154 MK. 1', J. COLE ON THE STEUCTURE AND MOEPHOLOGT OF 



(2) That this system is innervated by a series of lateral Hne nerves which are un- 

 doubtedly independent of miy of the recognised cranial nerves, and are in fact mori^ho- 

 logically branches of one trunk. 



(3) That the lateral line brancli of the glossopharyngeal belongs to the lateralis 

 lateral line nerve. 



(4) That the lateral sense-organs and nerves are precisely comparable to the auditory 

 organ, with which they should be associated — the lateral and auditory nerves arising from 

 a common centre in the brain which is further peculiar to this system of nerves. It is also 

 possible that the lateral sense organs, together with the auditory organ, in most forms 

 arise from a common sensory anlage from the skin. 



We have seen that the developmental evidence in favour of the metamerism of the 

 ]ateral line system is too conflicting to be made the basis of any discussion, but there is 

 no room for doubt that the sense organs of the body canal are often metameric in the 

 adult, as has been described above. It is hence necessary to enquire into the nature of 

 this regular occiu-rence of the body sense organs. I have previously pointed out that 

 the lateral sense organs primitively belong to the head, and have only secondarily 

 extended on to the trunk. This can be proved both by the development and adult 

 structure of the lateralis canal. Even in young adults of Gadiis the body canal posterior 

 to the shoulder girdle is still very imperfectly formed, and in fact is somewhat difficult 

 to detect in sections. In all known cases of development the body canal grows from 

 before backwards, and its most posterior part is the last to be formed of any of tlie 

 sensory canals. When the sensory canals are quite perfect in the head and have fully 

 reached their adult condition, the posterior section of the body canal is still nothing 

 more than a mere rudiment or anlage. It is hence impossible to regard any condition 

 of the body canal as representing a primitive condition, since it is itself only a secondary 

 structure. The innervation of this canal, however, settles the point. The nerve whicli 

 supplies it is in all fishes the lateralis lateral line nerve, which, as it is not a collector, 

 and is absolutely independent of the metameric nerves, must be held to disprove the 

 essential metamerism of the body canal, and indeed shows that such a condition of 

 this canal is purely superficial. 



Now it is obvious that the independent character of the lateral line system, which is 

 such a characteristic feature of the post-embryonic conaition of existing fishes and 

 amphibians, may be either primitive or acquired. It may be, either that the lateral 

 nerves were originally branches of the various cranial nerves, and that the present 

 apparent relations with the latter are the vestiges of that connection, or that the lateral 

 nerves were primitively independent and have already commenced to fuse with, and to 

 form an essential part of, the true cranial nerves. It seems to me that the latter conclusion, 

 although it was advocated by myself, is being hastily and injudiciously adopted, and 

 that further embryological investigations Avill have to be concluded before we liave 

 sufficient data to arrive at an accurate appreciation' of the subject. The question may 

 be viewed from the thi'ee standpoints of Embryology, Comparative Anatomy, and 

 PalEContology. With regard to the former, in spite of the numerous and bulky memoirs 

 that have been written on the subject by such naturalists as Dohrn, Goette, Balfoui', 



