1902 Encyclopedia > Myriapoda

Myriapoda




MYRIAPODA. The Myriapoda or Centipedes are usually treated of in text-books as one of the classes of the group Arthropoda, being associated in that group with the Crustacea, Arachnida, and Insecta as equivalent divi-sions of the animal kingdom. In consequence, however, of recent evidence which points to a community of origin of all the Tracheate forms apart from that of the Crustacea, it is probably more natural to divide the Arthropoda (with the exception of certain minor groups of obscure affinities) into two classes, one consisting of the Crustacea, the other of the Tracheata. If this plan of classification be adopted, the Myriapoda form a sub-class of the Tracheata. It is necessary that the peculiar contrast in structure between

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 " Pro-tracheata," 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—
CRUSTACEA.
f Group— Sub-class—
Class— I Protracheata. Peripatidea.
TRACHEATA. I /"Sub-class—
I Group— J Myriapoda.
\ Eutracheata. 1 Insecta.
\ Arachnida.
Linnaeus included the Myriapoda in his Insecta Aptera, together with the Crustacea and Arachnida. 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 Linnoeus's Insecta into three primary natural classes, creating the Crustacea and retaining as the two others the Arachnida 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 pah-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-cesophageal 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-

FIG. 1.—Large adult example of Peripatus capensis of natural size. (From Moseley.)
bodied animals very like lepidopterous caterpillars in fori) , 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 papillae 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
pair of simple eyes and a pair of antennae 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 a 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 papillae, — 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 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 wdiole 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 Flc" 3
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 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 FIG. 4.—Ventral view of a head of an embryo Peripatus ventral cords an- a^ an advanced stage of development (from Moseley).
f c. . ,'J\ The figure shows the jaws (the modified pair of claws pear at first with- 0f the first pair of appendages) turned in towards, out ganglionic en- the mouth, the dark spot between them, which to-laiirements but in gether with them is enclosed by the wall of the buc-voaliHr r,i!l;mon+ ral cavity. The second post-oral appendages or oral lediiiy i uuiuieiii- papjnaj are seen to be distinctly five-jointed, ary ganglionic
swellings are present on them corresponding with the origins of the nerves for the jaws, oral papillae, and all the legs. They

are connected by fine transverse commissures forming a so - called rope-ladder nerve - system. Visceral nerves are present, with an arrangement similar to that in Chaetopods rather than that found in Arthropoda. A dorsal and a ventral blood-vessel are present. The dorsal vessel is, according to Gaffron, a true muscular heart provided with a pair of dorsally-placed valvular openings on each segment of the body, but without any trace of vessels in connexion with these. It lies as in other Tracheata in a pericardial sinus formed by a horizontally-stretched septum, and especially approaches in its general structure the corresponding organ of Diplopoda.
The ovary in the female is closely similar to that of Lithobius in structure. The animal is viviparous, and the pair of long oviducts function as uterus. In the male the spermatozoa are long and filiform, exactly like those of Lithobius. They are actively mobile, and perform exactly the same movements as those of Lithobius, and, as in that genus, are formed into rod-like spermatophores.
In its embryological development, which cannot here be followed, conclusions as to the affinities of Peripatus with the Tracheates receive strong confirmation. Early embryos of Peripatus bear a remarkable resemblance to those of scorpions, spiders, and Myria-pods, and the mode of formation of the procephalic lobes, nerve-system, and limbs is closely similar. The five-jointing of the limbs of the embryo of Peripatus is remarkably similar to that occurring in embryo spiders, such as Agelena. In one important respect the embryonic history of Peripatus differs most remarkably from that of all other Tracheata, as far as is known. A large, open, slit-like blastopore is, as discovered by Balfour, formed in the very early embryo of P. capensis. This slit closes in its centre, and its ante-rior extremity apparently remains permanently open as the mouth, whilst its posterior region persists as the anus. In the embryos of other Tracheates the only representative of this no doubt ancestral mode of development surviving is the so-called mesoblastic groove.
The species of Peripatus live in moist places, in hollows in de-cayed wood, and under stones and logs. The animals walk with a gait similar to that of caterpillars, with their bodies completely supported from the ground by their legs. When irritated they eject from their oral papillae fine jets of the viscid slime secreted by their slime-glands. These fine jets form networks of fine threads, looking like a spider's web, which cling to the fingers with the tenacity of bird-lime. The New Zealand species is said to catch insects for food by means of these slime - jets. The animals roll themselves up like Julus when quiescent. They appear to be noc-turnal in habits.
The group EUTRACHEATA may be thus defined— Tracheataof various form,usually with completelyjointed appendages; never bearing diffuse stigmata indefinite in number, but with not more than two pairs on each somite ; ventral nerve-cords closely approximated, with well-marked infra-cesophageal ganglion, never united posteriorly dorsad of the anus; no segmental organs present; no blasto-pore formed in the embryo; mouth and anus formed as a stomodaeum and a proctodaaum respectively.





