Fig. 10.—Freshly hatched larva of Julus multistriatus? 3 mm. long: a, 5 pairs of rudimentary legs, one pair to a segment.
Sinclair (Heathcote) regards each double segment in the diplopods as not two original segments fused together, nor a single segment bearing two pairs of legs, but as “two complete segments perfect in all particulars, but united by a large dorsal plate which was originally two plates which have been fused together.” (Myriopods, 1895, p. 71.) That the segments were primitively separate is shown, he adds, by the double nature of the circulatory system, the nerve cord, and the first traces of segmentation in the mesoblast. Kenyon believes that from the conditions in pauropods, Lithobius, etc., there are indications of alternate plates (not segments) having disappeared, and of the remaining plates overgrowing the segments behind them, so as to give rise to the anomalous double segments.[[3]]
Fig. 11.—Sixth pair of legs of Polyzonium germanicum, ♀: cs, ventral sacs; cox, coxa; st, sternal plate; sp, spiracle.—After Haase.
Diplopods are also provided with eversible coxal sacs, in position like those of Symphyla and Synaptera; Meinert, Latzel, and also Haase having detected them in several species of Chordeumidæ, Lysiopetalidæ, and Polyzonidæ (Fig. 11). In Lysiopetalum anceps these blood-gills occur in both sexes between the coxæ of the third to sixteenth pair of limbs. In the Diplopods the blood-gills appear to be more or less permanently everted, while in Scolopendrella they are usually retracted within the body (Fig. 15, cg).
Diplopods also differ externally from insects in the genital armature, a complicated apparatus of male claspers and hooks apparently arising from the sternum of the sixth segment and being the modified seventh pair of legs. In myriopods there are no pleural pieces or “pleurites,” so characteristic of winged insects.
Perhaps the most fundamental difference between diplopods and insects is the fact that the paired genital openings of the former are situated not far behind the head between the second and third pair of legs. Both the oviducts and male ejaculatory ducts are paired, with separate openings. The genital glands lie beneath, while in chilopods they lie above the intestine; this, as Korschelt and Heider state, being a more primitive relation, since in Peripatus they also lie above the digestive canal.
The nervous system of diplopods is not only remarkable for the lack of the tendency towards a fusion of the ganglia observable in insects, but for the fact that the double segments are each provided with two ganglia. The brain also is very small in proportion to the ventral cord, the nervous system being in its general appearance somewhat as in caterpillars.
The arrangement of the tracheæ and stigmata is much as in insects, but in the Diplopoda the tracheary system is more primitive than in chilopods, a pair of stigmata and a pair of tracheal bundles occurring in each segment, while the bundles are not connected by anastomosing branches, branched tracheæ only occurring in the Glomeridæ. The tracheæ themselves are without spiral threads (tænidia). It is noteworthy that the tracheæ arise much later than in insects, not appearing until the animal is hatched; in this respect the myriopods approximate Peripatus.
In the Chilopoda also the parts of the head, except the epicranium, are not homologous with those of insects, neither are the mouth-parts, of which there are five pairs.