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Encyclopedia Britannica - Main :: CAU-CHA |
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CENTAURS , in Greek mythology, a race of beings part horse part man, dwelling in the mountains of Thessaly and Arcadia. The name has been derived (I) from KEVTEiv (goad) and Tai3pos (bull), implying a people who were primarily herdsmen, (2) from KEVTESV and the common termination -avpos or aiipa ("air ") i.e. " spearmen." The former is unsatisfactory partly from the philological standpoint, and the latter, though not certain, is preferable. The centaurs were the offspring of Ixion and Nephele (the rain-cloud), or of Kentauros (the son of these two) and some Magnesian mares or of Apollo and Hebei They are best known for their fight with the Lapithae, caused by their attempt to carry off Dezdameia on the day of her marriage to Peirithous, king of the Lapithae, himself the son of Ixion. Theseus, who happened to be present, assisted Peirithous, and the Centaurs were driven off (Plutarch, Theseus, 30; Ovid, Metam. xii. 210; Diod. Sic. iv. 69, 70). In later times they are often represented drawing the car of Dionysus, or bound and ridden by Eros, in allusion to their drunken and amorous habits. Their general character is that of wild, lawless and inhospitable beings, the slaves of their animal passions, with the exception of Pholus and Chiron. They are variously explained by a fancied resemblance to the shapes of clouds, or as spirits of the rushing mountain torrents or winds. As children of Apollo, they are taken to signify the rays of the sun. It is suggested as the origin of the legend, that the Greeks in early times, to whom riding was rank as distinct classes of Arthropods. Pocock, indeed, definitely associated the Chilopoda with the Hexapoda in a group, the Opisthogoneata (Opisthogonea), equivalent to a group, the Progoneata (Prosogonea), comprising the Diplopoda, Pauropoda and Symphyla. As the basis for this classification was taken the position of the generative orifices which open in the Opisthogonea at the posterior end and in the Prosogonea near the anterior end of the body
recent
Structure of the Chilopoda.The exoskeletal elements of a typical somite consist of a dorsal plate or tergum, a ventral plate or sternum, a lateral or pleural membrane, often strengthened with chitinous sclerites, and a pair of appendages. At the anterior extremity there is a head-shield or cephalite, which bears eyes, when present, and a pair of antennae. In all centipedes, except the Scutsgeridae, the preantennal portion of the cephalite is sharply reflexed, ventrally forrhing an area called the clypeus. The inferior edge of this bears the labrum, which is usually represented by a small median, and two large lateral plates. The appendages are modified as a single pair of antennae, four pairs. of jaws or gnathites, a variable number of walking legs and a single pair of generative limbs or gonopods. The antennae, articulated to the forepart of the head and preoral in position, are long and flexible and consist of fourteen or more segments. The jaws of the first pair of mandibles are stout and bisegmented, with a dentate cutting edge. Those of the second pair or maxillae vary considerably in structure in different groups. ,I hey are foliaceous and are usually regarded as biramous. In some genera (Scutigera, Lithobius) the inner branch consists of two distinct segments meeting those of the opposite side in the middle line. The outer branch, which is always larger, consists of three or four segments. Generally, however, the basal segments of the two branches are coalesced with each other and with the corresponding segments of the opposite side to form a single broad transverse plate. The above described condition seen in Sculigera suggests that two pairs of jaws may be involved in the formation of the maxillae in the Chilopoda. The jaws of the third pair, the palpognaths or second pair of maxillae, resemble dwarfed walking legs, and consist of five or six segments, of which the basal or coxa is united mesially to its fellow. The jaws of the fourth pair, the toxicognaths or poison-jaws, are long and powerful, and consist like the legs primarily of six segments, whereof the basal is large and usually fused with its fellow to form a large coxal plate, the second is small and generally suppressed by fusion with the third, the fourth and fifth are also small, while the sixth
The tergal elements of the somites bearing the antennae, mandibles and maxillae appear to be represented by the head-shield or cephalite. The tergal element
body
end of the body. They consist typically of six segments, of which the basal is termed the coxa and the apical the tarsus. The tarsus is armed with a single terminal claw, and, except in the Geophilomorpha and a few genera of other orders, is divided by a mesial transverse joint into two segments, as is the case in Scolopendra and Lithobius for example. But in some of the longer-legged, swift-footed centipedes of the order Lithobiomorpha (e.g. Henicops, Cermatobius) the tarsi are further subdivided. The multiplication of sub-segments reaches its maximum in Scutigera, where the tarsi are extremely long, slender, flexible and annulated. The legs of the last pair are directed backwards in a line parallel with the long axis of the body, so that their coxae, fused in some cases with the pleural sclerites (Scolopendra, Geophilus), or free and of large size (Scutigera, Lithobius), serve to protect the small genital and anal somites. They are often greatly modified. In the males of some species of Lithobius one or more of the segments is inflated or furnished with tubercle-bearing, tactile bristles; in some Geophilomorpha the whole limb is thickened in the male sex. In most Scolopendromorpha the basal segment is armed beneath with spines or spikes (Dacetum, Scolopocryptops) ; sometimes the whole appendage is thickened and terminated by a sharp
