CRANIAL NERVES OF CHIMERA MONSTROSA. 637 



however, is not a branch of the trunk mentioned above, but arises in close proximity to 

 it, i.e., an appreciable distance in front of and dorsal to the internal origin of the glosso- 

 pharyngeal and vagus. Notwithstanding his discovery of the above important facts, 

 Stiiong still continued to include the lateralis with the vagus, and apparently did not 

 observe that they lent valuable support to the view which I shall now proceed to 

 develop. 



The natural corollary of the above is, that the lateral line nerves form a single 

 complete system, and are not, for example, the representatives of the seven supra- 

 branchial nerves described by Beard. This view was first suggested, I believe, by 

 Pollard (60). He says (p. 528) : — " For convenience it is best to regard the lateral line 

 system in the adult Clarias as one organ sui generis, as much so as the auditory organ, 

 for example, and to think of the nerves merely as branches corresponding to the various 

 branches of the auditory nerve. They all arise from the same region of the brain, 

 though they take different directions." Further, Strong may be considered as implying 

 the same when he denies the segmental character of the lateral line system, and asserts 

 that it cannot be considered a guide to the study of the segmentation of the head. 



The proof that the lateral line system is a distinct formation innervated by a single 

 and specially developed system of nerves, is based upon the following facts : — 



(1) They have a distinct and characteristic development, in so far as they are developed 



from the skin, and sink down into their adult positions, and are not, like the 

 true cranial nerves, developed from the neural crest.* This is insisted on by 

 Pollard, who says :t — " The nerves of the lateral line system develop quite 

 differently from the trigeminal branches. Their ganglia are derived from cells 

 proliferating along certain tracts of the ectoderm, as shown by Beard, 

 Froriep and Kupffer. I have followed the process myself in Gohius. 

 The evidence of comparative anatomy is not less clear as to the distinctness 

 of the lateral line nerves in fish." 



(2) As far as it is known, the development of the sensory canals themselves supports 



this view. Thus Beard (35) describes the supra- and infra- orbital sensory 

 canals as arising from the splitting of a single sense organ. Compare 

 Beard's statements in this connection on p. 115. 



(3) When the lateral sense organs disappear, the whole of the nerves supplying them 



disappear also. This is completely proved by Strong (68), who studied the 

 natural extirpation of the lateral line system which takes place during 

 the metamorphosis of the Amphibian tadpole. Strong found that none 

 whatever of the lateral line nerves persisted in the adult frog. Hence 

 the attempts that have been made to homologise the auricular division 



* The origin of the neural crest itself, as described by Beard (43), obviously does not affect the cogency of my 

 argument. 



t " The Oral Cirri of Siluroids, and the Origin of the Head in Vertebrates," Zool. Jahr., Abth. f. Morph., Bd. 8, 

 p. 397, 1895. 



