96 THE ANCESTRY OF VERTEBRATES 



The spinal nerves lack the lateral and dorsal cutaneous- 

 branches, owing to the usurpation of the long trunk-branches 

 of the cranial nerves. A third component present in the head 

 ganglia but absent in those of the trunk is that derived- 

 from the epibranchial placodes. The latter circumstance- 

 can not well be considered as primitive, since the gill-slits 

 are formations of which it may be doubted if they ever- 

 have been present along the whole length of the body; 

 the same holds for the epibranchial placodes. 



3. The dorsal roots are very strong, the ventral roots' 

 either fail or are feebly developed. It is rendered probable, . 

 however, by the comparison of Gnathostomes with Amphi-- 

 oxus and Cyclostomes that the absence of ventral roots is 

 secondary and caused by the disappearance of the myoto- 

 mes which have been crushed out of existence by the- 

 development of the skull. 



4. There is no union of dorsal and ventral roots, as is" 

 the case in the whole trunk in Amphioxus and Petromyzon. 

 This too, however, is a somewhat doubtful point, since we 

 are not sure if the eye muscle nerves are to be compared 

 directly to ventral spinal nerves, while the so-called occi- 

 pital nerves have no dorsal roots, as we shall see below. 



The third segment the simplest head-segment. — The seg- 

 ment of the glossopharyngeus must be considered, no doubt, 

 as the most characteristic head segment. Here we have a 

 typical gill-slit with a prae- and post-trematic branch of the 

 segmental nerve, no myotome and no ventral root. All the 

 other segments show deviations from this simple scheme. 

 In the vagus region several nerves have fused or have been- 

 at least partly "collected" by another nerve in front of them, 

 the primary vagus. In the first or mandibular segnjent, the- 

 fusion of both the gill-slits belonging to it, to form the 

 mouth, and the innervation of the dermal sense-organs of 

 the prostomium by the rami ophthalmici, have caused a very 

 strong development of the trigeminus. The latter circum- 

 stance has exerted its influence upon the facialis also, 

 which innervates the prostomial continuation of the lateral 

 line system. In this and in the development of the eye-muscles 

 and their nerves we have all secondary modifications im- 

 printed upon the first (mandibular) and the second (hyoid)- 

 segment by the neighbourhood of the prostomium. The 

 second segment moreover is distinguished by the presence 

 of the auditory vesicle. Part of the second segmental nerve 



