152 THE ORIGIN OF VERTEBRATES 



respiratory in function. Of these two groups, I will consider the 

 latter group first. 



In Limulus the great characteristic of the branchial region is its 

 oronounced segmental arrangement, each pair of branchial appendages 

 belonging to a separate segment. This group of segments forms the 

 mesosoma, and these branchial appendages are the mesosomatic 

 appendages. Anterior to them are the segments of the prosoma, 

 which bear the prosomatic or locomotor appendages. The latter are 

 provided at their base with gnathites or masticating apparatus, so 

 that the prosomatic group of nerves, like the trigeminal group in the 

 vertebrate, comprises essentially the nerves subserving the important 

 function of mastication. As already pointed out, the brain-region 

 of the vertebrate is comparable to the supra-cesophageal and infra- 

 cesophageal ganglia of the invertebrate, and it has been shown (p. 54) 

 how. by a process of concentration and cephalization, the foremost 

 region of the infra-cesophageal ganglia becomes the prosomatic region, 

 and is directly comparable to the trigeminal region in the vertebrate ; 

 while the hindermost region is formed from the concentration of 

 the mesosomatic ganglia, and is directly comparable to the medulla 

 oblongata, i.e. to the vagus region of the vertebrate brain. 



As far, then, as concerns the centres of origin of these two groups 

 of nerves and their exits from the central nervous system, they are 

 markedly homologous in the two groups of animals. 



Comparison of the Cranial and Spinal Segmental Nerves. 



It has often been held that the arrangements of the vertebrate 

 nervous system differ from those of other segmented animals in one 

 important particular. The characteristic of the vertebrate is the 

 origin of every segmental nerve from two roots, of which one con- 

 tains the efferent fibres, while the other possesses a sensory ganglion, 

 and contains only afferent fibres. This arrangement, which is found 

 along the whole spinal cord of all vertebrates, is not found in the 

 segmental nerves of the invertebrates ; and as it is supposed that the 

 simpler arrangement of the spinal cord was the primitive arrange- 

 ment from which the vertebrate central nervous system was built up, 

 it is often concluded that the animal from which the vertebrate arose 

 must have possessed a series of nerve-segments, from each of which 

 there arose bilaterally ventral (efferent) and dorsal (afferent) roots. 



