PHYLOGENY 



Other Protozoa 



Flagellata 



Figure 50. Probable Main Lines of Animal Phylogeny. (Redrawn from Hyman. ) 



as respiratorv organs. Yet some respiration probabK- does occur in the 

 gills. But feeding mav have been the primary function in the gills of true 

 chordates also. Finallv, the notochord is a small structure developed as an 

 outgrowth from the digestive tract into the proboscis. It has also been 

 homologized with the pituitary gland, and so its taxonomic value is doubt- 

 ful. It seems probable that these worms are related to the chordates, but 

 how is not clear. Their relationship to the echinoderms, as indicated by 



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