1911 Encyclopædia Britannica/Millipede

by Reginald Innes Pocock

After Pocock in Max Weber's Zool. Ergebnisse, &c., IV., Pl. XXI., fig. 8, 1894.
Fig. 1.—Spirostreptus vittatus, an Oriental species of the Spirostreptoidea, lateral view, showing the repugnatorial pores on the sides of the segments.
c,	head with eyes and antennae.
tg1,	tergal plate of first segment.
a.tg,	tergal plate of last or anal segment.
a.st,	sternal plate of ditto.
a.v,	anal valve.

MILLIPEDE, the popular name of the best known members of a group of the Arthropoda, scientifically known as Diplopoda, and formerly united with the Chilopoda (see Centipede), the Pauropoda and the Symphyla as an order of the class Myriapoda. This classification, however, has of late years been abandoned on account of the recognition of closer affinity between the Chilopoda (centipedes) and the Hexapoda (insects) than between the Chilopoda and Diplopoda. By modern writers the above mentioned groups of “tracheate” Arthropoda are either regarded as independent classes of this phylum Arthropoda, or associated in two superclasses, the Opisthogonea or Opisthogoneata for the Chilopoda and Hexapoda; and the Prosogonea or Prosogoneata for the Diplopoda, Pauropoda and Symphyla. The structural character upon which these superclasses are based is the position of the generative apertures which open anteriorly in the Prosogonea and posteriorly in the Opisthogonea. Although the Pauropoda and Symphyla are not, strictly speaking, Diplopoda, these three groups of prosogoneate arthropods are here for convenience considered together.

After Silvestri, Ann. Mus. Genova, (2), xvi, figs. 17, 19, 25.

Fig. 2.—The Gnathochilarium or jaws of second pair of various Chilognatha.

A, of Spirostreptus.⁠B, of Julus.⁠C, of Glomeris.

c,	cardo.	⁠	m,	mentum.
st,	stipes.		pm,	promentum.
lg,	linguae.		h,	hypostoma.

CLASS DIPLOPODA.

Structure.—The anterior extremity is provided with a distinct head which by its general form and the nature of its appendages is as sharply marked off from the body as is the case in the Hexapoda. It always bears at least three pairs of appendages, the eyes when present and, in the Oniscomorpha a peculiar sense organ. The inferior edge of the head plate overhangs the mouth and is termed the labrum. The exoskeleton of a typical somite consists of the following elements: a dorsal plate, a ventral plate, and a pleural plate on each side. To the external margin of the ventral plate or sternum is articulated a pair of legs and between the leg and the pleural plate is situated the spiracle of the tracheal system. But the segmentation of the Diplopoda presents two marked peculiarities. The first is the fact that, with the exception of a few of the anterior leg-bearing segments and perhaps one or two of those at the posterior end of the body, a single dorsal plate or tergum with its pleural plates overlies two sternal plates, two pairs of legs and two pairs of spiracles. Hence the segments appear to be double and to be furnished with twice as many legs as is normal in the Arthropoda—a peculiarity which has suggested the term “Diplopod” or “double-footed,” for this group. It is generally believed that each tergal plate results from the coalescence of the terga of two originally distinct adjoining segments; but the same effect would be produced by the enlargement of one of a pair of terga and the complete excalation of the other. It is in favour of the latter view that there is only a single pair, and not two pairs, of stink-glands on each so-called double tergal plate. Unfortunately the history of the development of the segments does not clear up the difficulty since the terga of the double segments are single from the first, and no evidence either of fusion or excalation is supplied. The second of the two peculiarities above-mentioned is the great development of the tergal sclerite as compared with the sternal. Only very rarely (i.e. in Platydesmus) is there a broad sternal area. In the majority of cases the lateral edges of the tergum are bent downwards and inwards towards the mid ventral line; the sternum at the same time is so much reduced that the basal segments of the legs of opposite sides are almost in contact. The pleural plate on each side usually disappears either by suppression or by fusion with the tergum. The sterna with their attached legs often remain free. But quite commonly the coalescence of the skeletal elements is carried to such an extreme that each segment is a solid ring with two pairs of movable appendages. The last segment is differently constructed from the others. It is always limbless, and usually consists of a complete tergal ring, a single sternal plate, and a pair of movable anal valves which are normally closed, but are capable of being opened for the passage of faeces. These anal valves are possibly the homologues of the plural scutes of a normal segment. The appendages are modified as a single pair of antennae, two or three pairs of jaws and a variable number of walking-legs, of which one or more pairs may be transformed into gonopods. The antennae are short and very similar to the legs. They are preoral in position, and usually consist of seven segments, the seventh or distal segment being small, as a rule, and furnished with a sense organ which is probably olfactory or tactile in function. The mandibles or jaws of the first pair are the most anterior of the postoral appendages. They are large, powerful, and usually consist of three or two segments, a basal or cardo, which is sometimes absent, a second or stipes, and a third or mala, the latter being supplied with a strong tooth and pectinate lamellae. In all Diplopods, with the exception of the Pselaphognatha, there are only two pairs of jaws, those of the second pair forming a large plate, the gnathochilarium, which acts as a lower lip. It consists of several distinct sclerites, two external on each side, the proximal known as the cardo, the distal as the stipes, the latter being tipped with one or two lobes (malae) and far exceeding the cardo in size. Between the external plates there is a median proximal plate (mentum) generally of large size and often itself subdivided, and a pair of distal plates (linguae). Behind the base of the gnathochilarium there is a single large transverse plate, the hypostoma. In the Pselaphognatha, the jaws representing the gnathochilarium are differently constructed and an additional pair, the maxillulae, has been recently detected between the gnathochilarium and the mandibles. Behind the gnathochilarium, which from embryological data appears to result from the modification of a single pair of appendages, a legless somite has been detected in some embryos. Possibly the plate referred to above as the hypostoma is its sternal element.

