THE CR.\.NIAL NERVES AND LATERAL SEN.SE ORGANS OF FISHES. 197 



Savi's vesicles [!] and the ear in the majority of Vertebrates) comrauuioate with the 

 surface. Both forms may or may not be entirely enclosed in cartilage or in bone. The 

 semicircular canals of the ear are simply remnants of the canal system of the surface, 

 and although bent into shapes more or less semicircular, they still retain their communi- 

 cation Avith the (morphological) exterior by means of their terminal (/. e. surface) pores 

 or ojaenings in the walls of the utriculo-sacculus, which in its turn often communicates 



with the surface of the head by the endolymphatic duct or surface canal Thus it 



is true that the develoi^ment of the semicircular canals in the ear is due to phylogeuetic 

 and mechanical causes, and is not in the least a response to physiological necessities or 

 requirements, and conversely the canals have no active part in the auditory function. 

 They serve merely as chambers to hold the liquid necessary to tloat the auditory sensory 

 hairs " (p. 308). 



Ewart & Mitchell (1892, 69) remark (p. 100) :— " Restiug on the top of the hillock 

 there is often what Solger terms the ' Cujmlabildung.' This seems to consist of mucin. 

 In some cases we have seen long threads of mucin extending from the hillock into the 

 cupula or across the canal, the threads having frequently leucocytes entangled between 

 them." A similar condition of the cupula is described by Emery (18S0, 66). Willey 

 (189-4, 223) and Bashford Dean (1895, 58) agree that the auditory organ represents a 

 specialization of the lateral line system, and belongs to the same category as the sense 

 organs of the remainder of the lateral line. Locy (1895, 130) goes further, and regards 

 the ear, the nose, and probably the eyes as derived from the lateral line system, and states 

 on p. 51'7 that " there is now general agreement that the ears belong to tlie lateral line 

 series." Again, on p. 579, he says : — " The history of the auditory vesicle in sharks has 

 been worked out beyond this period by Ayers, and it is very clear from its mode of 

 growth that it is directly related to the canal organs of the lateral line." Finally, Miss 

 Piatt (1896, 158) remarks (p. 492): — "The ear has developed from the dorso-lateral 

 thickening in the hyobranchial intersegment, and it will be noticed that although the ear 

 undoubtedly belongs in [?to] the lateral line system, and is in fact the centre of that 

 system, it is not proj)erly a ' branchial sense organ,' as Beard suggests, for this term 

 cannot be acciu^ately applied to sense organs above the epibranchial line." 



The general fact that the auditory organ is a modified portion of the lateral line 

 system must hence be considered established on a secure basis of fact, and it accordingly 

 follows that the lateral system of the ancestral Vertebrate must have existed before 

 differentiated sense organs, and then diverged in at least two directions (omitting 

 mention of the nose and eye, the phylogeny of wdiich is doubtful) — one producing the 

 lateral canals of recent fishes and the other the vertebrate auditory organ. The associa- 

 tion of the two sets of organs is completely justified on the evidence of the innervation 

 alone, since ]\Iayser and Strong have estal:)lished (1) that the lateral line and auditory 

 nerve fibres constitvite a system by themselves, and are quite independent of the other 

 cranial nerves ; (2) that this system arises from a common central origin in the brain, 

 which is farther distinct from the fibres of any of the other cranial nerves. The argument 

 from the nerves is well backed up by the argument from the minute structure and 

 development of the two series of organs, and I have already drawn attention to the fact 



27* 



