THE CRANIAL NERVES AND LATERAL .SENSE ORdANS OE ELSHES. 179 



This tlieoiy, besides being obviously opposed to the facts, is based upon the jjroia5/e 

 mode of development of the "lateralis" nerve of the Lamprey, and assumes that this 

 nerve develoj)s in the Lamprey just as it does in Elasmobrauchs. 



It seems to me, therefore, that there is room for little doubt as to the morphological 

 value of the " lateralis " nerve of Tetroinyzon, since all the known facts of its anatomy 

 point to the conclusion that it Ijelougs to tlie accessory lateral series. First, we know 

 that its roots correspond to those of the accessory lateral system in the higher Teleosts, 

 and that besides its posterior or vagal rootlets it has also an anterior or (trigemino-?) 

 facial root ; second, its fibres are of the same nature, being somatic sensory in function ; 

 and third, it is connected with the spinal nerves in a manner characteristic of the 

 accessory lateral series, and such as to justify Kansoni & Thompson's description of it 

 as a commissural nerve. In these three conditions we recognise the most characteristic 

 features of the accessory lateral systeur of the Teleostean fishes — features which may 

 indeed be described as fully diagnostic of this series of nerves. Finally, the " lateralis " of 

 Fetromyzoii. doubtless does include a few lateral line fibres to the scattered sense organs 

 of the body, but these are probably contained in a small accompanying nerve and would 

 of course be morphologically distinct from the main bulk of the " lateralis." 



I had written the above before noting that Strong (1895, 204) discusses the same 

 question, and arrives at conclusions precisely identical with mine. He brings out some 

 additional points in favour of the view, which practically establisb it as a fact, and 

 which are included in the folloAving quotation (pp. 199-200; see also p. 157): — 

 '•Furthermore the N. lateralis [/. e. of Vctromyzoii\ is formed partly by a recurrent 

 branch from the facialis passing around outside the auditory capsule — a thing which 

 does not occur in the N. lateralis in the higher forms [except in Protopterus]. Again, 

 on comparing the course of the N. lateralis with the arrangement of the pits, it is evident 

 that only a small proportion of them loonlcl he innervated by this nerve, lohich has a 

 position near the mid-dorsal line. When these facts are considered — especially the 

 non-derivation of this nerve from the Acuslicus centre, thus differing from the origin so 

 universal for the N . lateralis in all other forms — it must be regarded as very probable 

 that this nerve does not represent the N. lateralis vagi of higher forms .... What it 

 does represent is probably the R. lateralis trigemini, so-called, of Teleosts — ^a nerve 

 which is formed principally, as we have seen, by a recurrent branch of the facialis, 

 derived from the lobus trigemini [?], and which is reinforced by a branch from the 

 vagus. It would then much more probably innervate the papilhe which are so numerous 

 on the dorsal fin, and which probably correspond to the structures innervated by the 

 so-called R. lateralis trigemini." (Italics mine.) 



P. Gadus compared with other Forms. 



Hyrtl (186G, 105) finds the sensory canals of Lota in a very ciu-ious condition. The 

 lateral or body canals open terminally on to the surface bebind, as also do the supra- 

 orbital canals in front, and these are the 0}ily communications between the sensory canals 

 and the exterior («. e. two on each side) ! These openings are situated on fine papilla-. 



SECOND SERIES. — ZOOLOGY, VOL. VII. 25 