After Voges.
Fig. 3.—Inner view of ventral area of a single segment of Julus, much enlarged to show the structure and arrangement of the tracheal organs. The two pairs of tracheae are seen in situ, the posterior pair overlapping the anterior.
h,	Posterior margin of the body-ring (tergum).	t,	Fine tracheae given off from it.
r,	Anterior border.	ms,	Respiratory muscle attached to tracheal sac.
st,	Tubular chamber of tracheae	m,	Ventral body muscle

The heart is a median dorsal vessel composed of a series of chambers each giving off a pair of arteries and furnished with a pair of orifices or ostia. According to Newport, the anterior chamber lying in the second segment is prolonged into an aortic trunk from which arise three pairs of lateral arteries dipping down on each side of the alimentary canal and uniting beneath it in a common ventral vessel. The heart is enveloped in a delicate pericardial membrane and is supported by lateral alary muscles. The alimentary canal is a simple tube extending usually straight through the body from mouth to anus. Only in the Oniscomorpha is it looped, thus suggesting the origin of this short-bodied group of millipedes from longer, more vermiform ancestors. A pair of so-called salivary glands opens into the fore-gut near its anterior extremity and one or two pairs of malpighian tubes communicate with the hind-gut at its junction with the broad mesenteric portion of the canal. Respiration is effected by means of tracheal tubes which communicate with the exterior by means of spiracles situated just above the bases of the walking limbs. Each spiracle leads into a longer or a shorter pouch whence the tracheae, which are of two kinds, arise. In the majority of the orders the tracheae are tufted, that is to say, they form two bundles of short simple tubules springing from the innermost corners of each pouch. In the Oniscomorpha, however, each pouch gives rise to a number of long tubes which extend through the body and somewhat resemble those of the Chilopoda except that they neither branch nor are extensive. As in the Chilopoda and Hexapoda the tracheae are strengthened and kept expanded by a slender spiral filament.

The ventral nerve cord consists of two strands so closely approximated as to be practically fused, with a small ganglionic enlargement for each pair of legs. Hence in the double segments there are two such ganglia, which in addition to the crural nerve give off on each side a large branching nerve to other organs in the segment. In the Opisthospermophora (Julus, Spirostreptus) and the Oniscomorpha (Glomeris, Sphaerotherium) the ganglia are spaced at equal distances on the cord, but in the Merochaeta (Polydesmus) they are grouped in pairs to correspond to the spacing of the legs. The apodous penultimate and anal segments are innervated from the last ganglion of the cord, as are also the gonopods of the males of the Oniscomorpha. The first (suboesophageal) ganglion of the cord supplies the mandibles and gnathochilarium and is connected by the oesophageal commissures with the bilobed cerebral nerve whence arises the nerves for the eyes, when present, and the antennae.

