Now, although the auditory apparatus is common to all vertebrates, the lateral line system is not found in any land-dwelling animals; it belongs essentially to the fishes, and is, therefore, an old system so far as concerns the vertebrate group. Its sense-organs are arranged along the lateral line of the fish, and, in addition, on the head-region in three well-marked lines known as the supra-orbital, infra-orbital, and mandibular line systems. These sense-organs lie in the skin in a system of canals, and are innervated by a special nervous system different to that innervating adjacent skin-areas. The great peculiarity of their innervation consists in the fact that their nerves all belong to the branchial system of nerves; no fibres arise in connection with the trigeminal, but all of them in connection with the facial, glossopharyngeal and vagus nerves. In other words, although organs in the skin, their nerve-supply belongs to the lateral nervous system which supplies splanchnic and not somatic segments, a system which, according to the theory advanced in this book, originated in the nerves supplying appendages. The conclusion, therefore, is that in order to obtain some clue as to the origin of the sense-organs of this system in the assumed palæostracan ancestor, we must examine the mesosomatic appendages and see whether they possess any special sense-organs of similar function. Further, considering that the auditory organ is to be regarded as a specially developed member of this system, we must especially look for an exceptionally developed organ in the region supplied by the auditory nerve. The question of the origin of this system of lateral line sense-organs possesses a special interest for all those who attempt to obtain a solution of the origin of vertebrates, for the upholders of the view that the vertebrates have descended from annelids have always {357}found its strongest support in the similarity of two sets of segmental organs found in annelids and vertebrates. On the one hand, great stress was laid upon the similarity of the segmental excretory organs in the two groups of animals, as will be discussed later; on the other, of the similarity of the segmentally arranged lateral sense-organs.