A class of terrestrial, tracheate, oviparous arthropods. Diplopods (millipeds) are largely cryptic in habits, are saprophytic feeders, and are characterized by the development of a compact head with a pair of short, simple, eight-jointed antennae, powerful mandibles, and a subbuccal gnathochilarium formed from embryonic maxillary elements. The body is not differentiated into thorax and abdomen, but is composed of a variable number of similar, cylindrical diplosomites, each of which (except the first two or three) bears two pairs of walking legs (see illus.). The body wall is chitinous, with a thick impregnation of calcium carbonate in the majority of species. Most millipeds are variously adapted for rolling into a closed spiral or nearly perfect sphere when threatened. Typically, each abdominal segment is provided with a pair of glands which effuse a volatile poisonous fluid. Respiration occurs through profuse fine tracheae opening through stigmata near the bases of the legs. The digestive tract is a straight tube, except in oniscomorph species in which it is somewhat coiled. The anal opening is in the last segment and is closed by two tightly fitting anal valves. Ducts from reproductive organs pass through or behind the second pair of legs to external openings.