After G. C. Bourne, J. Linn. Soc. xix., Pl. 29, 1886.

Fig. 4.—Diagram of the nervous and circulatory system of Sphaerotherium obtusum, a South African species of Oniscomorpha.
c, Head.
oc, Eye-cluster.
ant, Antenna.
md, Basal segment of mandible.
tg² and tg¹³, Part of the terga of the second and thirteenth segments.
cb, Cerebral ganglia supplying tr, Tracheal tubes with spiral the eyes and antennae.
oes, Oesophagus, cut through.
sb.gl, Suboesophageal or First ganglion of ventral chain.
gl² and gl²², Second and twenty second ganglia of chain, the posterior nerve of each
 ganglion, lg.n, supplies the leg, the anterior, tr.n, the tracheal sac and other organs.
n.gon, Nerve to gonopods.
tr.s, Tracheal sac.

Eyes are sometimes absent, as in all the genera of Merochaeta and in many genera of other groups, as in Siphonophora, one of the Colobognatha, and several of the Juloidea (Typhloblaniulus). In other cases they are represented by one or two ocelli on each side (Stemmiuloidea); or by a vertical series of ocelli as in the Glomeroidea and Polyzonium amongst the Colobognatha. But in the majority of the orders they are represented by triangular or subspherical aggregations of ocelli recalling in a certain degree those of the Lithobiomorpha amongst the Chilopoda. They are simple in structure and consist externally of a cuticular corneal thickening or lens and internally of a retinular layer of enlarged epidermic cells, the internal or proximal ends of which are continuous with the fibres of the optic nerve. The ovary is unpaired and extends almost the entire length of the body beneath the alimentary canal. The oviducts are sometimes separate tubes (Lysiopetalum), sometimes confluent and divided just before terminating in the two orifices behind the base of the legs of the second pair (Julus). The testes and seminal ducts occupy the same position and extent as the ovary and oviducts. The ducts are sometimes coiled, sometimes divided, sometimes united. The two testes are sometimes united by transverse branches across the middle line, and are sometimes branched posteriorly. They bear short caecal diverticula in which the semen is developed. There, are no accessory glands associated with the generative organs; but in some forms, e.g. Polyxenus, there is a pair of receptacula seminis extending backwards alongside the ovary and opening into the oviduct.

   After Pocock, J. Linn. Soc. xxi., Pl. 25.

Fig. 5.—Gonopods of Trigoniulus andersoni, one of the Opisthospermophora (Spiroboloidea).

 A, Anterior view, and B, lateral views of the apparatus. ant, anterior, and post, posterior portions of the coleopod ensheathing the phallopod, of which the proximal portion, ph, is shown.

 C, Phallopod removed from the coleopod.

The secondary sexual characters of the males are of great taxonomic importance. The seminal ducts, like the oviducts, open behind the legs of the second pair. Associated with them in the Limacomorpha (Glomeridesmus), there is a pair of very long retractile penes. In the Spirostreptoidea and Juloidea the penes are much shorter and have coalesced. Sometimes they are undeveloped (Spiroboloidea). In other cases, the Merochaeta, Oniscomorpha, &c., the ducts traverse the coxae of the legs of the second pair. But in all these groups, with the exception of the Oniscomorpha, semen is transferred from the genital orifices, with or without the aid of the penes, either into the first or second pairs of appendages of the seventh segment which are modified in various ways, and are termed phallopods. When the posterior legs are so modified the anterior are as a rule even more profoundly altered to form a protective sheath, or coleopod, for the phallopod; and as a further precaution the entire apparatus is usually withdrawn within the seventh segment. In the Oniscomorpha the semen is transferred into a pair of receptacles developed upon the coxae of the legs of the last pair, which are chelate. The male appendages that are modified in the above described ways are comprehensively spoken of as gonopods. Other secondary sexual characters, like the stridulating organs of the males of some Oniscomorpha, the suctorial pads on the legs of Spirostreptoidea, the development of angular processes upon the mandible or first tergal plate, or of fine ridges in the gnathochilarium—all of which are concerned in enabling the male to maintain a secure hold upon the female—are of great taxonomic use in distinguishing the genera and species.

