THE CRANIAL NERVES AND LATERAL .SENSE ORGANS OF FISHES. 151 



as to the laomology of the vertebrate avicUtory organ are incorrect "*. Although Avers' 

 views as to the innervation of the auditory organ cannot any longer be maintained, it is 

 possible that his latter statement is to some extent sufficiently near the truth to require 

 a reopening of the whole question of the " branchial " or " epiln-anchial " sense organs. 



In his L(cmargi(s j^aper (1892, 68), Evrart supports the view of the innervation of the 

 lateral sense organs taken by Triant and AUis. He does not consider that Beai-d's 

 scheme applies to the adult Elasmobranch, nor does he consider the lateral canal as 

 composed of ontogenetic metanieric sense organs (p. 79). In his paper with Mitchell 

 we find (69, p. 100) : — " But v/hile the sense organs and tubules have a nietamerlo 

 arrcmgement in the trunk, there is no relation between the sense organs and segments In 

 the head region; and, as already pointed out, some portions of the cranial canals, though 

 possessing numerous sense organs, have no tiilnxles connecting them with the exterior. 

 In all the cranial canals, l^cjth dorsal and ventral, there are far more sense or"-ans than 

 segments ; e. g. in the supra-orbital canal there are nearly ninety sense organs, and in the 

 infra-orbital there are over ninety." Pollard (1.892, 161), referring to the relation between 

 the lateral canals and the dermal bones, in which connection we should remember Allis's 

 statements above, says (p. 527) : — " In Clarias it is by no means a rule that pores should 

 open at sutures " ; but on p. 539 we find the absolutely contradictory statement that 

 " as the dermal bones are much reduced in Auchcnaspis the close relationship of pores to 

 sutures, which exists in Clarias, is not seen." Willey (189 i<, 223) makes a somewhat 

 remarkable statement. He says (pp. 41-i5) : " It seems certain that at first the sense 

 organs of the lateral line must have been iuncrv^ated by spinal nerves. This follows 

 both from ti priori considerations and also from the condition in Amphioxus, where the 

 ectoderm of the metapleural folds is innervated by the Rami ciitanel ventrales of the 

 dorsal spinal nerves. Under these circumstances it is necessary to supjiose with Eisig 

 that the lateral line nerve {Ramus lateralis vagi) arose as a collector." I am unacquainted 

 with any evidence in support of this statement. Eisig's views on the subject I have 

 already dealt Avith, but I may mention that his explanation of the lateralis nerve has 

 never been accepted by vertebrate morphologists. 



Bashford Dean (1895, 58) evidently considers the metamerism of the lateral canal a 

 secondary modification. He says (p. 51), after considering the probable phylogeny of 

 the system : — " The sensory cells are no longer scattered evenly along the floor of the 

 canal ; they now occur in metameral masses supplied with a distinct nerve branch, located 

 in the region immediately below the external tubules." Again on p. 52 he remarks : — 

 " The original significance of the lateral line system as yet remains undetermined. 

 As far as can be judged from its development, it appears intimately, if not genetically, 

 related to the sense organs of the head and gill region of the ancestral fish : in resj^onse 

 to special aquatic needs, it may thence have extended fu.rther and further backward along 

 the median line of the trunk, and in its later differentiation acquired its metameral 

 characters." Locy (1895, 130) agrees ndth other observers that the branchial sense 

 organs of Beard and Froriep are not the lateral sense organs, but may perhaps corre.spond 



* When Ayers wrote this he must have forgotten what lie had previously written on n. 213 I 



