160 THE ORIGIN OF VERTEBRATES 



indications of a corresponding somatic or mesomeric segmentation. 

 The nerve supply to these segments would consist of — 



1. The epimeral purely sensory nerves to the somatic surface, 

 equivalent in the vertebrate to the ascending root of the trigeminal. 



2. The mixed nerves to the internal branchial segments, equivalent 

 in the vertebrate to the vagus, glossopharyngeal, and facial. 



3. The motor nerves to the somatic muscles, equivalent in the 

 vertebrate to the original nerve- supply to the somatic muscles 

 belonging to these segments, i.e. to the muscles derived from van 

 Wijhe's 4th, 5th, and 6th somites. 



Further, the centres of origin of these appendage-nerves would 

 form centres in the central nervous system separate from the centres 

 of the motor nerves to the somatic muscles, just as the centres of 

 origin of the motor parts of the facial, vagus, and glossopharyngeal 

 nerves form groups of cells quite distinct from the centres for the 

 hypoglossal, abducens, trochlear, and oculomotor nerves. 



In fact, if the vertebrate branchial nerves are looked upon as the 

 descendants of nerves which originally supplied branchial appendages, 

 then every question connected with the branchial segmentation, with 

 the origin and distribution of these nerves, receives a simple and 

 adequate solution — a solution in exact agreement with the conclusion 

 that the vertebrate arose from a pakeostracan ancestor. 



It would, therefore, be natural to expect that the earliest fishes 

 breathed by means of branchial appendages situated internally, and 

 that the evidence for such appendages would be much stronger in 

 them than in more recent fishes. 



Although we know nothing of the nature of the respiratory appa- 

 ratus in the extinct fishes of Silurian times, we have still living, in 

 the shape of Ammoccetes, a possible representative of such types. 

 If, then, we find, as is the case, that the respiratory apparatus of 

 Ammocoetes differs markedly from that of the rest of the fishes, and, 

 indeed, from that of the adult form or Petromyzon, and that that 

 very difference consists in a greater resemblance to internal branchial 

 appendages in the case of Ammoccetes, then we may feel that the 

 proof of the origin of the branchial apparatus of the vertebrate from 

 the internal branchial appendages of the invertebrate has gained 

 enormously. 



