THE PROSOMATIC SEGMENTS OF AMMOCG@TES 311 
are strictly supplied by the nerve of that segment, and, as already 
described, each segment is as carefully mapped out in its innervation 
as it is in any arthropod appendage. One exception occurs to this 
orderly, symmetrical arrangement : a nerve arises in connection with 
the facial nerve, and passes tailwards throughout the whole of the 
branchial region, giving off a branch to each segment as it passes. 
This nerve (Br. prof., Fig. 123) is known by the name of the ramus 
branchialis profundus of the facial, and its extraordinary course has 
always aroused great curiosity in the minds of vertebrate anatomists. 
Miss Alcock, by the laborious method of following its course through- 
out a complete series of sections, finds that each of the segmental 
branches which is given off, passes into the tubular muscles of that 
segment (Fig. 124). The tubular muscles which belong to the velum, 
Ree. Vil n Lat.Vil-X 
Opth. Sup. ~ = wer 
Mu Z 5) 2: ‘ . a 
a s os Ne = 
0 0 0 0 SOS 
s 0) 
0 
Moext. ' 
n.Hy. a Tky. 
Fic. 123.—D1aGRAM SHOWING THE DISTRIBUTION OF THE Factau NERVE. 
Motor branches, red ; sensory branches, blue. ¢ 
i.e. those belonging to the lower lip-segment and to the hyoid segments, 
receive their innervation from the velar or mandibular nerve, and 
belong, therefore, to the trigeminal, not to the facial, system. 
The evidence presented by these muscles is as follows :— 
In the ancestor of the vertebrate there must have existed a seg- 
mentally arranged set of dorso-ventral muscles of peculiar structure, 
concerned with respiration, and confined to the mesosomatic segments 
and to the last prosomatic segment, yet differing from the other 
dorso-ventral muscles of respiration in their innervation and their 
attachment. 
Interpreting these facts with the aid of my theory of the origin 
of vertebrates, and remembering that the homologue of the vertebrate 
ventral aorta in such a palostracan as Limulus is the longitudinal 