The sub-class MYRIAPODA may be defined as follows—_ Eutracheata with a head distinctly separate from the numerous closely similar posterior somites, with a pair of antennae and two pairs of jaws; with numerous similar jointed walking legs ; Malpighian tubes present.
The Myriapoda, like Peripatus, approach the Annelids in having elongate bodies, either cylindrical or more or less flattened, composed of numerous similar joints or somites. They bear numerous pairs of walking legs on the somites posterior to the head, and in this particular differ most markedly from all insects except some of the most primitive forms, such as the Campodeidx, which approach them by the possession of rudimentary abdominal legs, and with which, as will be described, they are possibly connected to some extent by such a form as Scolopendrella. Some forms (Glomeridee) most remarkably resemble woodlice in shape, and hence Latreille's connexion, already referred to, of the Myriapoda with the Isopodous Crustacea. The head of Myriapoda is very like that of Insecta, and bears a single pair of antennae, the eyes when present, and two pairs of jaws. The first pair of jaws, the stout toothed mandibles, are, as in insects, always devoid of palps, but in nearly all cases they are jointed, a primitive condition not occurring in insects. The second pair, the maxillae, are fused together to form a sort of under-lip. In one aberrant family (Poly-zonidee) the mouth parts are formed into a tubular pricking
st

and sucking apparatus. The body behind the head is composed of distinctly separated similar segments, usually numerous (in Pauropus nine only). There is no division of the body into thoracic and abdominal regions. The ventral nerve-cord stretches the entire length of the body, and is provided with a ganglion more or less distinct for each somite. The first three of its ganglia are fused to-gether. Eyes are wanting in some forms, in others they are present on the sides of the head as simple ocelli or groups of ocelli. In one form only (Scutigera) are they compound and faceted. The structure of the ocelli differs remarkably from that of the simple eyes of spiders, in that the crystalline rods in each ocellus are so placed that their axes lie parallel to the plane of the equator of the simple lens, at right angles to the optic axis of the eye instead of in its direction. They are also not isolated by pigment. Grenadier hence concludes that it is impossible that any definite image can be formed on the rods, so as to convey any impression of it to the retinal cells with which they are in connexion. Hence these eyes must discrimi-nate only variations in intensity of light. In Scutigera the internal structure of the eyes, though closely simulating that of the compound eyes of insects in many details, is in reality very peculiar and different from that in all other Arthropods. The digestive canal is simple and straight, except in Glomeridx, and ends in a terminal anus. At the commencement of that portion which acts as a rectum, and which in the embryo is formed from the proctodeum, two or four long coiled urinary tubes (Malpighian tubes), homo-logous with those of insects, open into it. There is a dorsal tubular heart, divided into a series of chambers correspond-ing with the somites, and, as in insects, contained within a blood sinus (pericardium), formed by a horizontal septum stretched across the body-cavity, and provided with a series of pairs of wing-like muscles [aim cordis), by which the sinus-cavity is dilated. Respiration is effected by means of air-tubes or tracheae as in insects. In the Diplopoda, except-ing the Glomeridx, in which they are ramified as in the Chilopoda, these are of essentially the same structure as in Peripatus, viz., tufts of unbranched tubes with feebly-developed spiral filaments springing from a common short sac-like chamber (fig. 5). Four of these sac-like chambers
Fin. 5.—Inner view of the sterna of a single segment of Julus londinensis, much enlarged to show the structure and arrangement of the tracheal organs (after Voges). The two pairs of tracheae are seen in situ, the posterior pair over-lapping the anterior. ]i, posterior margin of the body-ring (tergum); r, anterior border ; between the two lie the two terga ; st, tubular chamber of tracheae; t, fine trachea; given otf from it; vis, respiratory muscle attached to tracheal sac ; m, ventral body muscle.
open on the ventral surface of the body by means of two pairs of stigmata on each somite. In the Chilopods wide tracheal tubes with well-developed spiral thickening of their lining membrane, springing directly from the stigmata, freely branching, and often anastomosing like those in in-sects, are always present. In all cases single pairs of laterally-

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 Polydesmidx, 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 Geophilus 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 Julidx may be after all to some extent homologous with the lateral stigmata and tracheae of the Scolopendridx, the ventral tracheae of the Julidx 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.


