the appendage of another segment

must be classed as an instance of “ homoeosis,” not only where such a change is obviously due to abnormal development or injury, but also where it constitutes a difference permanently established between allied orders or smaller groups, or between the two sexes. The most extreme disguise assumed by the Arthropod parapodium or appendage is that of becoming a mere

Fig. 10.—The first thoracic (fourth postoral) appendage of Apus cancriformis (right side). Ax1 to Ax*, the four segments of the axis with muscular bands; En1, gnathobase; En* to En^, the elongated jointed endites (rami); En®, the rudimentary sixth endite (exopodite of higher Crustacea); Fl, the flabellum which becomes the epipodite of higher forms ; Br, the bract devoid of muscles and respiratory in function. (After Lankester, Q. J. Mic. Sci. vol. xxi. 1881.)

stalk supporting an eye—a fact which did not obtain general credence until the experiments of Herbst in 1895, who found, on cutting off the eye-stalk of Palsemon, that a jointed antenna-like appendage was regenerated in its place. Since the eye-stalks of Podopthalmate Crustacea represent appendages, we are forced to the conclusion that the sessile eyes of other Crustacea and of other Arthropoda generally, indicate the position of appendages which have atrophied.1 From what has been said, it is apparent that we cannot, in attempting to discover the affinities and divergences of the various forms of Arthropoda, attach a very high phylogenetic value to the coincidence or divergence in form of the appendages belonging to the somites compared with one another. The principal forms assumed by the Arthropod parapodium and its rami may be thus enumerated :— (1) Axial corm well developed, unsegmented or with two to four segments ; lateral endites and exites (rami) numerous and of various lengths (certain limbs of lower Crustacea). (2) Corm, with short unsegmented rami, forming a flattened foliaceous appendage, adapted to swimming and respiration (trunklimbs of Phyllopods). (3) Corm alone developed; with no endites or exites, but provided with terminal chitinous claws (ordinary leg of Peripatus), with terminal jaw teeth (jaw of Peripatus), or with blunt extremity (oral papilla of same) (see Fig. 8). (4) Three of the rami of the primitive limb (endites 5 and 6, and exite 1) specially developed as endopodite, exopodite, and epipodite—the first two often as firm and strongly chitinized, segmented, leg-like structures ; the original axis or corm reduced to a basal piece, with or without a distinct gnathobase (endite 1) —typical tri-ramose limb of higher Crustacea. 1 H. Milne-Edwards, who was followed by Huxley, long ago formulated the conclusion that the eye-stalks of Crustacea are modified appendages, basing his argument on a specimen of Palinurus (figured in Bateson’s book (1), in which the eye-stalk of one side is replaced by an antenniform palp. Hofer (6) in 1894 described a similar case in Astacus.