Phylogenetic Considerations 645 



ductive cells. Differentiation of locomotor-perceptive and nutritive types 

 of cells proceeded from this through the movement of the nutritive cells 

 into the interior spaces. Within this central cavity, food was passed 

 from the outermost amoeboid cells for digestion. An anterior-posterior 

 axis then must have appeared as development continued. 



Three phyla, the Porifera, Coelenterata, and Ctenophora, are con- 

 sidered to be but slightly above this grade of construction. 



Origin of Bilaterality. — The next problem is that of the origin of 

 bilaterality from these radially symmetrical animals. Here again we 

 must speculate from the evidence of living, highly specialized forms. The 

 clue as to this possible origin is found among the flatworms. 



The members of one order of the Turbellaria, the Acoela, are ex- 

 tremely simple in their structure. In fact the living representatives are 

 scarcely more advanced than the planula type larva of the coelenterates. 

 The ventral mouth opens into a central mass of cells, but there is no 

 digestive tract. Food is taken in through the mouth, and digestion is 

 performed by the individual cells of the central mass. This central mass 

 of cells represents entoderm and mesoderm and is a condition found also 

 in the embryological stages of the higher flatworms. 



It is usually assumed that forms such as the acoela were able to 

 arise from forms similar to the coelenterates in the following manner : 

 the planuloid form flattened in the oral-aboral axis and there was a shift- 

 ing forward of the nerve centers. This would result in a flattened, bi- 

 laterally symmetrical flatworm such as the acoela. The higher turbel- 

 larians actually pass through these stages in their embryology and then 

 the central mass hollows out to form the archenteron, the anterior nerv- 

 ous system becomes more highly differentiated, and the head is formed. 



Relationships of the Phyla. — In the discussion of the various ani- 

 mal phyla, constant emphasis has been placed on increasing complexity 

 of construction rather than on possible phylogenetic relationships. There 

 are many methods which attempt to relate the various phyla to one 

 another, but all are handicapped by the fact that the connecting, ancestral 

 forms are lost. Those animals available for careful study actually repre- 

 sent end points of eons of evolution ; thus they are totally removed from, 

 and usually unlike, any possible ancestral forms. Often for studies of 

 this sort, larval forms and embryology give valuable clues, but again 

 caution must be exercised against overemphasizing seeming similarities. 



In spite of these difficulties, it is possible to demonstrate relation- 

 ships. At present the most widely accepted theory is that known as the 



