358 COMPARATIVE ZOOLOGY OF CHORDATES 



nerve and the 3rd segment. The blood-vessels in these two 

 arches disappear in the higher vertebrates, but that in the 3rd 

 visceral arch corresponding to the glossopharyngeal nerve 

 and the 4th segment of the body becomes the carotid. 

 Similarly, the systemic blood-vessel corresponds to the 4th 

 visceral arch (5th segment of the body) down which the first 

 branch of the vagus nerve runs. The pulmonary artery 

 corresponds to the 6th visceral arch (7th segment of the body) 

 down which the 3rd branch of the vagus runs. 



The segmentation of the head is now clear, and it may be 

 asked how many segments of the body does the head occupy ? 

 Before this can be answered it is necessary to be clear as to 

 whether " the head " is to be regarded as everything in front 

 of the hindmost part of the skull (occipital arch), or whether it 

 extends as far back as the gill-slits. In point of fact, it is 

 necessary to distinguish between the dorsal or " neural head," 

 and the ventral or " visceral head," for they differ in extent. 

 The hindmost region of the neural head is indicated by the 

 position of the occipital arch of the skull, that of the visceral 

 head by the position of the last visceral arch. It is interesting 

 to find that the number of segments in either kind of " head " 

 varies in different animals. The neural head of Petromyzon 

 occupies 4 segments, that of Scyllium 7, that of Squalus 9, 

 that of Amphibia 6, that of Amniotes probably 8. Similarly, 

 the number of segments in the visceral head varies from 10 

 in Petromyzon to 8 in Scyllium, 9 in Hexanchus, and 10 in 

 Heptanchus, while the number is reduced in land- vertebrates 

 which no longer breathe by gills. In the formation of the 

 neural head, more and more segments of the trunk are incor- 

 porated during evolution. The occipital arch is therefore not 

 formed by the same segment in different groups of vertebrates, 

 but this fact does not affect the homology of the occipital 

 arches. This structure has a representative in the common 

 ancestor of Craniates, whatever segment of the body it may be 

 in. In the more primitive forms the neural head is short, 

 and the occipital arch becomes displaced backwards. The 

 primitive extent of the visceral head is probably about 10 

 segments, for not only is this the number in Petromyzon and 



