CRANIAL NERVES AND SENSE ORGANS OF FISHES. 231 



is naturally a much debated point. That cannot, however, 

 be said to affect the cardinal fact that the vagus is a com- 

 pound nerve. Each of the four branchial divisions of the 

 vagus of a typical Elasmobranch fish is concerned primarily 

 with the innervation of the gill arches immediately before 

 and behind it, and consists of a general cutaneous, " visceral" 

 and pre- and post-branchial branches. Further, I have 

 shown that in Chimcera, and also largely in Torpedo, each 

 branchial nerve arises separately from the brain. It 

 would be impossible to find nerves more identical both in 

 their central origin and peripheral distribution than the 

 seventh, ninth, and the various branchial divisions of the 

 vagus. If the first two form separate cranial nerves, then 

 the four branchial divisions of the vagus have a perfectly 

 equal right to be ranked with them. And yet the seventh 

 and ninth are considered to be distinct cranial nerves, 

 whilst the last four are lumped together as the vagus. 

 That this was originally due to an error of observation 

 there cannot be the least doubt ; but that does not 

 palliate the inconsistency that whilst the error of obser- 

 vation has been corrected, the error of deduction, about 

 which there is no practical difficulty, has been allowed to 

 stand. 



The vertebrate head has undoubtedly been produced 

 largely by the development of two series of structures, 

 neither of which series nor their homologues have been 

 found outside vertebrata. These structures are the gill 

 clefts and the lateral sense organs. I use the latter term 

 to include both the epibranchial sense organs (or branchial 

 sense organs) and the lateral sense organs sensu stricto. 

 The gill clefts have given us possibly the nose cleft, and 

 undoubtedly the mouth and the Eustachian tube ; whilst 

 the two series of sense organs have produced probably the 

 nasal organ, possibly the paired eyes (but the phylogeny 



