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 Craniota. 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 (Räderorgan). Homologizing the “ Gehirnanschwellung” of 
Amphioxus with the “Gehirnblase” of the larvae of Ascidians, 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 postertor 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 nérves 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, 5tb, 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 conelndes 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. Fürbringer (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 entodermie diverticula (vordere Entodermsáckchen) of Amphioxus 
with at least part of the premandibular head cavities (Ist somite of 
yan 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 
