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160 THE ORIGIN OF VERTEBRATES 
indications of a corresponding somatic or mesomeric segiventatton. 
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, 7.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 paleostracan 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. 
Tf, then, we find, as is the case, that the respiratory apparatus of 
Ammoceetes 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. 
