44 PHYLUM CEPHALOCHORDA. 



the future ventral middle line of the pharyngeal region 

 is on the right side of the body as shown by the first 

 trace of the subpharyngeal vessel, the endostyle and the gill- 

 slits ; whereas in the buccal region later median structures are 

 on the left side. It results from this that, at the so-called 

 metamorphosis when the larva becomes symmetrical, the buccal 

 region of the body and the pharyngeal regions must rotate, so 

 to speak, in opposite directions. 



No satisfactory explanation of this extraordinary larval 

 asymmetry has even been suggested. Though largely rectified 

 in the adult a 'trace of it persists in the slightly asymmetrical 

 position of the olfactory pit, the anus, and the continuity of the 

 cephalic fin with the right side of the preoral hood, and in the 

 innervation of the preoral hood (p. 18). 



