172 ARTHUR WILLEY. 



tomical and physiological sense, but in a morphological sense 

 also, because while no zoologist would conclude from it that 

 the corresponding structures in the respective types were mor- 

 phologically diflferent, yet it serves to explain most of those 

 diflferences in detail which Spengel so elaborately enumerates. 



In correlation with the great size of the tongue-bars in the 

 pharynx of the Enteropneusta, it is not surprising to learn 

 the important fact from Spengel that they, rather than the 

 primary bars, are hollow, containing a wide prolongation of 

 the coelom. " In Folge dessen," says Spengel himself (loc. 

 cit., p. 725), " kann die Zunge der Amphioxus-Kieme nicht, 

 wie die der Enteropneusten, zwei Zungenzinken enthalten, 

 sondern uur eine, die allerdings aus zwei gleichen Halften 

 zusammengesetzt erscheint." 



Thus, according to Spengel's own assertion, the presence of 

 two skeletal rods instead of one only, in the tongue-bars of 

 the Euteropneusta, stands in correlation with their hollow 

 character; while the latter, in its turn, is correlated with the 

 great size of the tongue-bars. In consequence of the occur- 

 rence of two separated skeletal rods in the tongue-bars, the 

 dorsal arcuate extremities of the branchial skeletal structures 

 are not continuous as they are in Amphioxus, but are in- 

 terrupted at each tongue-bar (cf. Spengel, loc. cit., Taf. ii, 

 fig. 21). 



The fact that the skeletal rod of the tongue-bar is single in 

 Amphioxus and double in the Euteropneusta is an anatomical 

 diflFerence of importance, but not necessarily and, it may be 

 confidently asserted, not in fact a morphological difference. 

 But it accounts, on the principle of correlation, for other 

 differences upon which Spengel lays such stress. It fully 

 explains the difference which Spengel has had printed in spaced 

 type — namely, that " beim Amphioxus gehort jede Skeletgabel 

 einer einzigen, bei den Enteropneusten aber zwei Kiemen an." 



Before coming to the conclusion that " die Kiemen der 

 Enteropneusten und. des Amphioxus .... wesentlich ver- 

 schiedne, morphologisch einander nicht entsprechende Bild- 

 ungen siud," Spengel makes a serious attack upon the synap- 



