SCYLLIUM 47 



lateral-line canal, a pharyngeal branch to the gut, and a branchial 

 branch which divides ; a small branch running in the hyoid 

 arch in front of the first gill-slit, and the main branch running 

 behind the ist gill-slit in the ist branchial arch. The glosso- 

 pharyngeal thus bears the same relations to the ist gill-slit as 

 the facial does to the spiracle. 



There is no ventral root to the fourth segment ; the 

 somite which it would innervate disappears. 



The fifth segment has also lost its myotome and ventral 

 root during development. To each of the remaining gill- 

 slits, 2nd to 5th, there corresponds a branchial nerve, the 

 main branches running behind the slits and pretrematic 

 branches in front of them. These nerves are the dorsal roots 

 of the fifth, sixth, seventh, and eighth segments, which have 

 joined together to form the vagus (No. X). The lateral-line 

 organs in the occipital region of the head and all along the side 

 of the trunk to the tail, are innervated by branches of the vagus. 

 In addition, the vagus sends a visceral branch to the heart and 

 stomach, forming part of the parasympathetic system. 



The ventral roots of the sixth and following segments 

 innervate the myotomes of their segments, and also contribute 

 to a nerve — the hypoglossal — which runs back over the gill- 

 slits, down behind them and forwards beneath them to inner- 

 vate some muscles under the pharynx. 



The ninth is the first segment to have a fully formed mixed 

 spinal nerve. 



Skull. — The skull and all the skeleton is made of cartilage. 



The glossopharyngeal and vagus nerves emerge well in 

 front of the hind end of the skull. The latter therefore 

 occupies a larger number of segments than in Petromyzon, 

 namely seven. The skull encloses the brain completely except 

 for an aperture in its roof. The cranial nerves all emerge 

 through special holes or foramina. The auditory and olfactory 

 capsules are firmly fused on. The notochord disappears in 

 the skull-region, and a definite joint is formed between the 

 hind end of the skull and the front of the vertebral column. 



The jaws are formed by the skeleton of the first or mandi- 

 bular visceral arch, which separates the mouth from the 



