CONVERGENT METAMERISM 77 



presents itself to my mind is this : — The 

 integrity of the gut throughout the Triplo- 

 blastica cannot be assailed without invalidat- 

 ing the continuity of the archenteric cavity or 

 primitive gut throughout the Metazoa ; but this 

 is to strike at the root of the entire fabric of 

 comparative morphology, since the archenteron 

 is as much the palladium of animal morphology 

 as the primitive leaf-member is the root- 

 conception of botany. 



The Triploblastica are composed of two great 

 series representing distinct phyletic types, namely, 

 the Platyhelminthes and the Coelomata. The 

 highest flatworms, the Nemertina, show points 

 of convergence towards Coelomata just as the 

 highest Mollusca, the Cephalopoda, show points 

 of convergence towards Vertebrata. 



Inseparable from the question of the general 

 homology of the gut in coelomate animals is 

 that of the general homology of the coelom or 

 secondary body-cavity itself. It must suffice here 

 to affirm this general homology, again on first 

 principles ; but just as the divisions of the gut are 

 obviously not homologous throughout the coelo- 

 mate series, neither are the subdivisions of the 

 coelom, i.e., the somites, necessarily homologous. 

 A phenomenon analogous to metameric segmenta- 

 tion is that of strobilation, which is definitely 

 known to occur independently in different phyla, 

 e.g., Coelenterata, Platyhelminthes, Annelida. In 



