REPORT ON THE PTEROPODA. 



69 



Halopsyche is the most specialised genus of all the Gymnosomatous Pteropoda ; it 

 has lost, besides every vestige of a branchial apparatus, the cephalic hood and the 

 proboscis. 



These statements may be represented in the following diagram, which shows the 

 mutual relations of the Gymnosomata. This phylogenetic tree only exhibits, of 

 course, the grouping of the living forms according to their reciprocal affinities, and does 

 not at all suggest their mutual descent, which is quite impossible among cotemporary 

 organisms. 



Olionc Halopsyche 



Notobranchsm 



Spongiobranct 



Pnewrumoderma 

 ClionopsU 



Demobranchma 



