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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.