FINAL REMARKS 



495 



and grow ventral wards until they fuse in the ventral mid-line (cf m 

 Fig. 168). 



As far, then, as this one single striking similarity between 

 Amphioxus and the Enteropneusta is concerned it necessitates the 

 reversal of dorsal and ventral surfaces to bring the two branchial 

 chambers into harmony. 



In a mud-dwelling animal, like Balanoglossus, which possesses no 

 appendages, no special sense-organs, it seems likely enough that 

 ventral and dorsal may be terms of no particular meaning, and con- 

 sequently what is called ventral in Balanoglossus may correspond to 

 what is dorsal in Amphioxus ; in this way the branchial regions of the 



CN.S 



V.A 



V.A. 



Fig. 168. -Diagram illustrating the Position of the Pleural Folds and 

 Gonads in Ptychodera (A) and Amphioxus (B) respectively. 



Al., alimentary canal ; D.A., dorsal vessel; V.A., ventral vessel; cj., gonads; NC, 

 notochord ; C.N.S., central nervous system. 



two animals may be closely compared. Such comparison, however, 

 immediately upsets the whole argument of the vertebrate nature 

 of Balanoglossus based on the relative position of the central nervous 

 system and gut, for now that part of its nervous system which is 

 looked upon as the central nervous system in Balanoglossus is ventral 

 to the gut, just as in a worm-like animal, and not dorsal to it as 

 in a vertebrate. 



There is absolutely no possibility whatever of making such a 

 detailed comparison between Balanoglossus and any vertebrate, as 

 I have done between a particular kind of arthropod and Ammoccetes. 

 In the latter case not only the topographical anatomy of the organs 

 in the two animals is the same, but the comparison is valid even to 

 microscopical structure. In the former case the origin of almost all 