The most important glands in the Diplopoda are the repugnatorial or stink-glands, which, except in the Oniscomorpha, Limacomorpha and Ascospermophora, open by pores upon the sides of more or fewer of the segments. They secrete a fluid with an unpleasant odour, breaking up in one case into cyanide of potassium, and are practically the only means of protection, apart from the hard exoskeleton, which Diplopods possess. In some millipedes silk glands also exist and open upon papillae upon the posterior border of the last tergal plate. They are found in the Ascospermophora, Stemmiuloidea and Proterospermophora, and are used for spinning nests for the eggs and protective cases for the young during exuviation.

Classification.—The existing members of the class Diplopoda may be classified as follows:—

Subclass 1. Pselaphognatha.

Order: Penicillata (Polyxenus).

Subclass„ 2. Chilognatha.

Order: Oniscomorpha (Glomeris, Zephronia).

„  Limacomorpha (Glomeridesmus).

„  Colobognatha (Polyzonium, Siphonophora).

„  Ascospermophora (Chordeuma).

„  Proterospermophora (Lysiopetalum).

„  Merochaeta (Polydesmus).

„  Opisthospermophora.

Suborder: Stemmiuloidea (Stemmiulus).

„⁠Spiroboloidea (Spirobolus).

„⁠Spirostreptoidea (Spirostreptus).

„⁠Juloidea (Julus, Nemasoma).

Subclass Pselaphognatha.

Diplopods with the soft integument strengthened by weakly chitinized sclerites and furnished above and on the head with transverse rows of short, stout, somewhat squamiform bristles; laterally, on each side of the principal segments, with a thick tuft of long bristles and with a large, silky, white tuft projecting backwards from the posterior extremity. Mandibles one-jointed. Behind them a pair of small, one-jointed maxillulae, attached to a median membranous “lingua.” Behind the “lingua” and maxillulae, a large, double, transverse plate with a long, external sclerite bearing distally in Polyxenus an inner short-lobate process and an outer long spiny palpiform branch. The latter, however, is absent in Lophoproctus. These sclerites probably represent the gnathochilarium of the Chilognatha, but the homology between the skeletal elements of the jaws in question is not clearly understood. It has been suggested that they represent two pairs of jaws, but embryological proof of this does not exist.

A, after Carpenter, Q.J.M.S. 40, Pl. 28, fig. 1.
B, after Latzel, Die Myr. Ost. Ung. Mon. II., Pl. ii., 1884.

Fig. 6.—Jaws of Polyxenus lagurus.

 A, Jaws of second and third pairs. mxl, maxillula; mx.p, palpiform branch of maxilla; mx.lb, lobate process of maxilla; mx.ext, external plate of maxilla perhaps corresponding to the stipes of the gnathochilarium of the Chilognatha; mx.int, internal plate of maxilla, perhaps corresponding to the mentum and promentum of the gnathochilarium (by Carpenter mx.int is regarded as an appendage posterior to the maxilla); mb, membrane.
 B, Mandibles of Polyxenus lagurus

Order Penicillata (＝Ancyrotricha).

After Bode.

Fig. 7.—Ventral view of Polyxenus lagurus much enlarged, actual length a little over 1/12th of an inch.
a, Position of gener-
 ative openings.

Head large, usually with lateral eyes. Antennae eight-jointed, attached near the middle of the front of the head. On the dorsal side of the body there are eleven segments, simple and compound. The first four of these bear one pair of legs each, the succeeding four two pairs of legs, the ninth segment one pair, making a total of thirteen pairs of legs. The tenth and eleventh or anal segment are legless. There is a narrow sternal area separating the bases of the legs of the two sides. There are no repugnatorial glands. In the male none of the legs are modified as gonopods, but the coxa of each of the legs of the second pair is furnished with a conical penis, which during copulation, it may be supposed, is inserted into the genital orifice of the female, which occupies a corresponding position in that sex. The young when first hatched has only three pairs of legs and five segments. The millipedes of this order are all of small size, measuring at most only a few millimetres in length. The best-known genera are Polyxenus and Lophoproctus, both of which occur in Europe. Other forms have been discovered in the West Indies, North and South America, and Ceylon; and it is probable that the group has an almost cosmopolitan range. They live under stones or the loosened bark of trees. The carboniferous fossil, Palaeocampa, is usually referred to this subclass.

