APPENDICULATES AND VERTEBRATES 83 



convergence ; and there are other facts which 

 serve still more to complicate the issue and to 

 baffle the most competent judgment. Chief 

 amongst these is the apparent topographical 

 coincidence between the infundibulum in the 

 floor of the third ventricle of the vertebrate 

 brain and the appendiculate oesophagus. 



If we admit the convergence as tabulated 

 above, why should we not regard this further 

 coincidence as another case of convergence ? 

 There are at least two reasons why I think we 

 should not : firstly, because the structural con- 

 vergence in the two types of central nervous 

 system is accompanied by a certain degree of 

 functional equivalence, whereas the topographical 

 coincidence of infundibulum and oesophagus is 

 not accompanied by any functional equivalence ; 

 secondly, because the infundibulum can be brought 

 into interesting topographical correlation with 

 a primitive feature quite different from the 

 appendiculate oesophagus, namely, the anterior 

 neuropore or anterior neurenteric canal of pro- 

 tochordates. If the striking correspondence in 

 plan of composition of the central nervous 

 system in appendiculates and vertebrates had 

 been recognised as an instance of general 

 structural convergence thirty - five years ago, 

 when the late Dr Anton Dohrn, the founder of 

 the Stazione Zoologica at Naples, explained his 

 application of the principle of change of function 



