The Amphineura are divided into two orders: (1) the Polyplacophora, or Chitons; (2) the Aplacophora, or forms without shells, Neomenia, Chaetoderma and their allies.
- Aplacophora, von Jhering.
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 order Aplacophora is divided into two suborders.
- Aplacophora with a distinct longitudinal ventral groove; bisexual with paired genital glands and no distinct liver.