The Chitons and their allies were placed under the Gastropoda, as a distinct branch called Isopleura, and for the Acephala de Blainville's name Lamellibranchia was substituted.
In the Chitons or Polyplacophora, on the other hand, the two cavities are separate, and there are independent genital ducts.
Originally the Chitons were placed with the limpets, Patella, in Cuvier's Cyclobranchia, an order of the Gastropoda.
Of Neomenia and Chaetoderma, vermiform animals destitute of shell, with the Chitons, and placed them all in a division of worms which he named Amphineura.
The Amphineura are divided into two orders: (1) the Polyplacophora, or Chitons; (2) the Aplacophora, or forms without shells, Neomenia, Chaetoderma and their allies.
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.
The coelom differs from that of the Chitons in the fact that the cavities of the genital organs are continuous with it, and in the fact that there is only one pair of coelomoducts resembling the renal organs of Chitons, but serving also as genital ducts.
The aorta is not independent as in Chitons, but is a sinus like the other channels of the circulation.
Thus the independent metamerism of certain organs in the Chitons is not primitive but acquired within the group: e.g.