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-oesophageal and infra-oesophageal 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-oesophageal 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 contains 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 arrangement 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.