Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/128

Rh 116 MYRIAPODA the remarkable genus Peripatus and all other Tracheata should be clearly indicated in any systematic arrangement. Peripatus, an account of which, in consequence of its im portant relations to the Myriapoda, is given in the present article, has been variously placed by systematists as con stituting a separate class of the Arthropoda, the &quot;Pro- tracheata,&quot; or as worthy of higher or lower rank than indicated by such position. It will be regarded here as representing a special sub-group of the Tracheata the Protracheata as opposed to the remaining Tracheates or Eutracheata thus : /Class I CRUSTACEA. Arthropoda. &amp;lt; /Group Sub-class I Class ProtracJieata. Peripatidea. ^ TRACHEATA. &amp;lt; / Sub-class I Group I Myriapoda. Eutracheata. | Insecta. _ Araehnida. Linnaeus included the Myriapoda in his Insecta Aptera, together with the Crustacea and Araehnida. In 1779 Fabricius first separated the Myriapoda as a distinct order under the name Mitosata, but still retained several separate orders of Crustacea as equivalent. In 1796 Latreille divided the Aptera of Linnaeus into seven orders, one of which was constituted by the Myriapoda (so first named by him), but he included with them the Isopoda. In 1800 Cuvier and Lamarck first separated Linnaeus s Insecta into three primary natural classes, creating the Crustacea and retaining as the two others the Araehnida and Insecta. In 1825 Latreille, finally following Leach (1815), set up the Myriapoda as a fourth class, separating them from the Insecta connected with them. Subsequently the class Myriapoda was a constant source of controversy amongst naturalists, and many attempts were made to overthrow it altogether. Thus the Myriapoda were connected by Macleay with the Insecta, and also by Kirby in 1826, further by Burmeister in 1837, and by Von Siebold with Crustacea in 1848 The PROTRACHEATA may be defined as Tracheata with imperfectly -jointed appendages, and numerous stigmata indefinite in number, scattered in various regions of the body ; the first pair of post-oral appendages only modified to act as jaws ; the second pair rudimentary, bearing the opening of the duct of a slime- gland ; remaining pairs numerous, all alike ; ambulatory legs, each provided with a pair of claws ; no definite infra- oesophageal nerve-ganglion ; ventral nerve-cords imperfectly ganglionated, widely divaricated, united posteriorly dorsad of the rectum ; complicated segmental organs present, open ing at the bases of the legs ; arch-enteron in the embryo formed by invagination ; a wide slit-like blastopore formed in the embryo, which gives rise to the mouth anteriorly and anus posteriorly. The sole representatives of this group of the Tracheata are the seven or eight known species of the genus Peripatus (fig. 1 ). These are soft- - 1- Large adult example of Peripatus capensis of natural size. (From Moseley.) bo licd animals very like lepidopterous caterpillars in form, of a brown or blackish colour, with a series of pairs of short conical legs placed laterally at equal intervals along the entire length of the ventral surface behind the mouth. The legs in advanced embryos show a distinct division into five joints by transverse constrictions, but in the adults this jointing is much obscured. The skin not being protected by chitinous plates, but only by fine papilla armed with chitine, no definite hinge -joints are formed comparable to those so usually present in other Tracheata. The terminal joint of each leg or foot is provided with a pair of curved claws. The num ber of legs present varies in the different species. The head bears a coiled (from Balfour, after Moseley). a, antennae ; c, procephalic lobe ; i, intestine ; o, mouth ; 1, 2, 3, &c., post-oral append ages. pair of simple eyes and a pair of antennas composed of very numer ous joints. The first pair of legs, which in the embryo closely resembles those developed behind it (fig. 2), becomes in the process of development turned in in front of the mouth, and its claws become modified into a pair of sickle - shaped toothed jaws which work against one another in front of the mouth, and are completely enclosed in the adult in a wide buccal cavity. This cavity opens to the exterior ventrally on the under side of the head by the buccal aperture, which is oval in form and is surrounded by tumid lips, and has often been described as the mouth, although the true mouth lies within the buccal cavity underneath the jaws (figs. 3, 4). The second pair of appendages of the em bryo becomes converted in the adult into a pair of short papillse, the oral papillae, which bear at their tips the openings of a pair of large glands secreting a viscid sub stance. Respiration is effected in Peripatus by means of an immense number of small tracheal tufts. Each of these tufts consists of a short tubular chamber or sac, opening at one end, which is narrowed, to the ex terior by a minute simple aperture (stigma) in the cuticle, and provided at its opposite enlarged extremity with a tuft of very fine FIG. 2. Early embryo of air-tubes. In these fine tracheal tubes only Peripatus capensis un- a very faint indication of an imperfect spiral thickening of the chitinous lining mem brane can be detected. The tubes are, with very rare exceptions, unbranched ; they are freely distributed to the various muscles, viscera, &c. These tracheal tufts closely resemble in structure those of the Diplopod Myriapoda, but their disposition differs from that occurring in all other Tracheata. Instead of a definite small number of stigmata only being present, placed in definite positions on the successive somites, an in definite number is present in Peripatus. Certain of these are scattered irregularly over the whole body-surface, whilst others are concentrated more or less thickly in a double row on each side of the dorsal median line, in a correspond ing double ventral row, and FlG - 3- Ventral view of the head of a more f^vfl,. fl, Q ..,4..-:~J ,! advanced embryo of Peripatus capensis (from Balfour, after Moseley). e, eye ; I, thickening of epiblast of procephalic lobe to form supra-oesophageal ganglion ; m, process from procephalic lobe growing over the first post- oral appendage ; o, mouth ; 1 and 2, first and second pairs of post-oral appendages, becoming later the jaws and oral papillae respectively. end into the body cavity and at the other to the exterior at the bases of the legs on their inner or ventral aspects. The nervous sys tem consists of a pair of supra- oeso- phageal ganglia fused together in the middle line, from which arise the ventral cords, which remain widely divaricated throughout the length of the body to its hinder ex tremity, where they unite above the dorsal surface of the rectum. The Fro. 4. Ventral view of a head of an embryo Peripatus ventral cords ar&amp;gt;- a * an a(lvancec l stage of development (from Moseley). The figure shows the jaws (the modified pair of claws of the first pair of appendages) turned in towards out ganghonic en- the mouth, the dark spot between them, which to- largements but in &quot;ether with them is enclosed by the wall of the buc- roilitv rmlinipnr cal cavity. The second post-oral appendages or oral papilla; are seen to be distinctly five-jointed. ary ganghonic swellings are present on them corresponding with the origins of the nerves for the jaws, oral papilla?, and all the legs. They further on the anterior and posterior aspects of the legs and round the bases of the legs. A series of segmental organs is present, a pair for each pair of legs. They are coiled tubes opening at one