The Chitonidae and the Aplacophora are now separated from the Gastropoda and raised to the rank of a distinct class, under the name of Amphineura.
Among the Amphineura we find one pair of coelomic ducts in the Aplacophora, two pairs in the Chitons.
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.
- Aplacophora without distinct ventral groove, with single median unisexual gonad, with differentiated hepatic sac, and with cloacal chamber furnished with two bipectinate gills.
According to Paul Pelseneer the Polyplacophora are the most archaic, the Aplacophora being specialized in (1) the great reduction of the foot, (2) the disappearance of the shell (Cryptoplax among the Polyplacophora showing both reductions in progress), (3) the disappearance of the radula.
And although embryology seems to prove that the Neomeniomorphs are derived from forms with a series of shell-valves, nevertheless it seems probable that the calcareous spicules which alone are present in adult Aplacophora preceded the solid shell in evolution.
On this view then the Aplacophora are more primitive than the Polyplacophora in the relations of coelom, gonad and coelomoducts; and the genital ducts of the Chitons have arisen either by metameric repetition within the group, or by the gradual loss of an original connexion between the generative sac and the renal tube, as in Lamellibranchs and Gastropods, the generative sac acquiring a separate duct and opening to the exterior on each side.