The generative organs vary in structural details in different centipedes. In the male of Lithobius the testes consist of a single coiled tube lying above the alimentary canal. The slender vas deferens which proceeds from its hinder end divides posteriorly into a right and left branch, embracing the gut and uniting beneath it to form a common chamber or atrium within the genital orifice. The atrium receives the secretion of two pairs of large accessory glands; and a pair of tubes, or vesiculae seminales, open, one on each side, into the divided sperm ducts close to their point of origin above the intestine. The organs of the female are very similar. There is a large median ovary followed by a short oviduct forming a circum-intestinal collar and a common atrium. Into the latter open a pair of short receptacula seminis and the slender duct of two pairs of large accessory glands. There is nothing in the female corresponding to the supra-intestinal vesiculae seminales of the male. In the male of Scolopendra, on the contrary, there are as many as twelve pairs of somewhat sausage-shaped testes, approximated two by two. From each pair proceed two slender ducts which open into a median duct coiled in the posterior third of the body and much expanded in the last three of the leg-bearing segments. The right and left portions of the intestinal ring of the genital duct are unequally developed, and there are no vesiculae seminales, but two pairs of accessory glands communicate with the genital atrium as in Lithobius. In the female Scolopendra the right and left portions of the intestinal collar are also unequally developed, and only a single pair of accessory glands besides the receptacula seminis open into the atrium.

After Heymons, Bibl. Zool, 1901, by permission of E. Nagele.
Fig. 4.—Posterior portion of generative organs of male of Scolopendra (A), of female (B). t, Testes; v.d, vas deferens; ov, ovary; r.s, receptaculum seminis; gl. acc, accessory glands; g.o, generative orifice.

The heart is tubular and lies in the middle dorsal line immediately beneath the integument. It consists of a series of chambers corresponding roughly to the leg-bearing segments, and lies in a blood-sinus formed by a pericardial membrane whence large alary muscles extend to the sides of the body. Each chamber gives off in Scolopendra a pair of fine lateral vessels, and is furnished at its posterior extremity with a pair of orifices by which the blood re-enters the organ from the pericardial space. From the anterior chamber, which lies in the first or second leg-bearing segment, proceed three arteries, a median which runs forwards into the head to supply the brain and other organs, and a lateral which with its fellow of the opposite side forms an oesophageal aortic collar. From the sides of the latter arise vessels to the gnathites, and from its inferior portion an unpaired vessel passes forwards into the head and another backwards above the nerve chord to the posterior end of the body, supplying each segment in its course with a delicate lateral branch. In Scolopendra the chambers of the heart, excepting the first and last, which are small, are subequal in size; but in forms like Scutigera where the terga are very unequal in size a corresponding inequality in the size of the chambers is manifested.

A after Newport, Phil. Trans., 1843.B after Haase, Zool. Beitrage, i. pt. 65, 1884, by permission of J.N. Kern.C after Haase, loc. cit.
A, Anterior extremity ofScolopendra, showing twochambers of the heart (h), theaortic ring (a), the alae cordis(a.m) and a cardiac orifice (o).B, Two segments of Scolopendra,showing the branchingand anastomosing tracheae and aspiracle (sp).
Fig 5.
C, A pair of tufted tracheaeof Scutigera. d, Dorsal plate;t.s, tracheal sac; tr, trachealtubes.

In all centipedes, except Scutigera, respiration is effected by chitinized tracheal tubes which extend with their ramifications throughout the body and open to the exterior by means of spiracles perforating the lateral or pleural membrane of more or fewer of the somites below the edge of the terga. Spiracles are never present upon the anal, genital and last leg-bearing somites, and only rarely, as in Henicops, upon the somite bearing the legs of the first pair. In the majority of cases the spiracles are circular, sigmoid or slit-like orifices, with chitinized rim, leading into a pocket-like integumental infolding, from which emanate numerous small tracheal tubes which soon anastomose to form the main tracheal trunks. In Dacetum, one of the Scolopendridae, there is no pocket-like infolding, the small tracheal tubes opening direct to the exterior on a large subcircular plate where their apertures fuse to form a complicated network. The apertures, as in the case of other genera, are protected by fine hairs; and the tracheae themselves are strengthened by a fine spiral filament. In the Lithobiidae the tracheae do not anastomose; but in Scolopendra and Geophilus the main trunks in each segment fuse transversely with those of the opposite side and also longitudinally with those of the preceding and succeeding segments.

In Scutigera the tracheae differ both in structure and position from those of all other Chilopoda. The spiracles, unpaired and seven in number, open in the median dorsal line. Each leads into a short sac from which five tracheal tubes depend into the pericardial blood-sinus.

Existing Chilopoda may be classified as follows, into five orders referable to two subclasses—

Subclass I.Pleurostigma.
  Order 1  Geophilomorpha.
  Order 2  Scolopendromorpha.
  Order 3  Craterostigmomorpha.
  Order 4  Lithobiomorpha.
Subclass II.Notostigma.
  Order 5  Scutigeromorpha.

Subclass 1, Pleurostigma.—Chilopods furnished with a rich system of branching tracheal tubes, the spiracles of which are paired and open upon the pleural area of more or fewer of the somites. Each leg-bearing somite contains a distinct tergum and sternum, the number of sterna never exceeding that of the terga. Eyes are either preserved or lost; when preserved they are represented either by a single one-lensed ocellus or by an aggregation of such ocelli on each side of the head. The anterior portion of the head, bearing the labrum, is bent sharply downwards and backwards beneath the larger posterior portion lying behind the antennae, so that these appendages, approximated in the middle line, project directly forwards from the margin of the head formed by this retroversion of the labral area. The maxillae are short and have no sensory organ; the palpognaths consist of four segments, and the toxicognaths have their basal segments fused to form a single coxal plate.

Fig. 6.
A, Upper view of anterior extremity in Geophilus. a, Basal segments of antennae. c, Cephalic plate. t.palp, Tergal plate of somite, bearing palpognaths. t.tox, Tergal plate of somite, bearing toxicognaths (tox). t.lg.1, Tergal plate of somite, bearing legs of first pair.
B, Toxicognaths of Scolopendra, showing the large coxal plate and the reduced penultimate and antepenultimate segments.
C, Terminal segment or fang of the same, showing the orifice of the poison gland.
(After Latzel, Die Myr. öst.-ung. Mon. vol. i. “Chilopoda,” Vienna, 1880.)