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
