The Cranial Nerve Components of Petromyzon. 185 
arch, as compared with that of other vertebrates, is greatly com- 
pressed and pushed backwards, coming to form the obliquely placed 
velum, with its ventral end far caudad. At the same time the first 
gill sae is obliterated; but, since the expansion of the buccal funnel 
while it pushes the pharynx back tends rather to draw the skin 
forward, the eutaneous nerve (r. hyomandibularis) which runs behind 
the spiracle seems to have shifted forward. It would appear that 
this nerve then innervates, in addition to the pit organs, a part of 
the skin which in fishes is supplied by the ramus mandibularis. It 
is, however, possible that some of the ventral twigs of the maxillaris 
innervate this area, since it is impossible to do more than conjeeture 
what area of skin may be called the »mandibular area«. In any 
case, it seems probable that a large part of the mandibularis is 
ineorporated into the maxillaris in Petromyzon, and this eonclusion 
is supported by the presence of a motor component in this nerve. 
The presence or absence of a general cutaneous component in 
the facialis nerve has been a question of great interest for several 
years. This is directly eonneeted with the question of the source of 
eutaneous innervation for the hyoid segment in non-operculated forms 
such as cycelostomes and selachians. In only one animal has a 
general ceutaneous component in the VII‘: root been described; 
namely, Amia, and here Kınasgury (27) was doubtful whether it 
.would prove a constant character. When the ectodermal area of the 
hyoid segment is covered in by an opereulum, and perhaps reduced, 
general cutaneous innervation is not to be expected. In forms without 
an opereulum, however, it is certainly to be expected that the skin 
of this area shall have fibers for general sensibility. The origin and 
course of such fibers becomes of importance for questions of seg- 
mentation and of the evolution of the nervous system of the head. 
The hyomandibularis in P. dorsatus has a large general eutaneous 
component derived from the spinal V '* traet and forming an intimate 
part of the VII!" root. The position of the nerve in ammocoetes 
of this size scarcely shows whether it is pre- or post-trematie, but 
since in its development (P. planeri, 26) it is elearly posttrematie, 
we must regard it as such here. Both in its development as shown 
in Kontzorr’s diagrams and in its relations in the ammocoetes this 
nerve corresponds celosely to the following branchial nerves. It would 
seem clear that this nerve originally innervated the skin of the 
hyoid segment, but that even in Petromyzon the dorsal and lateral 
portions of this area are being encroached upon by the branches of 
