A class of vermiform marine mollusks covered by a cuticle invested with innumerable calcareous spicules; also known as solenogasters. There are 277 named species. Most species are less than 0.4 in. (10 mm) in length, but a few attain 12 in. (300 mm). The head with a mouth is poorly differentiated; there is a small posterior mantle cavity, or cloaca. Despite their specialized shape (see illus.),Aplacophora retain a primitive molluscan condition in the radula, integument, gonad-pericardium, muscles, and nervous system. There are two distinct subclasses, the creeping neomenioids (Neomeniomorpha) with a narrow foot within a ventral groove, and the burrowing chaetoderms (Chaetodermomorpha) without either a groove or foot, and with a cuticular shield around the mouth. Aplacophora are continental-shelf and deep-sea forms found from subtidal depths to hadal depths (30,000 ft or 9000 m), and in some rare localities may be dominant organisms.