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 
pronounced 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, ¢.¢. 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. 
Tt 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. 
