THE CH^TOGNATHA, OR PRIMITIVE MOLLUSCA. 387 



Polyplacophor-Grastropod-Lamellibranch section, and that 

 the Cephalopoda have never had a creeping ancestor at all ; 

 and, in onr opinion, the Creeping Arcliimollusc itself has 

 been derived from a Swimming Archimollusc from which 

 the Amphineura, Aplacophora, and Cephalopoda have been 

 independently evolved, and of which the Chaetoguatha are 

 the nearest living representatives. 



Several groups of Mollnsca descended from the creeping 

 Archimollusc have again taken to a predatory life among 

 the Necton and Plankton, and have apparently thrown back 

 to the earlier swimming Archimolluscan ancestor in several 

 respects. It is thus that characteristics common to the 

 Chastognatha and the Pteropoda are to be explained. The 

 hereditary taint of asymmetry, however, is never lost ; even 

 in Phyllirhoe the anus and the genital organs remain on one 

 side of the body. 



In the light of the undoubtedly primitive nature of the 

 Cha3tognatha, the negative characteristics of the Aplacophora 

 may well be reconsidered. Pelseneer and others consider 

 that the Polyplacophora present the most archaic characters 

 among the Amphineura, but until we have good evidence to 

 the contrary there is at least equal justification for the view 

 that certain structures in the Aplacophora are in an incipient 

 rather than in a reduced condition. Foot, radula, and shell 

 may never have attained to a higher grade of development 

 among their ancestors than that which they have reached 

 among the living Aplacophora. 



The association of the Cheetognatha with the Mollusca does 

 not throw much light on the position of Rhodope. The 

 absence of heart, shell, radula, and foot are not, in our 

 opinion, insuperable obstacles to including this curious minute 

 form in the Melius can phylum, but the fact of the presence 

 of flame cells opens a vista of new difficulties. 



Phylogeny and Classification. 

 By the inclusion of the Cheetognatha within the Molluscan 

 phylum no new suggestion is indicated as to the connection 



