The chief points in which the Aplacophora differ from the Polyplacophora are: (1) they are worm-like in shape; (2) there is no distinct foot, and the mantle bears no shell-valves, but only numerous calcareous spicules; (3) the digestive tube is straight.
The cuticle, in some species very thick, contains numerous spicules which are long, hollow and calcified; they are secreted by epithelial papillae.
The ectoderm behind the ciliated ring develops spicules, and the post-oral region of the larva elongates.
Later the ciliated ring or velum disappears and seven imbricated calcareous plates, made up of flattened spicules, are formed on the dorsal surface.
Slender, tapering behind, with subventral cloacal orifice; thin cuticle without papillae; flattened spicules; no gills.