ON THE DIFLOCHORDA. 333 



aud other suitable coiulitious, arc capable of more elaborate 

 diftcrcntiatiou to suit a more complex euviroumeut tliau those 

 which are limited in the number of their morphological units, 

 i.e. segments in this case. 



The same environmental factors acting on the raetameric- 

 ally and the archimerically segmented animals produce a close 

 homoplastic resemblance in the two groups of organisms. 

 Thus the metamerically segmented Arthropoda, aud even 

 Vertebrata, show a grouping of segments into three regions, 

 the head, thorax, and abdomen, which in the general plan of 

 their several form and functions have a close resemblance 

 to the protomere, mesomere, and metamere of the Archi- 

 coelomata. 



On the other hand, in the parallel evolution of metameric 

 segmentation in the Eu- chorda, the tracing of archimeric 

 segments in them is fraught with interest. 



