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CLASS PAIISOPODA

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Originally appearing in Volume V18, Page 474 of the 1911 Encyclopedia Britannica.
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CLASS PAIISOPODA. As in the Diplopoda there is a distinct head bearing a pair of antennae and two pairs of jaws. On each side of the head there is an eye-like spot which may conceivably represent a degenerate eye, although the external cuticle shows no corneal thickening nor the epidermis retinular specialization, and optic nerves are absent from the brain. The antennae are structurally unique in the Arthropoda. There are four short basal segments from the distal of which arise two one-jointed branches, an external thinner and an internal thicker. The external or postaxial branch is tipped with a single long annulate flagelliform bristle with a rounded apical knob; and the internal or preaxial branch with two similar but shorter bristles and a globular, usually pedunculated, sense organ between them. The mandibles or jaws of the first pair are large and one-jointed. Those of the second pair are very short, piriform, and attached to the ventral side of the head by a long, rod-like sclerite. Between these two pairs of jaws there is a horny framework forming a kind of lower lip to the mouth. The correspondence between these mouth parts and those of the Diplopoda is not understood. No doubt the mandibles are homologous in the two groups; but whether the jaws of the second pair in the Pauropoda correspond to the maxillulae of the Pselaphognatha, or to part of the gnathochilarium in the Chilognatha, or whether the chitinous framework alone or in conjunction with the pair of jaws answers to the gnathochilarium Platyrhachus mirandus, a Sumatran species of Polydesmidae, to show the form characteristic of the order Merochaeta. c, Head. ant, Antenna. tgi, Tergal plate of first body segment. tg', Ditto of 'seventh, a.tg, Tergal plate of anal segment. The figure also shows the repugnatorial pores which are present upon the majority of the segments, the laterally expanded tergal plates, and the presence of two pairs of legs for each of the segments except the two last, the four first and the seventh; the latter, since the figured specimen was a male, has the anterior leg converted into a phallopod which is concealed beneath the body. are questions to which no answer can as yet be given. Judging from the segmentation and the appendages the body is composed of twelve somites, including the last or anal, which, like the pen-ultimate somite, is limbless. Each somite in front of the penultimate a V. A and B, after Kenyon, Tufts Coll. Studies, iv., 5895; C, after Hansen, Vid. Meddel., Igor, Pl. VI., fig. 3a; D. and E, after Kenyon. A. Pauro pus huxleyi (?). c, head; ant. antenna; tgi and 1g5, first and fifth double tergal plates ; lgi, first walking-leg (= 2nd post-cephalic appendage); 1g9, ninth walking-leg; a.sg, anal segment; st, setae. B. Eurypauropus spinosus. Lettering as in A. C. Brachypauropus superbus. Lettering as in A and B ; (igr) = first and second terga; tg',=ninth and tenth terga. D. Jaws of Pauropus huxleyi; md, mandible; mx, maxilla; lb, labial framework. E. Antenna of Eurypauropus spinosus; fl, flagella; gl, sensory organ. bears a single pair of legs, nine pairs of which are fully developed ambulatory limbs, while those of the first segment are reduced to After Lubbock. a pair of bud-like processes. The first and last pairs of ambulatory limbs consist of five segments; in the remaining pairs the terminal segment may be subdivided into two, so that there may be six segments in all. The ambulatory limbs are usually terminated by three claws, a principal and two subsidiary, each claw being accompanied by a membranous pad. Between these limbs, which are relatively longer and stronger than in the Diplopoda and evenly spaced on each side of the body, extends a soft-skinned sternal area. The distensible pleural region of the body is also membranous, but the dorsal area is covered by chitinous plates or terga, usually six in number, excluding that of the anal segment; each of the anterior five of these overlies two limb-bearing somites, the first covering the somite of the rudimentary limbs and of the first pair of locomotor legs, the second those of the second and third pairs of locomotor legs, and so on. This condition is an adumbration of the far completer fusion of somites seen in the Diplopoda. The sixth tergal plate belongs to the limbless penultimate somite. The duplex character of the first five terga is suggested in Pauropus by the presence of two rows of sensory bristles; there being only one such row upon the sixth tergum. In the aberrant genus Brachypauropus the evidence is practically completed by the correspondence in number between the terga and pairs of legs, there being a divisional line between the two rows of setae. On each side of the body there are five long pubescent tactile setae situated on the second to the sixth terga in Pauro pus, and on the pleural area corresponding to these terga in Brachypauropus. The cerebral mass of the nervous system is large and when viewed from above is seen to consist of two lobes defined by a median groove. In the absence of eyes no optic nerves are given off. Beneath these are two antennal lobes whence arise, close together, the antennal nerves: Two short commissural cords connect the cerebral mass with the suboesophageal ganglion, a composite mass formed of the nervous centres which supply the two pairs of jaws and the rudimentary legs of the first pair. Behind this large ganglion the cord, which shows superficially no trace of its double origin, presents a ganglionic swelling for each pair of legs. No circulatory or respiratory organs have been detected. The alimentary canal consists of a short, narrow fore-gut, a large, straight mid-gut, and a moderately long hind-gut which is itself composed of two parts, an anterior narrow tube which opens into s • l av tyr y' ps After Kenyon, Tufts Coll. Studies, iv, 1895. FIG._14.