_sp. c._, spinal cord; _gl._, glandular tissue filling the spinal canal; _sk._, Gegenbaur's skeletogenous cells; _p._, pigment.] {183}SUMMARY. From the close similarity of structure and position between the branchial skeleton of Limulus and of Ammocoetes, as given in the preceding chapter, it logically follows that the branchiæ of Ammocoetes must be homologous with the branchiæ of Limulus. But the respiratory apparatus of Limulus consists of branchial appendages. It follows, therefore, that the branchiæ of Ammocoetes, and consequently of the vertebrates, must have been derived from branchial appendages, and as they are internal, not external, such branchial appendages must have been of the nature of 'sunk-in' branchial appendages. Such internal appendages are characteristic of the scorpion tribe, and of, perhaps, the majority of the Palæostraca, for no external respiratory appendages have been discovered in any of the sea-scorpions. In the vertebrates--and it is especially well shown in Ammocoetes--a double segmentation exists in the head-region, a body or somatic segmentation, and a branchial or splanchnic segmentation, respectively expressed by the terms mesomeric and branchiomeric segmentations. The nerves which supply the latter segments form a very well-marked group (Charles Bell's system of lateral or respiratory nerves) which do not conform to the system of spinal nerves, for they do not arise from separate motor and sensory roots, but are mixed nerves from the very beginning. The system of cranial segmental nerves is older than the spinal system, and cannot, therefore, be derived from it, but can be arranged as a system supplying two segments, somatic and splanchnic, which differ in the following way: Each somatic segment is supplied by two roots, motor and sensory respectively, as in the spinal cord segments, while each splanchnic segment possesses only one root, which is mixed in function. The peculiarities of the grouping of the cranial segmental nerves, which have hitherto been unexplained, immediately receive a straightforward and satisfactory explanation if the splanchnic or branchiomeric segments owe their origin to a system of appendages after the style of those of Limulus. In Limulus and all the Arthropoda, the segmentation is double, being composed of (1) somatic or body-segments, constituting the mesomeric segmentation; (2) appendage-segments, which, seeing that they carry the branchiæ, constitute a branchiomeric segmentation. Similarly to the cranial region of the vertebrate, the nerves which supply the somatic segments arise from separate sensory and motor roots, while the single nerve which supplies each appendage contains all the fibres for the appendage, both motor and sensory.