388 K. T. GUNTHER. 



of the phylum with any other division of the animal kingdouij 

 but the classes within the phylum will require some re- 

 adjustment. 



Our phylogenetic speculations are influenced by the follow- 

 ing facts : 



A. Mollusca typically pass through a free-swimming, 

 bilaterally, symmetrical, "veliger" stage. 



B. In creeping and sessile forms the foot and shell reach 

 their highest development. 



c. In free-swimming marine forms the shell tends to atrophy, 

 and the creeping foot tends to become either a swimming 

 organ or to disappear. 



It may therefore be fairly argued that if a race of Mollusca 

 had always been free swimming, either foot or shell might be 

 absent. 



The Chaetognatha may therefore be fairly regarded as the 

 living adult representatives of the phyletic stage indicated 

 by the veliger larva, and it is from such an ancestor that we 

 conceive the creeping Polyplacophora, the worm-like Aplaco- 

 phora, and the swimming Cephalopoda to have been inde- 

 pendently derived. 



A scheme of classification which would represent this view 

 would be — 

 Phylum.— MOLLUSCA. 



Grrade A. — Nectomalacia or Mollusca natantia. 



Mollusca in which the primitive free-swimming habit 

 has been retained : '' foot '^ circumoral, a propodium 

 developed from paired lateral Anlage. 

 Class 1. Cha)tognatha. 



Without shell. 

 ,, 2. Cephalopoda. 



With shell. 



Grade b. — Herpetomalacia or Mollusca reptantia. 

 Mollusca in which a creeping habit has been developed : 

 foot postoral, a metapodium, developed as an 

 unpaired median structure. 



