THE EVIDENCE OF THE AUDITORY APPARATUS ^8 



J U J 



the surface of the animal, comes phylogenetically to form the lining 

 wall of an internally situated membranous capsule is given by the 

 ontogeny of this capsule, which shows step by step how the sense- 

 organ sinks in and forms a capsule, and finally is entirely removed 

 from the surface except as regards the ductus endolymphaticus. 



Summary. 



The special apparatus for hearing is of a very different character from that 

 for vision or for smell, for its nerve belongs to the infra-infundibular group of 

 nerves, and not to the supra-infundibular, as do those of the other two special 

 senses. Of the five special senses the nerves for touch, taste, and hearing, all 

 belong to the infra-infundibular seg'inental nerve-groups. The invertebrate 

 origin, then, of the vertebrate auditory nerve must be sought for in the infra- 

 oesophageal segmental group of nerves, and not in the supra-cesophageal. 



The organs supplied by the auditory nerve are only partly for the purpose 

 of hearing ; there is always present also an apparatus — the semicircular canals 

 — concerned with equilibration and co-ordination of movements. Such equili- 

 bration org'ans are not confined to the auditory nerve, but in the water-living 

 vertebrates are arranged segmentally along the body, forming the organs of the 

 lateral line in fishes ; the auditory organ is but one of these lateral line organs, 

 which has been specially developed. 



These lateral line organs have been compared to similar segmental organs 

 found in connection with the appendages in worms, especially the respiratory 

 appendages. In accordance with this suggestion we see that they are all 

 innervated from the region of the respiratory nerves — the vag-us, glosso- 

 pharyngeal, and facial — nerves which originally supplied the respiratory 

 appendages of the palaeostracan ancestor. 



The logical conclusion is that the appendages of the Palaeostraca possessed 

 special sense-organs concerned with the perception of special vibrations, 

 especially in the mesosomatic or respiratory region, and that somewhere at the 

 junction of the prosoma and mesosoma, one of these sense-organs was specially 

 developed to form the origin of the vertebrate auditory apparatus. 



Impressed by this reasoning - I made search for some specially striking 

 sense-organ at the base of one of the appendages of Liniulus. at the junction of 

 the prosoma and mesosoma, and was immediately rewarded by the discovery 

 of the extraordinary nature of the flabellum. which revealed itself as an 

 elaborate sense-organ supplied with a nerve out of all proportion to its size. 

 Up to this time no one had the slightest conception that this flabellum was 

 a special sense-organ ; the discovery of its nature was entirely due to the 

 logical following- out of the theory of the origin of vertebrates described in 

 this book. 



The structure of this large sense-organ is comparable with that of the 

 sense-org-ans of the pectens of the scorpion, and of many other organs found 

 on the appendages of various members of the scorpion group, of arachnids and 



