FINAL REMARKS 495 
and grow ventralwards until they fuse in the ventral mid-line (cf. 
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 
B ‘4 C.N.S. 
Fic. 168.--D1aGRAM ILLUSTRATING THE PosITION oF THE PLEURAL FoLps anp 
Gonaps IN PrycHopEerRa (A) anp AmpHioxus (B) RESPECTIVELY. 
Al., alimentary canal; D.A., dorsal vessel; V.A., ventral vessel; g., gonads; NC., 
apiodhords 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 
