234 T M£ ORIGIN OF VERTEBRATES 



mastication being performed in Limulns 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 Illrd 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 



