The Myriapoda are defined as articulate animals in which the head is distinct, and the remainder of the body is divided into nearly similar segments, the thorax exhibiting no clear line of demarcation from the abdomen. There is one pair of antennae, and the number of the legs is always more than eight pairs. Respiration is by tracheae.
In this class - comprising the Centipedes (figs. 169, 170) and the Millepedes - the integument is chitinous, the body is divided into a number of somites provided with articulated appendages, and the nervous and circulatory organs are constructed upon a plan similar to what we have seen in Crustacea and Arachnida. The head is invariably distinct, and there is no marked line of demarcation between the segments of the thorax and those of the abdomen. The body, except in Pauropus, always consists of more than twenty somites, and those which correspond to the abdomen in the Arachnida and Insecta are always provided with locomotive limbs. "The head consists of at least five, and probably of six, coalescent and modified somites; and some of the anterior segments of the body are, in many genera, coalescent, and have their appendages specially modified to subserve prehension" (Huxley). Pauropus has only nine pairs of legs; but, with this exception, eleven pairs of legs is the smallest number known in the order.
The respiratory organs, with one exception (i.e., Pauropus), agree with those of the Insecta and of many of the Arachnida in being "tracheae" - that is to say, tubes, which open upon the surface of the body by minute apertures, or "stigmata," and the walls of which are strengthened by a spirally-coiled filament of chitine. The tracheae may or may not anastomose with one another as they do in Insects.
Fig. 169. - A, Lithobius forficatus, enlarged and viewed from above: an Antennae; f Foot-jaws ; h Head. B, Head of Lithobius Leachii, viewed from below (after Newport) : an Antennae ;f Hooked foot-jaws; l Lower lip, composed of two pieces. C, Head of Lithobius forficatus, viewed from above (after Gervais): an Antenna ; e Eye.
The somites, with the exception of the head and the last abdominal segment, are usually undistinguishable from one another, and each generally bears a single pair of limbs. In some cases, however, each segment appears to be provided with two pairs of appendages (fig. 171). This is really due to the coalescence of the somites in pairs, each apparent segment being in reality composed of two amalgamated somites. This is shown, not only by the bigeminal limbs, but also by the arrangement of the stigmata, which in the normal forms occur on every alternate ring only, whereas in these aberrant forms they are found upon every ring.
The head always bears a pair of jointed antennae, resembling those of many Insects, and behind the antennae there is generally a variable number of simple sessile eyes. In one species (Scutigera) compound faceted eyes are present; and in Pauropus the antennae are bifid, and carry many-jointed appendages, thus differing wholly from the antennae of Insects, and presenting a decided approximation to those of the Crustacea.
The young in some cases, on escaping from the egg, possess nearly all the characters of the parents, except that the number of somites, and consequently of limbs, is always less, and increases at every change of skin ("moult" or "ecdysis"). In most cases, however, there is a species of metamorphosis, the embryo being at first either devoid of locomotive appendages, or possessed of no more than three pairs of legs, thus resembling the true hexapod Insects. It is believed, however, that the legs of these hexapod larvae do not correspond homo-logically with the three pairs of legs proper to adult Insects. In these cases the number of legs proper to the adult is not obtained until after several moults, the entire process being stated to occupy in some species as much as two years, before maturity is reached.
The Myriapoda are divided into three orders - viz., the Chi-lopoda, the Chilognatha, and the Pauropoda, to which a fourth, under the name of Onychophora, must be provisionally added for the reception of the genus Peripatus.