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CLASS SYMPHYLA

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Originally appearing in Volume V18, Page 475 of the 1911 Encyclopedia Britannica.
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CLASS SYMPHYLA. Prosogoneate Arthropods, differing in many important particulars from the Diplopoda and Pauropoda. The axis of the head lies in the same straight line as that of the body, as in the Chilopoda, and not at right angles to it as in the Diplopoda and Pauropoda. There are no eyes. The antennae are very long and many-jointed. Four pairs of gnathites attached to the under-side of the head have been detected. The first pair (mandibles) are two-jointed, as in many Diplopods. The second pair (maxillulae) are minute, one jointed and articulated to a median lobe or hypopharynx which is supported by two chitinous skeletal rods. The third pair (maxill-e) consist of a long, basal segment terminating distally in two lobes; near the distal end of the basal segments there is externally a minute one- or two-jointed process, regarded as a palpus. Between the maxillae lies a large, double plate (labium or maxillae of second pair) which is attached proximally to two rod-like basal segments and terminates distally in two pairs of short lobes. The body is long and narrow and bears on its dorsal side fifteen tergal plates. The first of these, immediately succeeding the head, is very short; the remainder are large and sub-equal in size. The adult animal is furnished with twelve pairs of walking legs, which, with the exception of the first pair, are alike in size and segmentation. Each consists of five segments, the distal of which is long and terminates with two powerful claws. The proximal segment bears internally a slender, cylindrical process which may be termed the parapod. It has been asserted that the segment bearing this parapod is in reality the second and that the true basal segment or coxa is embedded in the ventral integument. The legs of the first and second pairs never have the parapod, but they are invariably present in the remaining ten pairs. The legs of the first pairare never more than four-jointed; they are always smaller than the others, and are sometimes reduced to mere bud-like processes. They belong to the first segment behind the head. The segment represented by the last tergal plate has no ambulatory limbs; but articulated to its posterior A border is a pair of large, backwardly directed sclerites, which are perforated by the ducts of two spinning glands. These segments are regarded by some authors as the appendages of the last segment, and have been compared to the cercopods of insects. Attached also to the sides of the last segment-in front of the spinning mamilla there is a sub-conical papilla bearing an apical seta arising from a cuplike depression. It has been suggested that these papillae also represent a pair of appendages. In that case the last segment must be double and bear two pairs of appendages. Thus there may be as many as fourteen pairs of trunk append-ages. There are, however, only twelve pairs known to exist with certainty. These are represented by as many segments on the ventral side; but are numerically less by two than the terga. It is not known whether this very unusual phenomenon is to be accounted for by the addition of two supernumerary terga or by the excalation of two pairs of appendages. The legs of the first pair are basally in contact; the rest are separated by a triangular sternal area. At the base of the legs, with the exception of those of the first and last pair, there is a slit-like orifice recalling the coxal sacs of certain Diplopoda (e.g. Lysiopetalum, Platydesmus). In internal anatomy the Symphyla closely resemble the Diplopoda. The alimentary canal is straight and simple, with a pair of " salivary" glands opening into the fore-gut, and a pair of malpighian tubes joining the hind-gut close to its communication with the mid-gut. There is a dorsal heart with segmental ostia and valves, and also a supraneural vessel. The silk glands, which occur in both sexes, are situated as in Lysiopetalum. The generative glands and ducts, which are paired, lie between the alimentary canal above and the normally constructed nerve-cord below, and are accompanied in the male by a pair of seminal vesicles; and the orifice lies ventrally in the third segment behind the head. A peculiarity in which the Symphyla differ from all " tracheate' arthropoda is the presence of a single pair of tracheal tubes opening by a pair of spiracles on the lower surface of the head behind the antennae. The newly hatched young has a smaller number of appendages than the adult, the full complement of legs being reached only after successive moults. The known species of Symphyla Scolopendrella and Scutigerella, which Scolopendrellidae. The chief difference between the two lies in the form of the tergal plates, which in Scolopendrella have the posterior After Latzel, Die Myr. Ost. Ung. Mon. II. Pl. L. 1884. A. Mandibles or jaws of first pair of Scolopendrella; md md 2, first and second segments; t, tendon; c, part of ventral skeleton of head. B. Jaws of second pair; mxl, maxillula; hyp, hypopharynx. C. Jaws of third and fourth pairs; nix, maxilla; p.mx, maxillary palp; lb.mx, maxillary lobes; lb.st, sternal plate of jaw of fourth pair or labium; lb', 1b2, first and second segments of labium. (Figs. A, B, C modified from Hansen, Q.J.M.S., 47, PI, I.) D. Posterior end of body from below; lg", leg of Iith pair: lg12, rudimentary leg of 12th pair of immature specimen; sc, exsertile sac; ent., parapod; pap, sensory papilla; cerc, cercus or spinning sclerite: dl, duct of silkgland; a, anus. a 2, One of the functional legs further enlarged (from Wood Mason), showing the five joints and terminal pair of claws; b, parapod. are referred to two genera, together constitute the family 1 angles produced and angular,whereas in Scutigerella they are rounded. Both genera are widely distributed and are represented, in Europe, South America, Siam, &c. Large specimens reach a limit of between six and seven millimetres. They live in earth, beneath stones, dead leaves or fallen branches, and resemble diminutive centipedes (Scolo- endra or Lithobius) both in appearance and movements. The Symphyla have frequently been compared with the Thysanurous Hexapods, the parapods with their adjacent exsertile vesicles in Scolopendrella being very sirnilar to the abdominal appendages and vesicles of such an insect as Machilis; while the posterior spinning sclerites or cerci of the former bear much resemblance to the cercopods of Japyx. It must be remembered, however, that the spinning glands of certain Diplopods occupy the same position as those of the Symphyla and open upon papilliform processes of the last tergal plate, which are certainly not appendages. Hence, if the papillae are the homologues of the cerci in Scolopendrelia, these cerci cannot be morphologically comparable to the cercopods of Japyx or other insects. But even if the full force of the arguments in favour of relationship between the Symphyla and the Hexapoda be admitted, the Symphyla, nevertheless, differ essentially from the Hexapoda in the anterior position of the generative orifice, and in the presence of twelve pairs of similar ambulatory limbs. (R. I. P.)
End of Article: CLASS SYMPHYLA
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