THE CRANIAL NEEVES AND LATERAL SENSE ORGANS OF FISHES. 155 



Van Wijhe, and Beard, much has yet to be done. From an embvyological standpoint, 

 the first alternative above is still a possible, if not a probable, explanation of the facts, 

 and imless the evidence from Embryology is to be rejected altogether, which is out of the 

 question, it is necessary to wait until further investigation from this standpoint 

 definitely vipholds one view or the other. Whatever result is finally arrived at, it 

 should be one in which both embryologists and anatomists concur, and it seems to me 

 that this result will most probably be one that is adverse to the metamerism of the 

 lateral organs. 



Comparative anatomy helps us but little. It is true that in CUmcBra, which shows 

 us the innervation of the lateral sense organs in its simplest known condition, all the 

 lateral nerves, except the external mandibular, arise separately from the brain, and are 

 not in any way connected with the true cranial nerves. It is also true that as we go 

 higher in the vertebrate scale we get every possible gradation between this comparatively 

 independent condition and the very complex one found in the highly specialised recent 

 Teleosteans — where the mingling between the lateral and cranial nerves is at its 

 maximum. It is now a question of whether it is permissible to argue from this that 

 the primitive condition must have been that in which the nerves were concentrated 

 and independent, and therefore not metameric, since the cartilaginous fishes are simpler 

 than the Teleostean forms, and fossil Ichthyology teUs us that they have departed less 

 from the primitive type. Such a contention of course accepts as granted that the 

 soft parts have advanced pari passu with the specialisation of the skeleton. It is 

 evidence of perhaps little or doubtful value, but it may, I think, be used to confirm a 

 view already resting on a more solid basis of fact. 



From Palaeontology we learn still less, and it is most valuable when it is able to 

 throw light on the ancestry of a type that we have other reasons to believe is 

 primitive. It shows us also that the lateral line system is an extremely archaic 

 structure, and further that as regards the geograpliy of the sensory canals it has 

 remained in a largely unmodified condition for untold ages. What bearing this has 

 on the innervation of the system it would be hazardous to conjecture. 



Briefly, the question at issue between embryologists and anatomists is whether 

 the lateral line system is metameric or not. The most valuable evidence that has 

 been advanced from the latter point of view is that in which the lateral nerves have 

 been microscopically traced both to their central origin and peripheral distribution. 

 Considering this evidence, and pending agreement among embryologists, I take up the 

 provisional position (which, however, I believe to be a very strong one) that it is not. 

 I shall therefore in the present communication describe the lateral nerves as purely 

 independent structures having no connection, other than purely secondary, with the 

 true cranial nerves. 



The following description of the innervation of the lateral canals of Gadus is based 

 partly on dissections and partly on an examination of serial transverse sections of young 

 adult Gadus virens (="(?. carbouarms"). 



SECOND SERIES. — ZOOLOGY, VOL. VII. 22 



