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.
| After Lankester, Q. J. Mic. Sci. vol. xxi., 1881. |
| Fig. 10.—The second thoracic (fifth post-oral) appendage of the left side of Apus cancriformis, placed with its ventral or neural surface uppermost to compare with figs. 8 and 9. |
| 1, 2, The two segments of the axis. en1, The gnathobase. en2 to en6, The five following “endites.” fl, The flabellum or anterior exite. br, The bract or posterior exite. |
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 (trunk-limbs 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. 9).
(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.
(5) One ramus (the endopodite) alone developed—the original axis or corm serving as its basal joint with or without gnathobase. This is the usual uni-ramose limb found in the various classes of Arthropoda. It varies as to the presence or absence of the jaw-process and as to the stoutness of the segments of the ramus, their number (frequently six, plus the basal corm), and the modification of the free end. This may be filiform or brush-like or lamellate when it is an antenna or palp; a simple spike (walking leg of Crustacea, of other aquatic forms, and of Chilopods and Diplopods); the terminal joint flattened (swimming leg of Crustacea and Gigantostraca); the terminal joint provided with two or with three recurved claws (walking leg of many terrestrial forms—e.g. Hexapoda and Arachnida); the penultimate joint with a process equal in length to the last joint, so as to form a nipping organ (chelae of Crustaceans and Arachnids); the last joint reflected and movable on the penultimate, as the blade of a clasp-knife on its handle (the retrovert, toothed so as to act as a biting jaw in the Hexapod Mantis, the Crustacean Squilla and others); with the last joint produced into a needle-like stabbing process in spiders.
| After Lankester, Q. J. Mic. Sci. vol. xxi., 1881. |
| Fig. 11.—The first thoracic (fourth post-oral) appendage of Apus cancriformis (right side). |
| Ax1 to Ax4, the four segments of the axis with muscular bands. En1, Gnathobase. En2 to En5, The elongated jointed endites (rami). En6, 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. |
(6) Two rami developed (usually, but perhaps not always, the equivalents of the endopodite and exopodite) supported on the somewhat elongated corm (basal segment). This is the typical “bi-ramose limb” often found in Crustacea. The rami may be flattened for swimming, when it is “a bi-ramose swimmeret,” or both or only one may be filiform and finely annulate; this is the form often presented by the antennae of Crustacea, and rarely by prae-oral appendages in other Arthropods.