THE PROSOMATIC SEGMENTS OF AMMOClETES 



II 



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, 



r.Ree.VII 



n L at VII -X 



Me*t. j 



n'.%. n-.TLj. 



Fig. 123.— Diagram showing the Distribution of the Facial 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 palseostracan as Limulus is the longitudinal 



