234 THE ORIGIN OF VERTEBRATES 
mastication being performed in Limulus and its allies by the muscles 
which move the foot-jaws or gnathites, which are portions of the 
prosomatic appendages specially modified for that purpose, and in the 
vertebrates by the masticatory muscles, which are always innervated 
by the trigeminal or Vth cranial nerve. This comparison implies 
that the motor part of the trigeminal nerve originally supplied the 
prosomatic appendages. 
The investigations of van Wijhe and of all observers since the 
publication of his paper prove that in this trigeminal region, as in the 
vagus region, a double segmentation exists, of which the ventral or 
splanchnic segments, corresponding to the appendages in the inver- 
tebrate, are supplied by the trigeminal nerves, while the dorsal or 
somatic segments, corresponding to the somatic segments in the 
invertebrate, are supplied by the IIIrd or oculomotor and the IVth 
or trochlear nerves—nerves which supply muscles moving the lateral 
eyes. 
In accordance, then, with the evidence afforded by the nerves of 
the branchial segments, it follows that the muscles supplied by the 
motor part of the trigeminal ought originally to have moved the ap- 
pendages belonging to a series of prosomatic segments. On the other 
hand, the eye-muscles ought to have belonged to the body-part of the 
prosomatic segments, and must therefore have been grouped origi- 
nally in a segmental series corresponding to the prosomatic appendages. 
The evidence for and against this conclusion will be the subject 
of consideration in this and the succeeding chapters. At the outset 
it is evident that any such comparison necessitates an accurate know- 
ledge of the number of the prosomatic segments in the Gigantostraca 
and of the nature of the corresponding appendages. 
In all this group of animals, the evidence as to the number of 
segments in either the prosomatic or mesosomatic regions is given 
by— 
1. The number of appendages. 
2. The segmental arrangement of the muscles of the prosoma or 
mesosoma respectively. 
3. The segmental-arrangement of the ccelomic or head-cavities. 
4, The divisions of the central nervous system, or neuromeres, 
together with their outgoing segmental nerves. 
It follows, therefore, that if from any cause the appendages are 
not apparent, as is the case in many fossil remains, or have dwindled 
