Primarily a deep water family. Small disc; long, narrow arms that taper towards the tips. The main characteristic is a pair of longitudinal muscles on the aboral side of each arm from the level of the third to ninth superomarginal to the arm tip. Papulae confined to papularia on the arms or disc. Supero- and inferomarginals alternate and have long spines. The tube feet have suckers. The pedicellariae are pectinate.