Subclass Chilognatha.

Diplopods with firmly chitinized exoskeleton, sometimes thickly, sometimes sparsely covered with short, simple hairs, but never decorated with tufts or rows of peculiarly modified bristles. Mandibles, two- or three-jointed; maxillulae absent, the jaws of the second pair being represented by the gnathochilarium described above.

Order Oniscomorpha.

Body short and broad, hemispherical in transverse section; convex above, flat below, and capable of being spherically coiled. The exoskeleton of a typical compound segment consists of a vaulted tergum, a pair of free pleural sclerites, two pairs of small tracheal sclerites and two pairs of legs, the latter attached to the ventral membrane, which has no sternal plates. The tergal plates are twelve or thirteen in number, whereof the first is very small, the second enormously expanded laterally, and the last, also enlarged and probably representing at least three segments, extends laterally and posteriorly like a hood over the posterior end of the body without forming a chitinous ring round the anal valves and sternum. In the male the legs of the penultimate pair are sometimes modified as claspers; those of the last pair are always enlarged and prehensile, and bear on their coalesced basal segments a pair of sperm-carrying processes analogous to the phallopods of other groups. Apart from these organs the male has no penis, the seminal ducts perforating the coxae of the legs of the second pair. This order contains two well-marked suborders, the Glomeroidea and the Zephronioidea. The Glomeroidea, comprising the families Glomeridae, Gervaisiidae, Onomeridae, have the antennae approximated on the head, the eyes uniserial and twelve (rarely eleven) tergal plates. To this group belong the common pill-millipedes of Europe and North Africa. In North America the Onomeridae alone are found. The Zephronioidea, with the single family Zephroniidae, have the antennae at the sides of the head, the eyes composed of a spherical cluster of ocelli, and always thirteen tergal plates. This group is common in the tropical and southern continents of the Old World, having representative genera in South Africa, Madagascar, India, Malaysia, Australia and New Zealand. They are much larger forms than the Glomeroidea, large specimens reaching two or three inches in length. In addition to the characters mentioned above the Oniscomorpha differ from all other Diplopods in having long tubular tracheae and the alimentary canal bent upon itself.

After Pocock, in Max Weber's Zool. Ergebnisse, &c., IV., Pl. xx.

Fig. 8.—Sphaeropoeus hercules, a Sumatran species of the Oniscomorpha.

 A, Lateral view of the entire animal. c, head; ant, antenna; tgl¹, tg² and tg¹³, tergal plates of first, second and thirteenth segments; lg, extremities of some of the anterior legs.
 B, Gonopods of the male. gp¹ and gp², anterior and posterior pairs of gonopods, both being chelate claspers; pen, processes arising from the basal segments of the gonopods of the second pair, which act as penes.
 C, Vulvae or genital plates attached to the basal segments of the legs of the second pair in the female. g.o, genital orifice.

Order Limacamorpha

Resembling the Oniscomorpha in the shape and structure of a typical segment, except that the tracheal plates are unrepresented; in the facts that the last tergal plate does not form a complete ring round the anal area, and that the last pair of legs in the male are modified; but differing from them in that the body consists of nineteen or twenty segments, is elongate, and tapers anteriorly and posteriorly, the second and last tergal plates being small; in the presence in the male of a pair of long hairy protrusible penes between the legs of the second and third pairs, and in the structure of the gonopods, which, instead of being chelate, terminate in a slender, tapering tarsal segment. This order contains two families: Zephroniodesmidae (Zephroniodesmus) and Glomeridesmidae (Glomerisdesmus), the former from tropical Asia, the latter from tropical America. The largest of these millipedes reach a length of only about 7 mm. Nothing special is known of their habits.

Order Colobognatha.

