178 DEVELOPMENT OF AMFEIIOXUS. 



instinct to assume that position, but rather because they cannot 

 help it. It is apparently in consequence of a misunderstanding 

 of this observation that Korschelt and Heider ascribe the larval 

 asymmetry of Amphioxus to the same causes which brought about 

 the asymmetry of the Pleuronectidae. Another, and, as it appears, 

 a still more impossible view, has recently been expressed by van 

 WijHE. According to van Wijhe, the left-sided mouth occupies its 

 normal and primitive position in the larva of Amphioxus, and in 

 that position it represents a gill-slit, whose antimere is the club- 

 shaped gland. Van Wijhe arrived at this view as a result of his 

 very important discoveries as to the musculature and innervation 

 of the adult mouth. These discoveries may be summarised as 

 follows : — 



1. The outer muscle of the oral hood represents the anterior 

 continuation of the left half only of the transverse and subatrial 

 muscles. 



2. The inner nerve-plexus of the oral hood is formed on both 

 sides, exclusively from nerves which arise from the left side of the 

 central nervous system. 



3. The velum is innervated entirely from nerves of the left side. 



From these observations van Wijhe concludes that the mouth of 

 Amphioxus, even in the adult, is essentially an organ of the left 

 side, and is neither homologous with the Ascidian nor with the 

 craniate mouth. 



It would seem, however, that the more obvious and iustifiable 

 interpretation of these facts is that the asymmetrical musculature 

 and innervation described by van Wijhe are merely the partial 

 persistence in the adult of the more complete asymmetry of the 

 larva. 



Van Wijhe's observations, therefore, do not affect the question 

 of the cause of the asymmetry in any degree. 



14. (p. 165.) As first shown by Dohrn, the hypophysis of 

 Ammocoetes first arises from the roof of the stomodceum, from 

 which it is subsequently removed to the dorsal surface of the head 

 by the enormous development of the upper lip. 



15. (p. 169.) The ciliated tracts in the pharynx of Ammo- 

 coetes were first described and figured by Anton Schneider in 



