There are around a dozen known species of Mystacocarida divided between two genera, Derocheilocaris and Ctenocheilocaris. Most are less than 0.5 mm long, although D. ingens reaches 1 mm. The head is marked by a transverse "cephalic constriction" between the origins of the first and second antennae. Mystacocarids are marine interstitial (living between grains of sand) crustaceans found in littoral and sublittoral sands (i.e., in waters along and close to shore) throughout the world's temperate and subtropical seas. Their vermiform (worm-like) body form and small size are evident adaptations to life among sand grains. Mystacocarids are believed to feed by scraping organic material from the surfaces of sand grains with their setose (setae [bristle]- bearing) mouthparts.