FIG. 6.—Diagrams of the struc-ture of recent and certain Pal-aeozoic 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.
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 (" terga ") 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 ('' ster-na ") 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 tracheae. The mandibles mostly have broad chewing surfaces suited to mastication of vegetable mat-ter. The maxillae 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

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 londiuensis (original), a, modified tergum of first post-oral somite (dorsal-plate or collum); b, 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 ; e, 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 Julidee coil themselves up spirally when at rest or when attacked, like Peripatus. The whole of the Diplopods are vegetable

feeders. In the tropics very large representatives of the Julidse occur, especially species of the genus Spirobolus, which do consider-able damage in gardens by devouring leaves. In the Polyzonidas (fig. 7) the mouth parts are formed into a pricking and sucking beak or tube. In the Polydesmidee the body is semi-cylindrical in section, with the lateral regions of each tergum broadened out into a pair of horizontal projecting plates. In the Palaeozoic Diplopoda, EupAorberia and its allies, the terga bore each six stout projecting spines, forming rows of spines along the body (fig. 6) ; the sterna en-closed one-third of the circuit of the body instead of only an insignificant ventral streak as in the Julidse. In addition to the stigmata there are found on the ven-tral aspect of these fossil forms certain paired organs supposed by Scudder, who conjectures that these ancient Diplopoda may have been amphibious in habits, to have given support to gills. In the Glo-meridse the body is shortened, of twelve or thirteen somites only, and closely simu-lates that of the woodlouse in appearance. The males of Sphmrotherium, a genus of this family, possess a stridulating appara-tus at the hinder end of the body
the first post-cephalic somite bears a single pair of legs, the second post-cephalic a single
Very important from a zoological point of view are the genera Polyxenus and Pau-ropus, the species of both of which are ex-tremely small. Polyxenus (fig. 8) is about one-twelfth of an inch in length, and has fourteen pairs of legs, only the first four FIG. pairs of which are borne on the first four post-cephalic somites. The body is covered with bundles of hairy scales. In Pauropus (fig. 9) only nine pairs of legs are present. The antennae are branched at their tips ;
with a pair of curved claws perforated at their extremities by the
ducts of poison-glands embedded in the claws and their penultimate
joints. The basilar somite may bear posteriorly in addition a pair
v / of walking legs, but this pair is frequently
\ / aborted in the adult animal. The last pair of \ / c legs at the hinder extremity of the body is y elongated and directed backwards in the line f- of the body posteriorly. The generative ducts , open posteriorly beneath the anus.
The Chilopoda are all carnivorous, catching their prey and killing it by means of the poison-claws. Eucorybas crotalus of S. Africa makes with its hind legs a rattling noise like that of the rattlesnake. Arthronomatus longicornis of Europe is phosphorescent in the dark. The
Geophilidm, which are without eyes, have very long worm-like bodies, composed of very numerous segments. They live gregari-ously in moist earth. In Lithobius, the commonest British Cen-tipede, the somites are unequal in size, there being nine larger and six smaller terga and fifteen legs composed each of six joints and a claw. The genus Scutigera and its allies form a remarkable family, Scidigeridse (fig. 12), in which the antennae are bristle-lika and as long as the body, and the legs are extremely long, increasing in length towards the hinder end of the body. The peculiarities
pair of legs, and the remaining somites, except the posterior which bears a single pair, two pairs each. Sir John Lubbock, the dis-coverer of this form, which has many aberrant features besides those described above, referred it to a separate order of Myriapods, Pauropoda.
Order Chilopoda.
Myriapoda mostly of dorso-ventrally compressed form, with long multi-articulate antennae; with the second pair of post - cephalic appendages applied to the mouth as poison-claws; only one sternum and one pair of legs to each somite; stigmata lateral (in Scutigera dorsal); tracheal organs ramified, not tuft - shaped, often anastomosing; generative openings posterior ; larvae born with more than three pairs of functionally active legs.
In Scolopendra (fig. 10) the body is band-like and flattened dorso-ventrally. The terga and sterna are nearly flat broad plates of chitinous material, connected laterally by more flexible softer skin, in which in each somite lies on either side a single stigma. The corresponding large ramified tracheal trunks, which are provided internally with well-developed spiral filaments, are connected on each side by lateral longitudinal anastomosing tubes. The antennae are many-jointed, long, and tapering ; the head is followed by a second compound somite formed of four embryonic somites fused, termed the "basilar somite," which is covered by a single enlarged shield-like tergum. The legs are borne at the lateral margin of the ventral surface, their bases being wide apart; one pair is present to every post-basilar somite. The mandibles are provided with sharp cutting teeth ; the maxillae are fused together in the middle line as in Diplopods. They do not form a plate, but in the centre a small bilobed process only, and bear a pair of soft palps laterally (fig. 11). The basilar somite bears as the first pair of post-cephalic appendages a pair of palp-like legs turned forwards in front of the mouth, and as the second a pair of large powerful limbs, the poison-claws, provided of their eyes and respiratory organs have already been referred to above. Most important is the aberrant genus Scolopendrella, which has lately been shown to have certain marked features indicating alliance to the primitive insects, Thysanura. The species of Scolo-pendrella are minute forms five or six mm. in length, appearing (fig. 12), as indicated by the name; at first sight like small Scolo-pendras (fig. 13). The head and antennae nearly resemble those of Oampodea. The body bears, according to Wood Mason, dorsally fifteen tergites behind the head, the first of these being quite rudi-mentary. Ventrally thirteen corresponding indications of somites only can be detected ; and these bear twelve pairs of functional appendages. Two of the tergites appear to be devoid of sternites and appendages. The first post-cephalic appendages ( = third post-orals) are small and turned towards the mouth ; the eleven following ventrally-indicated somites bear each, besides a pair of functional