A study of the development of Scolopendra has shown that the antennae of the adult are the appendages of the second postoral metamere and the mandibles those of the fourth, the first postoral metamere, which has a pair of transient preantennal appendages, and the third, which has no appendages, being excalated at an early stage of embryonic growth. Further-more, behind the legs of the last pair two pairs of appendages are present. The second of these persists as the gonopods of the adult, but the first is suppressed. Possibly, however, it is represented in the male of Scutigera by the anterior branches of the gonopods. The cerebral or cephalic portion of the nervous system consists of a quadrilobate mass. From the two upper lobes, which are set transversely, arise the ocular nerves; from the two lower lobes, which are united by a transverse commis-sure, spring the antennal nerves in front and the chords which form the oesophageal collar be-hind. These chords unite below the oesophagus to form the compound suboesophageal ganglion, whence the nerves for the four pairs of jaws arise. The ventral system consists of a double chord uniting in each of the leg-bearing segments in a ganglionic swelling which gives off four pairs of nerves to the limbs and tissues of the somite. There is a single ganglion in the genital segment.Eyes are frequently absent. When present they may be either simple or compound, i.e. consisting externally of a single lens (monomeniscous) of or an aggregation of lenses (polymeniscous). Simple eyes vary in number on each side of the head from one, as in Henicops, to as many as forty, as in some species of Lithobius. In cL Modified from Heymons, Bib. Zool., Igor, by permission of E. Nagele. FIG. I. A, Diagram of anterior extremity of an early embryo of Scolopendra, ventral view; cl, clypeus; lb, labrum; m, mouth; p.a, preantennal appendage; a, antenna; int, premandibular rudiment; mdl, mandible; mx, maxilla; p.g, palpognath; t.g, toxicognath; lg. I, first pair of walking legs. B, Posterior end of a later embryo of Scolopendra, ventral view, showing the anal segment or telson (t) ; the legs of the last pair in the adult (lg. 21) and the two rudimentary pairs of legs (lg. 22, lg. 23). Scolopendra,where there are four, the corneal lens is a biconvex thickening of the cuticle. The soft or retinal portion of the eye beneath the lens consists of an aggregation of large cells forming a single layer continuous with the epidermic cells of the circumocular area. Thus the eye is monostichous, The arrangement of the cells, however, is peculiar. They are invaginated to form what may be described as a very deep cup with exceedingly thick walls and correspondingly narrow median space, the outer surface of the cup being formed by the inner or proximal ends of the cells and the inner surface by their outer or distal ends. It results from this arrangement that the cells forming all but the bottom of the invagination lie horizon-tally, i.e. at right angles to the vertical axis of the eye. From the distal ends of the cells are secreted chitinous rhabdomeres, forming a rhabdom which occupies and fills up the central portion of the cup beneath the middle of the corneal lens. The outer ends of the cells are nucleated and are continuous with the fibres of the optic nerve, large accessory glands; and a pair of tubes, or vesiculae seminaie. , open, one on each side, into the divided sperm ducts close to then point of origin above the intestine. The organs of the female are very similar. There is a large median ovary followed by a short oviduct forming a circum-intestinal collar and a common atrium. Into the latter open a pair of short receptacula seminis and the slender duct of two pairs of large accessory glands. There is nothing in the female corresponding to the supra-intestinal vesiculae seminales of the male. In the male of Scolopendra,on the contrary, there Len, B C. Zertti .................. B, Section of Eye of Scolopendra. len, C, Ocular unit or ommatidium Corneal lens; ret, retinal or visual cells; of compound Eye of .Scutigera. n.opt, optic nerve: len, corneal lenticle ; c.c, crystal- line cone
which passes from the outer surface of the bottom of the cup to the are as many as twelve pairs of outermost tier; 2, 3, retinular brain. A and B after Heymons, Bibl. Zool., 19or, by permission of E, Nagele. A, Brain of Scolopendra. n.ant, Antennal nerves; n.opt, ocular nerves; n.pr.ant, preantennal nerves'; oes.comm, oesophageal commissure. Compound eyes are found only in the Stutigeridae. Externally the eye consists of one hundred or more little lenses or lenticles. The retinal portion is composed of a corresponding number of ocular units or ommatidia. Each ommatidium is an elongated cone
