A R T H R (5) One ramus (the endopodite) alone developed—the original axis or conn 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 filifonn or brushlike 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 ot 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 (chelse 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). (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 “ biramose limb” often found in Crustacea. The rami maybe flattened for swimming, when it is “a biramose 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. (7) The endopoditic ramus is greatly enlarged and flattened, without or with only one jointing, the corm (basal segment) is evanescent; often the plate-like endopodites of a pair of such appendages unite in the middle line with one another or by the intermediary of a sternal up-growth and form a single broad plate. (These are the plate-like swimmerets and opercula of Gigantostraca and Limulus among Arachnids and of Isopod Crustaceans. They may have rudimentary exopodites, and may or may not have branchial filaments or lamellae developed on their posterior faces. The simplest form to which they may be reduced is seen in the genital operculum of the Scorpion). (8) The gnathobase becomes greatly enlarged and not separated by a joint from the corm; it acts as a hemignath or half-jaw working against its fellow of the opposite side. The endopodite may be retained as a small segmented palp at the side of the gnathobase or disappear (mandible of Crustacea, Chilopoda, and Hexapoda). (9) The corm becomes the seat of a development of a special visual organ, the Arthropod eye (as opposed to the Chsetopod eye). Its jointing (segmentation) may be retained, but its rami disappear (PodophthalmousCrustacea). Usuallyit becomes atrophied, leaving the eye as a sessile organ upon the prse-oral region of the body. (The eye-stalk and sessile lateral eyes of Arthropoda generally, exclusive of Peripatus). (10) The forms assumed by special modification of the elements ■of the parapodium in the maxillse, labium, &c., of Hexapods, Chilopods, Diplopods, and of various Crustacea, deserve special enumeration, but cannot be dealt with without ample space and illustration. It may be pointed out that the most radical difference presented in this list is that between appendages consisting of the corm alone without rami (Onychophora) and those with more or less developed rami (the rest of the Arthropoda). In the latter class we should distinguish three phases: {a) those with numerous and comparatively undeveloped rami; {b) those with three, or two highly developed rami, or with only one—the corm being reduced to the dimensions of a mere basal segment; (c) those reduced to a secondary simplicity (degeneration) by overwhelming development of one segment {e.g., the isolated gnathobase often seen as “mandible” and the genital operculum). There is no reason to suppose that any of the forms of limb observed in Arthropoda may not have been independently developed in two or more separate diverging lines of descent. Branchial.—In connexion with the discussion of the limbs of Arthropods, a few words should be devoted to the gill-processes. It seems probable that there are branchial plumes or filaments in some Arthropoda (some Crustacea) which can be identified with the distinct branchial organs of Chsetopoda, which lie dorsad of the parapodia and are not part of the parapodium. On the other hand, we cannot refuse to admit that any of the processes of an Arthropod parapodium may become modified as branchial organs, and that, as a rule, branchial out-growths are easily developed, de novo, in all the higher groups of animals. Therefore, it seems to be, with our present knowledge, a hopeless task to analyse the branchial organs of Arthropoda and to identify them genetically in groups. A brief notice must suffice of the structure and history of the Ayes, the Tracheae, and the so-called Malpighian tubes of Arthropoda, though special importance attaches j
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to each in regard to the determination of the affinities of the various animals included in this great sub-phylum. The Eyes.-—The Arthropod eye appears to be an organ of special character developed in the common ancestor of the Euarthropoda, and distinct from the Chaitopod eye, which is found only in the Onychophora where the true Arthropod eye is absent. The essential difference between these two kinds of eye appears to be that the Chaetopod eye (in its higher developments) is a vesicle enclosing the lens, whereas the Arthropod eye is a pit or series of pits into which the heavy chitinous cuticle dips and enlarges knobwise as a lens. Two distinct forms of the Arthropod eye are observed—the monomeniscous (simple) and the polymeniscous (compound). The nerve-end-cells, which lie below the lens, are part of the general epidermis. They show in the monomeniscous eye (see article Arachnida, Fig. 26) a tendency to group themselves into “retinulm,” consisting of five to twelve cells united by vertical deposits of chitin (rhabdoms). In the case of the polymeniscous eye (Fig. 23, article Arachnida) a single retinula or group of nerve-end-cells is grouped beneath each associated lens. A further complication occurs in each of these two classes of eye. The monomeniscous eye is rarely provided with a single layer of cells beneath its lens; when it is so, it is called monostichous (simple lateral eye of Scorpion, Fig. 22, article Arachnida). More usually, by an infolding of the layer of cells in development, we get three layers under the lens; the front layer is the corneagen layer, and is separated by a membrane from the other two which, more or less, fuse and contain the nerve-endcells (retinal layer). These eyes are called diplostichous, and occur in Arachnida and Hexapoda (Fig. 24, in article Arachnida). On the other hand, the polymeniscous eye undergoes special elaboration on its lines. The retinulse become elongated as deep and very narrow pits (Fig. 11 and Fio. 11.—Diagram to show the derivation of the unit or “ommatidium" of the compound eye of Crustacea and Hexapoda, C, from a simple monomeniscous monostichous eye resembling the lateral eye of a Scorpion, A, or the unit of the compound lateral eye of Limulus (see article Akachnida, Figs. 22 and 23). B represents an intermediate hypothetical form in which the cells beneath the lens are beginning to be superimposed as corneagen, vitrella, and retinula, instead of standing side by side in horizontal series. The black represents the cuticular product of the epidermal cells of the ocular area, taking the form either of lens, d, of crystalline body, cry, or of rhabdom, rhab; hy, hypodermis or epidermal cells; corn1, laterallyplaced cells in the simpler stage, A, which like the nerve-end cells, vifl and refl, are corneagens or lens-producing ; corn, specialized corneagen or lensproducing cells ; vifl, potential vitrella cells with cryi, potential crystalline body now indistinguishable from retinula cells and rhabdomeres; vit, ' vitrella cell with cry, its contained cuticular product, the crystalline cone or body; refi, rhafA, retinula cells and rhabdom of scorpion undifferentiated from adjacent cells, via ; ret, retinula cell; rhab, rhabdom : nf, optic nervefibres. (Modified from Watase.) explanation), and develop additional cells near the mouth of the narrow pit. Those nearest to the lens are the corneagen cells of this more elaborated eye, and those between the original retinula cells and the corneagen cells become firm and transparent. They are the crystalline cells or vitrella (see Watase, 7). Each such complex of cells underlying the lenticle of a compound eye is called an “ ommatidium ” ; the entire -mass of cells underlying a monomeniscous eye is an “ ommatseum.” The ommatseum, as already stated, tends to segregate into retinulae which correspond potentially each to an ommatidium of the compound eye. The ommatidium is from the first segregate and consists of few cells. The compound eye S. I. — 88
