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 A R T H R O P O D A (i) A pair of ccelomoducts is developed in every somite including the prosthomere, in which alone it atrophies in later development. (j) The ventral nerve-cords are widely separated—in fact, lateral in position. (k) There are no masses of nerve-cells forming a ganglion (neuromere) in each somite. (In this respect the Protarthropoda are at a lower stage than most of the existing Chfetopoda.) (l) The genital ducts are formed by the enlargement of the ccelomoducts of the penultimate somite. Class (Unica).—Oxychophora. With the characters of the grade: add the presence within the body of fine unbranched tracheal tubes, devoid of spiral thickening, opening to the exterior by numerous irregularly scattered tracheal pits. Genera—Eoperipatus, Peripatopsis, Opisthopatus, &e. Grade C (of the Arthropoda).—Euarthropoda. (a) Integument heavily plated with firm chitinous cuticle, allowing no expansion and retraction of regions of the body nor change of dimensions, except, in some cases, a dorso-ventral bellows movement. The separation of the heavier plates of chitin by grooves of delicate cuticle results in the hinging or jointing of the body and its appendages, and the consequent flexing and extending of the jointed pieces. (b) Claws and fangs are developed on the branches or rami of the parapodia, not on the end of the axis or conn. (c) The head is either deuterognathous, tritognathous, or tetartognathous. (d) Rarely only one, and usually at least two, of the somites following the mandibular somite carry appendages modified as jaws (with exceptions of a secondary origin). (e) The rest of the somites may all carry appendages, or only a limited number may carry appendages. In all cases the appendages primarily develop rami or branches which form the limbs, the primitive axis or corm being reduced and of insignificant size. In the most primitive stock all the post-oral appendages had gnathobasic outgrowths. (/) The segmentation of the body is anomomeristic in the more archaic members of each class, nomomeristic in the higher members. (g) The two eyes of Chaetopod structure have disappeared, and are replaced by the Euarthropod eyes. (h) The muscles in all parts of the body consist of striped muscular fibre, never of unstriped muscular tissue. (i) The ccelomoducts are suppressed in most somites, and retained only as the single pair of genital ducts (very rarely more numerous) and in some also as the excretory glands (one or two pairs). (J) The ventral nerve-cords approach one another in the midventral line behind the mouth. (k) The nerve-cells of the ventral nerve cords are segregated as paired ganglia in each somite, often united by meristic dislocation into composite ganglia. (l) The genital ducts maybe the coelomoducts of the penultimate or antepenultimate or adjacent somite, or of a somite placed near the middle of the series, or of a somite far forward in the series. Class 1 (of the Euarthropoda).—Diproroda. The head has but one prosthomere (monoprosthomerous), and is accordingly deuterognathous. This carries short-jointed antennae (in one case biramose) and eyes, the structure and development of which require further elucidation. Only one somite following the first post-oral or mandibular segment lias its appendages modified as jaws. The somites of the body, except in Pauropus, either fuse after eariy development and form double somites with two pairs of appendages (Julus, &c.), or present legless and leg-bearing somites alternating. Somites, anomomeristic, from 12 to 150 in the post-cephalic series. The genital ducts open in the fourth, or between the fourth and fifth post-oral somite. Terrestrial forms with small-jointed legs formed by adaptation of a rsingle ramus of the appendage. Tracheae are present. A ote. —The Diplopoda include the Juliformia, the Symphyla (Scolopendrella), and Pauropoda (Paurojius). They were until recently classified with the Chilopoda (Centipedes), with which they have no close affinity, but only a superficial resemblance. (Compare the definition of the class Chilopoda.) The movement of the legs in Diplopoda is like that of those of Peripatus, of the Phyllopod Crustacea, and of the Parapodia of Cluetopoda, symmetrical and identical on the two sides of the body. The legs of Chilopoda move in alternating groups on the two sides of the body. This implies a very much higher development of nerves and muscles in the latter. Class 2 (of the Euarthropoda).-—Arachnida. Head tritognathous and diprosthomerous—that is to say, with two prosthomeres, the first bearing typical eyes, the second a pair

