a class, or subclass, of arthropods, related to the hexapod insects, from which they differ in having the body made up of numerous similar segments, nearly all of which bear true jointed legs. they have one pair of antennae, three pairs of mouth organs, and numerous trachaae, similar to those of true insects. the larvae, when first hatched, often have but three pairs of legs. see centiped, galleyworm, milliped.