THE EVIDENCE OF THE AUDITORY APPARATUS 383 
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 segmental nerve-groups. The invertebrate 
origin, then, of the vertebrate auditory nerve must be sought for in the infra- 
cesophageal 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 organs 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 vagus, glosso- 
pharyngeal, and facial—nerves which originally supplied the respiratory 
appendages of the paleostracan ancestor. 
The logical conclusion is that the appendages of the Paleostraca 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 Limulus, 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-organs 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 