Body elongate, capable of being spirally coiled, consisting of a large and indefinite number of segments, each being furnished with a distinct often large sternal area, and with the pleural sclerite or membrane distinct from the tergum. The last tergal plate forms a complete ring round the anal valves. Legs with coxal pouches; those of the seventh segment transformed into gonopods of a very simple type in the male, which is also furnished with a double penis completely or partially confluent with the coxae of the legs of the second pair. Head always small, frequently triangular or piriform, in the latter case the gnathites reduced in size and complexity. Repugnatorial pores present and lateral. The genera of this order are divisible into three families: the Platydesmidae (Platydesmus, Pseudodesmus), Polyzonidae (Polyzonium, Siphonotus), Siphonophoridae (Siphonophora). Of these the Platydesmidae have departed least and the Siphonophoridae most from the typical Diplopod in the structure of the mouth parts. The group is for the most part tropical, one genus only, Polyzonium, extending as far north as Central Europe.

  After Pocock, J. Linn. Soc. xxiv., Pl. 37.

Fig. 9.—Glomeridesmus marmoreus, one of the Limacomorpha.

 A, Lateral view. c, head with antennae; tg¹, tergal plate of first segment; an.tg, tergal plate of last or anal segment.
 B, Lower view of one of the segments. tg, inferior edge of the tergal plate; pl, pleural sclerite; lg¹, basal segment of leg.
 C, Posterior extremity of body. an.tg, tergal plate of anal segment; cop.lg. gonopod or copulatory leg.
 D, Legs of the third pair with extruded penes, pen, in front of them.

Order Ascospermophora.

Body elongate, consisting of from twenty-six to thirty-two segments, but not varying within specific limits; the pleurae coalesced with the terga, the sterna free. More or fewer of the anterior ten pairs of legs may be modified in the males, but no true phallopods are differentiated, the function of seminal receptacles being performed (according to C. Verhoeff) by the exsertile coxal pouches of the two pairs of legs of the eighth segment. The seminal ducts in the male perforate the coxae of the legs of the second pair. There are no repugnatorial pores, and the terga are furnished with three pairs of symmetrically placed hairs or bristles. On the posterior border of the last tergal plate there is a pair of spinning papillae. The millipedes of this order, also called Coelochoeta, are referable to several families: Chordeumidae (Chordeuma), Craspedosomidae (Craspedosoma), Heterochordeumidae (Heterochordeuma), &c. The Heterochordeumidae belong to the Oriental region, extending from India to New Zealand. The others are particularly abundant in genera and species in North and Central America and Europe; but are unknown in Africa, south of the Sahara.

Order Proterospermophora.

Differing from the Ascospermophora in that the number of segments is large and variable; they are furnished with repugnatorial pores, and not with the three pairs of setae. In the males the anterior appendages of the seventh segment are modified as phallopods, and the seminal ducts perforate the coxae of the legs of the second pair.

This order, containing the family Lysiopetalidae (Lysiopetalum), is widely distributed in Europe and North America. Large examples of some of the species, e.g. L. xanthinum, reach a length of 4 or 5 ins.

Order Merochaeta.

Resembling the Proterospermophora in having only the anterior appendages of the seventh segment converted into phallopods and the seminal ducts perforating the coxae of the second legs in the males; but differing essentially in that the sterna are solidly welded to the rest of the exoskeleton of the segments, which are either nineteen or twenty in number, in the absence of eyes and of spinning papillae, and in having six-jointed legs. This order is cosmopolitan in distribution and consists of a very large number of genera which by some authors are referred to the single family Polydesmidae; by others to numerous families. Many species are brightly coloured, and some individuals of the Oriental genus Platyrhachus may reach a length of 5 ins. The segments are usually provided with lateral laminate or tubercular expansions bearing the repugnatorial pores, which are only very rarely absent.

After Pocock, in Max Weber's Zool. Ergebnisse, &c., IV., Pl .xx.

Fig. 10.

Platyrhachus mirandus, a Sumatran species of Polydesmidae, to show the form characteristic of the order Merochaeta.

c, Head.

ant, Antenna.

tg¹, Tergal plate of first body segment.

tg⁷, 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.

Order Opisthospermophora.

