442 Julia B. Platt 
of the hyoid. Oddly enough, WiLbdER (’96, page 295) says that he 
has sought in vain for the levator arcus I in Necturus, and 
coneludes that it is »failing«. The muscle is not small, and is both 
described and figured by Mivarr. 
From figs. 5a and 6a, pl. XVII, it is seen that as the stems of 
the external gills bud out, the small band of muscle, derived from 
the mesothelium seen in figs. 1a—1g on the postero-lateral wall 
of each of the three posterior arches, expands into the growing gill 
bud. Ultimately this muscle band, primarily continuous, breaks into 
separate parts, and from it also individual muscle cells migrate 
outwards and find their attachment in the skin.. Thus the three 
systems of gill-tuft muscles arise, dorsally the adductores and 
levatores branchiarum, and ventrally the depressores bran- 
chiarum, supplied by the proper nerve of the arch from which the 
gill-tuft arises, glossopharyngeal or vagus. The first adductor 
branchiarum is composed of muscle elements from the glossopha- 
ryngeal and first vagus arches which unite above the third gill 
cleft, and is supplied by a nerve formed from the union of a prae- 
trematic branch of the vagus with a branch of the glossophar- 
yngeus. 
There are three constrictores arcuum in Necturus. The 
first grows forwards from the mesothelium of the first vagus arch 
near the point where this mesothelium joins the wall of the peri- 
cardium, in the same manner as the muscle cerato-hyoideus in- 
ternus grows forwards from the mesothelium of the glossopharyngeal 
arch. The first constrictor arcuum is supplied by a branch of 
the vagus nerve which comes through the first vagus arch. The 
second and third constrictores arcuum arise as a single muscle 
from the wall of the pericardium in the region where the meso- 
thelium of the second vagus arch unites with the pericardial wall. 
This muscle is innervated by three branches of the vagus. One 
from each of the two vagus arches and also by a branch which 
descends through the trunk posterior to the last branchial cleft, and 
then turns forwards in the ventral gill region. 
The constrictor pharyngis arises from mesothelial tissue 
posterior to the branchial arches, but similarly related to the walls 
of the coelom. 
We thus find that all of the proper branchial muscles (part of 
the hypoglossus musculature is secondarily associated with the 
branchial region) in Necturus, may be traced directly to meso- 