ambulatory legs, a second pair of rudimentary appendages lying in-ternally to these latter. The legs are five-jointed and bear each a pair of claws as in Gampodea and Peripatus instead of a single claw as in other Myriapoda (Lithobius sometimes bears a pair on the anal legs only). There is a pair of caudal stylets on the last somite per-forated by the ducts of silk glands. The arrangement of the stigmata is uncertain. Hasse finds only a single pair on the head, Wood Mason and others many pairs on the body so-mites. Peculiar paired segmental organs are present on the ventral surface which may be excretory. It appears not impossible that Scolo-pendrella may have originally pos-sessed two pairs of appendages to each somite, and may thus represent to some extent a transition form be-tween the ancient Diplopoda and the more recent Chilopodous type ; but as the anatomy of Scolopendrella is as yet imperfectly known, and nothing is known as to its embryonic development, its place in classifica-tion must remain for the present an open question. By Kyder a sepa-rate order (Symphyla) has been formed for it, whilst Packard has placed it amongst the Thysanwra notwithstanding its numerous jointed legs, which constitute the most essential distinction of Myria-poda from Insecta.
Remains of representatives of the Ghilopoda have not been found in Palaeozoic strata. The earliest known are Secondary from the Solenhofen slate.
Development of the Myriapoda. —Scolopendra is viviparous like Peripatus. Lithobius lays its eggs loosely amongst earth; they are very hard to find there, and nothing is thus known of its development. The female Geophilus lays her eggs in heaps and watches over them; in the case of _Julus the process is similar. Information as to the develop-ment of the Myriapoda gener-ally is at present very imper-fect. In no case as far as yet known is a blastopore formed as in Peripatus. The first struc-ture to appear in Strongylo-soma (Polydesmidx) is a ventral thickened plate. The append- PlCJ ages are formed in succession from before backwards, and the mouth and anus as stomodaeum and proctodeum. The embryo when hatched (fig. 14) has nine post-cephalic somites, the second of which is without appendages, whilst the first, third, and fourth each bear a functional pair of limbs, the fifth bears two pairs of rudimentary limbs be-neath the larval skin, and the sixth a single pair, which very soon becomes double. The young Strongylosoma is thus provided with three pairs of functionally active legs. The young Julus has a similar number, the third post-cephalic somite being apodous, and it was formerly considered that this fact established a connexion between the Myriapoda and the Insecta, the three pairs of larval legs of the Myriapod being supposed to correspond with the three pairs of legs of insects. Such, however, is not the case; there being no second maxillae in Myriapoda, the first pair of legs in the larva must correspond with the second maxillae of insects ; and even if this could be shown to be incorrect, the three pairs of legs would still not corre-spond with those of insects, because in the young Stron-gylosoma the second and in Julus the third post-cephalic somites are devoid of legs. Moreover, the larvae are only apparently hexapodous, not in reality so. In Pauropus there is a hexapodous larval stage. In the Chilopoda no functional or real hexapodous larval stage occurs, as far as is known.
Bibliography.--Newport, " On the organs of Reproduction and Development
of the Myriapoda," in Phil. Trans., 1841; "On the Nervous and Circulatory
Systems," ibid., 1843 ; Koch, System der Myriapoden, 1847 ; Karsch, "Zur For-
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