Resembling the Proterospermophora in possessing a large and variable number of segments, each of which, with the exception of the last and the anterior four or five, is furnished with a pair of repugnatorial pores, but differing essentially from them in that the posterior pair of appendages of the seventh segment are converted into phallopods, and the anterior into protective coleopods in the male, and that the seminal ducts in this sex do not perforate the coxae of the legs of the second pair but are usually associated with a distinct penis situated immediately behind them. The genera of this order present greater diversity of structure than is found in the other orders and are referred to four suborders, which by some zoologists are erected to ordinal rank, namely, the Stemmiuloidea (Monochaeta); the Spiroboloidea (Anochaeta); the Spirostreptoidea (Diplochaeta); and the Juloidea (Zygochaeta).

In the Stemmiuloidea the sterna are free and the pleurae partially so; the terminal segment of the legs is bisegmented; there are two pairs of spinning papillae on the last tergite; the penis is a single long tube, and the eyes are represented by one or two large lenses on each side of the head. The genus Stemmiulus, constituting the Stemmiulidae, is represented by a few species recorded from the Oriental, Ethiopian and Neotropical regions. In the possession of silk-glands this suborder resembles the Ascospermophora and Proterospermophora, and should perhaps rank as an order apart from the Opisthospermophora.

The Spiroboloidea, containing one family, the Spirobolidae (Spirobolus, Rhinocricus, &c.), have the sterna and pleurae coalesced, the tarsi undivided; no spinning papillae, no penis, the eyes represented by an aggregation of ocelli; and the first five segments each with a single pair of legs, the sixth carrying two pairs. This group attains its maximum of development in the tropics, where species and genera are numerous and specimens of large size, i.e. 6 ins. or over, are met with.

The Spirostreptoidea resemble the Spiroboloidea in many particulars, but the fourth segment is footless, and the fifth has two pairs of limbs; the male has a distinct and double penis, and in both sexes the stipites of the gnathochilarium extend to the proximal end of the mentum, which is relatively small. The distribution of this order, which contains several families: Spirostreptidae (Spirostreptus, Rhynchoproctus), Cambalidae (Cambala, Julomorpha), &c., is practically the same as that of the Spiroboloidea. Specimens over 6 ins. in length are met with in the tropics of Africa and Asia.

The Juloidea differ from the Spirostreptoidea in having the third segment limbless, the first, second and fourth with a single pair of appendages, and the stipites of the gnathochilarium much expanded and meeting for a considerable distance in the middle line behind the very small promentum.

The best marked family of this group is the Julidae, which is widely distributed in the northern hemisphere. Its species and genera (Julus, Pachyiulus) are abundant in Europe. Another European family, the Nemasomidae, is founded for the genus Nemasoma, which is remarkable for having the sterna free.

Habits, &c.—Millipedes are principally cryptozoic, living under stones or logs of wood in damp, secluded localities. They feed almost wholly upon decaying vegetable matter, and drink a considerable quantity of water. Some of the tropical species emerge in numbers from their hiding-places after heavy rains, and crawl over the ground and bushes in search of moisture in broad daylight. Their method of progression over level ground is quite peculiar. The body is held in a straight line and is propelled by a succession of wave-like movements of the legs, which are moved in groups, the groups on the right and left side exactly corresponding. Some forms, e.g. Stemmiulus, have been described as attempting to evade capture by a hopping action caused by vigorous jerking and wriggling of the body. Many of the species are very conspicuously coloured and the association of brilliant colouring with the existence of the nauseous secretion of the repugnatorial glands suggests that the coloration is aposematic or of warning significance.

Copulation between the sexes takes place before oviposition. In the Opisthospermophora the males and females coil together with the ventral surface of the anterior ends of their bodies opposed, the male holding the female securely by the head while the extended phallopods carrying the semen are brought into contact with her genital orifice. In the Polydesmidae pairing is effected in the same way except that the male and female instead of intercoiling remain extended, the male clasping the female with his legs. In the Oniscomorpha the sexes also pair front to front, not head to head, however, but head to tail, so that the gonopods in the anal segment of the male can be applied to the second pair of postoral appendages in the female. Some males of this group, e.g. Sphaerotherium, have a stridulating organ on their posterior gonopods and stridulate when finding the females.

 From Balfour, after Metschnikov.

Fig. 11.—Larva of Strongylosoma Guerinii, one of the Polydesmidae, just hatched.