The alimentary canal is a simple tube running without convolutions from the mouth to the anus. Its anterior portion or pharynx, which arises from the stomodaeal "invagination in the embryo, is short; a pair of large, so-called salivary glands open into it. The mesenteric part of the canal is relatively wide and receives at its junction with the hind-gut a,' the excretory products of a pair of very long and slender malpighian tubes of Alimentary Canal of the canal, arising from the proctodaeum, Lithobius. is relatively short and narrow. a, Anus. The generative organs vary in struc- mg, Mid-gut. tural details in different centipedes. In hg, Hind-gut. the male of Lithobius the testes consist mt, Malpighian tubule. of a single coiled tube lying above the 's.gl, Salivary gland. alimentary canal. The slender vas de- 1g.15, Legs of first ferens which proceeds from its, hinder and fifteenth pairs. end divides posteriorly into a right and left branch, embracing the gut and uniting beneath it to form a common chamber or atrium within the genital orifice. The atrium receives the secretion of two pairs ofsomewhat sausage-shaped testes, cells of middle and innermost approximated two by two. From tiers; rbd, rhabdomeres; n.opt, each pair proceed two slender optic nerve; pg, pigment cells. ducts which open into a median duct coiled in the posterior third of the body and much expanded in the last three of the leg-bearing segments. The right and left portions of the intestinal ring of the genital duct are unequally developed, and there are no vesiculae seminales, but two pairs of After Heymons, Bibl. Zool., 19or, by permission of E. Nagele. accessory glands communicate with the genital atrium as in Lithobius. In the female Scolopendra ,the right and left portions of the intestinal collar are also unequally developed, and only a single pair of accessory glands besides the receptacula seminis open into the atrium. The heart is tubular and lies in the middle dorsal line immediately g6. arc beneath the integument. It consists of a series of chambers corresponding roughly to the leg-bearing segments, and lies in a blood-sinus formed by a pericardial membrane whence large alary muscles extend to the sides of the body. Each chamber gives off in Scolopendra a pair of fine lateral vessels, and is furnished at its posteriorA, Anterior extremity of B, Two segments of Scolo-Scolopendra, showing two pendra, showing the branching chambers of the heart (h), the and anastomosing tracheae and a aortic ring (a), the alae cordis spiracle (sp). (a.m) and a cardiac orifice (o). , extremity with a pair of orifices by which the blood re-enters the organ from the pericardial space. From the anterior chamber, which lies in the first or second leg-bearing segment, proceed three arteries, a median which runs forwards into the head to supply the brain and other organs, and a lateral which with its fellow of the opposite side forms an oesophageal aortic collar. From the sides of the latter arise vessels to the gnathites, and from its inferior portion an unpaired vessel passes forwards into the head and another back-wards above the nerve chord to the posterior end of the body, supplying each segment in its course with a delicate lateral branch. In Scolopendra the chambers of the heart, excepting the first and last, which are small, are subequal in size; but in forms like Scutigera where the terga are very unequal in size a corresponding inequality in the size of the chambers is manifested.In all centipedes, except Scutigera, respiration is effected by chitinized tracheal tubes which extend with their ramifications throughout the body and open to the exterior by means of spiracles perforating the lateral or pleural membrane of more or fewer of the somites below the edge of the terga. Spiracles are never present upon the anal, genital and last leg-bearing somites, and only rarely, as in Henicops, upon the somite bearing the legs of the first pair. In the majority of cases the spiracles are circular, sigmoid or slit-like orifices, with chitinized rim, leading into a pocket-like integumental infolding, from which emanate numerous small tracheal tubes which soon anastomose to form the main tracheal trunks. In Dacetum, one of the Scolopendridae, there is no pocket-like in-folding, the small tracheal tubes opening direct to the exterior on a large subcircular plate where their apertures fuse to form a complicated network. The apertures, as in the case of other genera, are protected by fine hairs; and the tracheae themselves are strengthened by a fine spiral filament. In the Lithobiidae the tracheae do not anastomose; but in Scolopendra and Geophilus the main trunks in each segment fuse transversely with those of the opposite side and also longitudinally with those of the preceding and succeeding segments. In Scutigera the tracheae differ both in structure and position from those of all other Chilopoda. The spiracles, unpaired and seven in number, open in the median dorsal line. Each leads into a short sac from which five tracheal tubes depend into the pericardial blood-sinus. Existing Chilopoda may be classified as follows, into five orders referable to two subclasses Subclass I. Pleurostigma. Order i Geophilomorpha. 2 Scolopendromorpha. 3 Craterostigmomorpha. 4 Lithobiomorpha. Subclass II. Notostigma. Order 5 Scutigeromorpha. End of Article: CENTAURS If you wish, you can link directly to this article.
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