Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/130

Rh 118 MYRIAPODA placed stigmata only are present on each somite. In the aberrant Scolopendrella possibly a single pair of stigmata only are present on the sides of the head, and in Scutigera there are only seven unpaired stigmata present in the middle dorsal line. In this latter form the tracheae proceeding from the stigmata are very short ; they branch a few times dichotomously, and then terminate in blind enlargements. From these the air is conveyed throughout the body in connexion with the fat bodies and peculiar folds of mem brane as far as into the tarsi, the arrangement thus approach ing somewhat that of the tracheal lungs of Arachnida. Re markable glandular structures provided with ducts opening to the exterior occur in various Myriapoda. In all the Diplopoda there are rows of apertures placed laterally one on each somite on either side of the body, known as foramina repugnatoria, because each acts as the opening of the duct of a gland producing an acrid offensive fluid, which is dis charged by the animal on irritation. In a species of Fontaria, one of the Polydesmidee, as has been recently discovered, this secretion contains a chemical body, probably benzol and petroleum ether, which readily breaks up into prussic acid, and another substance, probably benzaldehyde. The animal thus, when irritated, smells strongly of prussic acid. Similar glands are wanting in most Chilopods, but in Geopkilus Gabrielis there are a series of glands opening to the exterior by means of a series of perforated chitinous plates placed ventrally in the median line, which discharge a red fluid, probably of a similar defensive nature. The foramina repugnatoria of the Diplopods were by early observers mistaken for stigmata, as they correspond in position with the stigmata of the Chilopods. It is worthy of note that, if Moseley s hypothesis that tracheal organs have arisen in evolution as modifications of univer sally scattered cutaneous glands is correct, the lateral fora mina repugnatoria and glands of the Julidse may be after all to some extent homologous with the lateral stigmata and tracheae of the Scolopendridse, the ventral tracheae of the Julidse with the ventral glands of Geophilus Gabrielis, whilst the dorsal stigmata of Scutigera represent the sur vival of part of the dorsal tracheae of an ancestral form, with scattered tracheae like Peripatus. In some derived forms the glands have survived as tracheae in one region of the body, in others in another region, in some certain of them have remained as glands or reverted to that con dition, in others they have developed into enlarged tracheae. The generative organs of the Myriapoda are usually elongate, unpaired, tubular organs, often with paired ducts, always provided with accessory glands, and in the female often with a receptaculum seminis. In the Chilopods the ducts open at the hinder extremity of the body ; in the Diplopods on the ventral surface of the third somite pos terior to the head. The Myriapoda are usually divided into two orders the Diplopoda and the Chilopoda. Of these the former appears to be the most ancient and primitive, as proved by its general structure, and especially by that of its tracheae, and by geological evidence so far as knowledge extends. They may be thus characterized. Order Diplopoda ( = Chilognatha). Myriapoda with bodies circular or semicircular in section ; antennae short, of no more than seven joints ; no append ages acting as poison-claws; each somite in the middle and hinder region of the body bearing two pairs of legs ; a variable small number of anterior somites always bearing single pairs only; stigmata ventral, two pairs to each somite; tracheal organs tuft -shaped with sac -like main tube, not ^branching or anastomosing; laterally -placed repugnatorial glands present ; a single pair of Malpighian tubes present; generative organs opening on the third post-cephalic somite ; larvae at birth provided with only three pairs of functionally active legs. In the genus Julus, the well - known Millepedes, which may be taken as types of this order, the body is nearly cylindrical, slightly flattened beneath, and composed of a series of chitinous rings, one to each somite, which are bevelled off at their posterior borders for a certain part of their breadth so as to fit each within the next succeeding ring (fig. 6). The rings ( &quot; terga &quot;) are not com plete, but interrupted by a narrow in terval corresponding with the ventral median line, which is closed in each ring by a pair of ventral plates (&quot; ster na &quot;) placed one in front of the other. Each of these ventral plates, except in the first four post-oral somites, bears a pair of short jointed legs composed each of five joints and a single terminal claw, the bases of the legs of opposite sides being so closely approximated in the middle line as to be in contact. Just in front of the base of each leg is a simple stigma communicating with a tuft -shaped tracheal organ (fig. 5). There are thus two pairs of legs to all except the most anterior somites, and two pairs of trachese. The mandibles mostly have broad chewing surfaces suited to mastication of vpwtahlp mif Fla 6. Diagrams of the struc- u vegetame mat- ture of recent and certain Pal. ter. The maxilla of the two sides are fused together to form a four-lobed plate acting as an under-lip. There are no poison -claws as in the Chilo pods, but the leg -shaped appendages of the first post - cephalic somite, the tergum of which is in most forms en larged and shield-like, are turned to wards the mouth, and probably assist in the process of feeding. In the males of some forms these appendages are shaped into peculiar short blunt grasp ing hooks, bearing spines on their bases (fig. 7). Of the succeeding three somites one (the third post-cephalic) is devoid of legs, and also of sternal plates ; the other two bear a single pair of legs each only. The remainder of the somites bear each two pairs of legs (hence the term Diplopoda), except the seventh post-oral in the male, on which a complicated paired copulatory organ, formed out of modifications seozoic Diplopoda (after Scud- der). 1, cross section of a recent ; 3, of a Palaeozoic Dip- lopod ; 2, side view of a somite of a recent ; 4, of a Palaeozoic Diplopod. In 1 and 3 the space occupied by the sterna is indicated by fine lines pro jecting within the rings ; it is very narrow in 1, broad in 3. In 2 and 4 the posterior be velled border of the somites lies to the right. In 1 and 2 the foramina repugnatoria are shown. Fio. 7. 1, head and anterior somites of Siphonophora portoricensis (after Koch). 2, diagram of the arrangement of the anterior somites and appendages of the female Julus lorulinensis (original), a, modified tergum of first post-oral somite (dorsal-plate or collum) ; 6, short single appendage of same somite, of four joints and a claw only, turned towards the mouth ; c, single appendage of second somite of five joints and a claw like the remaining appendages ; d, third or generative somite devoid of appendages and sterna, but bearing the gener ative apertures ;, single appendage of fourth somite ; /, g, dual appendages of succeeding somites. 3, hook-like first post-cephalic appendage of male of same attached to its plate of support (= one-half of modified sternum ?). 4, mandible of same. 5, the four-lobed plate formed by the fused single pair of maxilla;. of the sterna and other components of the normal somite, is pre sent. The form of these copulatory organs varies very much in species and genera of Diplopoda, and is of great systematic value. In both male and female Julus the generative ducts open by a pair of apertures on the ventral surface of the third segment, just behind the bases of the second pair of legs, the copulatory organs in the male being without internal connexion with the ducts of the testis. The Julidse, coil themselves up spirally when at rest or when attacked, like Peripatus. The whole of the Diplopods are vegetable