—PAUROPODA. A. Alimentary canal of Pauropus; fg, fore-gut; sg, salivary gland; mg, mid-gut; hg, anterior portion of hind-gut; a, anus; m.p.l., malpighian tubule. B. Female genital organs of Eurypauropus; ov, ovary; ovid, oviduct; rs, receptaculum seminis; go, genital orifice. C. Male genital organs of Pauropus; ti and 12, anterior and posterior portions of testes; vdi, vd2, vd3, vasa deferentia; vs.:, vesicula seminalis; cd, common duct; go, genital orifices. D. Lateral view of Pauropus; c, head; ant. antenna; tgi,tg', first and fifth tergal plates; a.sg, anal segment; st, lateral bristles; lg.r, rudimentary leg; lgi and lg,9 first and ninth fully formed walking legs; p, penis. a dilated, piriform, posterior portion, narrowing gradually to terminate in the anus. Opening into the anterior extremity of the fore-gut there is a pair of " salivary " glands. Malpighian tubes have been found in some forms, i.e. females of Eurypauropus spinosus, but not in any examples, male or female, of Pauropus huxleyi. Where present they open at the point of union of the mid- 474 and hind-guts. The generative organs in the female are very simple, and much like those of the Diplopoda. In the male they are highly complex, and unlike those of any known Arthropod in certain particulars. The wide, unpaired ovary extends nearly to the posterior end of the body. Anteriorly it passes into an oviduct which is unpaired throughout its length. The posterior portion of the duct is wide. The anterior, an abruptly narrowed tube, curves round the nerve-cord and opens by a single sub-median orifice in the third segment. Just within the orifice there opens into the oviduct the short duct of a spherical receptaculum seminis. In the male the testis is never paired. Sometimes it is single, sometimes divided into an anterior and a posterior mass, and sometimes merely constricted. It lies above the intestine in the posterior half of the body in the adult, but at least in the young in some cases, where as many as four divisions have been detected, its position is more lateral. Leading from the sperm masses there may be as many as three slender short ducts which soon expand into wider tubes. These tubes, regarded as seminal vesicles, after forming a complex of loops, coils and caecal prolongations, ultimately unite beneath the intestine in a single tube which passing forwards divides on each side of the alimentary canal to terminate in the two penes situated just behind the bases of the second pair of complete legs, that is to say, the legs of the third segment. Just at the root of the penis there is an accessory gland on the duct, and a little farther back a'much larger glandular swelling. The Pauropoda are divided into three rather sharply defined groups or families which may be briefly characterized as follows : Pauropodidae.—Head not covered by the first tergal plate. Anal segment not covered by the sixth tergal plate. Terga of the first ten body segments fused in couples. Tactile setae situated on the lateral portions of the terga which are neither sculptured nor spinous. (Pauropus, Stylopauropus.) Brachypauropodidae.—Head and anal segment free and the terga smooth as in the last ; but each of the double terga of the Pauropodidae divided into an anterior and posterior plate by a transverse band of membrane and each of these into a pair of plates by a longitudinal integumental strip. The tactile setae arising from the pleural area of the segments. (Brachypauropus.) Eurypauropodidae.—Body wide and onisciform, the head and the anal segment concealed dorsally by the first and penultimate terga respectively. Terga fused as in the Pauropodidae, but thickly spinous or sculptured. The tactile setae situated beyond the edge of the terga, as in the Brachypauropodidae. (Eurypauropus.) The genus Pauropus is probably world-wide in distribution, since it has been discovered in Europe, North and South America, and in Siam. The two known species of Brachypauropus were found respectively in Italy and Austria. Eurypauropus has representatives in North America and Europe. Examples of Pauropus are extremely agile, recalling the centipede Lithobius in their movements; those of Eurypauropus, on the contrary, are extremely slow and quite comparable in lack of agility to the common pill-millipede. They are usually found in woods, under stones, fallen branches, dead leaves or other damp situations. They are believed to be vegetable feeders and are oviparous. The young upon hatching has four segments and three pairs of legs representing the first three pairs of ambulatory legs of the adult. The two last segments are apodous, the first bears the first pair of legs, and the second the second and third pairs. The young passes through four successive moults, and gradually acquires its full complement of segments and limbs.
End of Article: CLASS PAIISOPODA
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