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. The structure of the head of centipedes is shown in part in Fig. 12, compare also Fig. 8. It will be seen that it differs much from that of the diplopods, though the mandibles (protomalæ) are homologous; they are divided into a cardo and stipes, thus being at least two-jointed. The second pair of postoral appendages is in centipedes very different from the gnathochilarium of diplopods. As seen in Fig. 12 2, they are separate, cylindrical, fleshy, five-jointed appendages, the maxillary appendages of Newport, which are “connected transversely at their base with a pair of soft appendages” (_c_), the lingua of Newport. The third and fourth pair are foot-jaws, and we have called them _malipedes_, as they have of course no homology with the maxillipedes of Crustacea. The second pair of these malipedes, forming the last pair of mouth-appendages, is the poison-fangs (4), which are intermediate between the malipedes and the feet; Meinert does not allow that these are mouth-appendages. [Illustration: FIG. 12.—Structure of a chilopod. _A_, _Lithobius americanus_, natural size. _B_, under side of head and first two body-segments and legs, enlarged: _ant_, antenna; 1, jaws; 2, first accessory jaw; _c_, lingua; 3, second accessory jaw and palpus; 4, poison-jaw. (Kingsley del.) _C_, side view of head (after Newport): _ep_, epicranium; _l_, frontal plate; _sc_, scute; 1, first leg; _sp_, spiracle. ]