A class of the Myriapoda. The symphylans, like the pauropods, are tiny, pale, centipedelike creatures that inhabit humus or soil, or live under debris; in general, they live wherever there is sufficient moisture to preclude excessive water loss. They are similar to the Pauropoda and Diplopoda in being progoneate (having reproductive tracts open at anterior end of body) and anamorphic (gradually increasing in complexity of form and function). Each of their mandibles, like those of millipedes, but unlike the simple pauropod mandible, bears a movable gnathal lobe; at the same time their two pairs of maxillae are more reminiscent of the chilopods and lower insects than of the singly maxillate millipedes and pauropods. Additional signal characteristics include the following: The antennae are unbranched and simple; there is 1 pair of spiracles arising in the head and opening into tracheae; there are 12 pairs of legs, 1 pair per body segment; most of the legs have peculiar basal eversible vesicles with associated styli: the tergites number at least 15 and do not form diplotergites; there is a prominent pair of terminal spinnerets.