The method of disposing of the young, which usually have only three pairs of legs at hatching, differs in various groups. In Julus and Polydesmus the female burrows below the surface and makes a subspherical nest of small blocks of earth which are moistened with the salivary secretion and moulded to the proper shape between her jaws and anterior legs. When the receptacle is nearly finished she deposits her eggs in it, and, closing the aperture, leaves the whole to its fate. On the other hand, a female specimen of the South African species, Archispirostreptus erythrocephalus, that lived in the London Zoological Gardens, buried herself, coiled round the eggs, and remained with her young for some time after they were hatched. Again, millipedes, like the Stemmiuloidea and Ascospermophora, which possess silk-glands, spin silken cases for the protection of their eggs. Immature specimens of these groups spin similar silken cases at the time of exuviation; and cases, resembling the nests, are likewise made for purposes of moulting by immature forms of some exotic species of Polydesmidae, e.g. by the tropical African Oxydesmus. There is good reason to think, however, that the animal makes use of its own voided excrement in the formation of these receptacles.

A considerable number of Chilognatha of doubtful systematic position have been recorded from beds of the carboniferous formation. The best known are Acantherpestes and Euphoberia. Specimens referred to existing genera have been discovered in amber beds of Oligocene age.

CLASS PAUROPODA.

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 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 penultimate somite, is limbless. Each somite in front of the penultimate bears a single pair of legs, nine pairs of which are fully developed ambulatory limbs, while those of the first segment are reduced to a pair of bud-like processes.

A and B, after Kenyon, Tufts Coll. Studies, iv., 1895; C, after Hansen, Vid. Meddd., 1901, Pl. VI., fig. 34; D. and E, after Kenyon.

Fig. 12.—Pauropus.

A. Pauropus huxleyi (?). c, head; ant. antenna; tg¹ and tg⁵, first and fifth double tergal plates; lg¹, first walking-leg (＝2nd post cephalic appendage); lg⁹, 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; (tg¹)＝first and second terga; tg⁵,＝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.

After Lubbock.

Fig. 13.—Enlarged view of Pauropus huxleyi, from ventral side.

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 Pauropus, 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.

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.t., malpighian tubule.

B. Female genital organs of Eurypauropus; ov, ovary; ovid, oviduct; rs, receptaculum seminis; go, genital orifice.

C. Male genital organs of Pauropus; t¹ and t², anterior and posterior portions of testes; vd¹, vd², vd³, vasa deferentia; vs.s, vesicula seminalis; cd, common duct; go, genital orifices.

D. Lateral view of Pauropus; c, head; ant. antenna; tg¹, tg⁵, first and fifth tergal plates; a.sg, anal segment; st, lateral bristles; lg.r, rudimentary leg; lg¹ and lg⁹, first and ninth fully formed walking legs; p, penis.

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 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 midand 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.

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 (maxillae) 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 pair are 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 border is a pair of large, backwardly directed sclerites, which are perforated by the ducts of two spinning glands.

After Latzel, Die Myr. Ost. Ung. Mon. II Pl. L. 1884.
A. Mandibles or jaws of first pair of Scolopendrella; md1, and md2, 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; mx, maxilla; p.mx, maxillary palp; lb.mx, maxillary lobes; lb.st, sternal plate of jaw of fourth pair or labium; lb1, lb2, first and second segments of labium. (Figs. A, B, C modified from Hansen, Q.J.M.S., 47. pl. I.)
D. Posterior end of body from below; lg11, leg of 11th pair: lg12, rudimentary leg of 12th pair of immature specimen; sc, exsertile sac; ent., parapod; pap, sensory papilla; cert, cercus or spinning sclerite: dl, duct of silk gland; a, anus.
	Fig.15.

 Fig. 16.—1, Scutigerella sp? highly magnified (slightly modified from Packard); a, spinning sclerites; b, b, legs of first pair; c, antennae.

 2, One of the functional legs further enlarged (from Wood Mason), showing the five joints and terminal pair of claws; b, parapod.

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 appendages. 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 are referred to two genera, Scolopendrella and Scutigerella, which together constitute the family Scolopendrellidae. The chief difference between the two lies in the form of the tergal plates, which in Scolopendrella have the posterior 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 (Scolopendra 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 similar 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 Scolopendrella, 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.)