356 THE ORIGIN 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 paleostracan 
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 
