262 bulletin: museum of comparative zoology. 



deny its gill-cleft nature (Dohrn), van Wijhe regards it as a visceral 

 cleft on the left side, antimeric to the club-shaped gland, which with 

 Willey he regards as a modified visceral cleft, exactly homologous with 

 the hyomandibular (spiracular) cleft of Crauiota. Van Wijhe ('93, 

 p. 155) finds evidence of a primary unpaired mouth in the external 

 opening of the left anterior entodermic diverticulum known as the pre- 

 oral pit (Eaderorgan). Homologizing the " Gehirnanschwellung " of 

 Amphioxus with the " Gehirnblase " of the larvae of Ascidlans, he con- 

 siders it impossible to homologize the mouth (tremostoma) of Am- 

 phioxus with the median dorsal mouth of Tunicates, since in the former 

 the mouth and its antimere are laid down immediately posterior to the 

 brain vesicle, whereas in the latter the mouth arises in the median 

 plane immediately anterior to the brain vesicle ; however, the visceral 

 clefts of the young Ascidian larva are laid down, like the mouth of 

 Amphioxus, immediately behind the brain vesicle. Moreover, van 

 Wijhe holds that the mouth of Amphioxus is an organ of the left side 

 only, and on the following grounds (quoted from Willey, '94, p. 178) : 

 " The outer muscle of the oral hood represents the anterior continua- 

 tion of the left half only of the transverse and subatrial muscles. 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. 

 The velum is innervated entirely from nerves of the left side," viz. 

 branches from the 4th, 5th, and 6th left dorsal nerves. 



Willey ('94) finds evidence to support his view, that the mouth of 

 Amphioxus represents the median dorsal mouth of Ascidians, in the 

 marked asymmetrical conditions of the larva, for which van Wijhe's 

 observations and conclusions afford no explanation. Affirming the 

 asymmetry to be non-adaptive and non-advantageous {contra Korschelt 

 und Heider), he concludes that it is the mechanical result of the (phy- 

 logenetic) forward extension of the notochord, an extension which is 

 advantageous to an animal which bores in the sand. Hatschek ('92) 

 and M. Fiirbringer ('97) agree with Willey in this explanation as to the 

 homology of the mouth of Amphioxus, but bring forward no evidence to 

 support their view. There is no disagreement in homologizing the an- 

 terior entodermic diverticula (vordere Entodermsackchen) of Amphioxus 

 with at least part of the premandibular head cavities (1st somite of 

 van Wijhe) in Craniota. 



From the foregoing review it will be seen that two very important 

 questions concerning the nature and homologies of the Vertebrate mouth 

 remain in dispute, viz. : — 1. Is or is not the mouth of Amphioxus to be 