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of appendages reduced to a single ramus, w'hich is in more primitive forms antenniform, in higher forms chelate or retrovert. The ancestral stock was panto-gnathobasic-—i.e., had a gnathobase or jaw process on every parapodium. As many as six pairs of appendages following the mouth may have an enlarged gnathobase actually functional as a jaw or hemignath, but a ramus is well developed on each of these appendages either as a simple walking leg, a palp, or a chela. In the more primitive forms the appendage of every post-oral somite has a gnathobase and two rami; in higher specialized forms the gnathobases may be atrophied in every appendage, even in the first post-oral. The more primitive forms are anomomeristic ; the higher forms nomomeristic, showing typically three groups or tagmata of six somites each. The genital apertures are placed on the first somite of the second tagma or mesosoma. Their position is unknown in the more primitive forms. The more primitive forms have branchial respiratory processes developed on a ramus of each of the post-oral appendages. In higher specialized forms these branchial processes become first of all limited to five segments of the mesosoma, then sunk beneath the surface as pulmonary organs, and finally atrophied, their place being taken by a well-developed tracheal system. A character of great diagnostic value in the more primitive Arachnida is the tendency of the chitinous investment of the tergal surface of the telson to unite during growth with that of the free somites in front of it, so as to form a pygidial shield or posterior carapace, often comprising as many as fifteen somites (Trilobites, Limulus). A pair of central monomeniscous diplostichous eyes is often present on the head. Lateral eyes also are often present which are monostichous with aggregated lenses (Limulus) or with isolated lenses (Scorpio), or are diplostichous with simple lens (Pcdipaljri, Aranefe, etc. Class 3 (of the Euarthropoda).—Crustacea. Head tetartognathous and triprosthomerous—that is to say, with three prosthomeres: the first bearing typical eyes, the second a pair of antenniform appendages (often biramose), the third a pair of appendages usually antenniform, sometimes claw - like. The ancestral stock wras (as in the Arachnida) pantognathobasic, that is to say, had a gnathobase or jaw-process on the base of every post-oral appendage. Besides the first post-oral or mandibular pair, at least two succeeding pairs of appendages are modified as jaws. These have small and insignificant rami, or none at all, a feature in which the Arachnida differ from them. The appendages of four or more additional following somites may be turned upwards towards the mouth and assist in the taking of food. The more primitive forms (Entomostraca) are anomomeristic, presenting great variety as to number of somites, form of appendages, and tagmatic grouping; the higher forms (Malacostraca) are nomomeristic, showing in front of the telson twenty somites, of which the six hinder carry swimmerets and the five next in front ambulatory limbs. The genital apertures are neither far forward nor far backward in the series of somites, e.g., ori the fourteenth post-oral in Apus, on the ninth post-oral in female Astacns and in Cyclops. With rare exceptions, branchial plates are developed either by modification of a ramus of the limbs or as processes on a ramus, or upon the sides of the body. No tracheate Crustacea are known, but some terrestrial Isopoda develop pulmonary in-sinkings of the integument. A characteristic, comparable in value to that presented by the pygidial shield of Arachnida, is the frequent development of a pair of long appendages by the penultimate somite, which with the telson form a trifid, or, when that is small, a bifid termination to the body. The lateral eyes of Crustacea are polymeniscous, with highly specialized retinulse like those of Hexapoda, and unlike the simpler compound lateral eyes of lower Arachnida. Monomeniscous eyes are rarely present, and when present, single, minute, and central in position. Note.—The Crustacea exhibit a longer and more complete series of forms than any other class of Arthropoda, and may be regarded as preserving the most completely represented line of descent. Class 4.—Chilopoda. Head triprosthomerous1 and tetartognathous. The two somites following the mandibular or first post-oral or buccal somite carry 1 Embryological evidence of this is still wanting. In the other classes of Arthropoda we have more or less complete embryological evidence on the subject. It appears from observation of the embryo that whilst the first prosthomere of Centipedes has its appendages reduced and represented only by eye - patches (as in Arachnida, Crustacea, and Hexapoda), the second has a rudimentary antenna, which disappears, whilst the third carries the permanent antenna?, which accordingly correspond to the second antenna? of Crustacea, and are absent in Hexapoda.