The morphology of Astrobrachion constrictum, a representative of the little known Euryalida, was examined by light and electron microscopy. The tegument is smooth and lacks calcified elements. Many features of A. constrictum were found to conform to the usual pattern of ophiuroid anatomy. The skeletal elements are generally composed of uniform labyrinthic stereom. The radial shields, however, consist of overlapping stereom plates. The 5 oral shields serve as madreporites, each with a U-shaped canal leading into an axial sinus. A 3-lobed axial organ is associated with each madreporite and may play some sensory or secretory role.