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56 THE RIG IX OF VERTEBRATES 



as to form a category by themselves. These nerves cannot be classed 

 among the afferent nerves of the skin any more than the nerves of the 

 optic and olfactory apparatus ; they require separate consideration. 

 A very extensive literature has grown up on the subject of this 

 system of lateral line sense-organs and their innervation, the outcome 

 of which is decisively in favour of this system being classed with the 

 sense-organs supplied by the auditory nerve, so that in endeavouring 

 to understand the position of the auditory nerve, we must always 

 bear in mind that any theory as to its origin must apply to the 

 system of lateral line nerves as well. 



Now, although the auditory apparatus is common to all verte- 

 brates, 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 pal^ostracan 

 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 



