THE EVIDENCE OF THE OLFACTORY APPARATUS 231 
gives a satisfactory reason for its appearance in the lowest verte- 
brates as an unpaired median organ; equally so, the history of the 
camerostome itself supplies the reason why the olfactory nerves are 
double, why the organ is in reality a paired organ and not a single 
Fie, 102.—Thelyphonus. (From the Royal Natural History.) 
median one. Thus, in a sense, the grouping of the fishes into Mono- 
rhine and Amphirhine has not much meaning, seeing that the 
olfactory organ is in all cases double. 
The evidence of the olfactory organs in the vertebrate not only 
confirms, in a most striking manner, the theory of the origin of the 
