358 THE ORIGIN OF VERTEBRATES 
solid bodies falling into the water, are accompanied by disturbances 
which are stimuli for the lateral line organs. 
One of these segmental organs has become especially important 
and exists throughout the whole vertebrate group, whether the animal 
lives on land or in water—this is the auditory organ. Throughout, 
the auditory organ has a double function—the function of hearing 
and the function of equilibration. If, then, this is, as is generally 
supposed, a specialized member of the group, it follows that the 
less specialized members must possess the commencement of both 
these functions, just as the experimental evidence suggests. 
In our search, then, for the origin of the auditory organ of verte- 
brates, we must look for special organs for the estimation of vibra- 
tions and for the maintenance of the equilibrium of the animal, 
situated on the appendages, especially the branchial or mesosomatic 
appendages; and, further, we must specially look for an exceptional 
development of such segmental organs at the junction of the pro- 
somatic and mesosomatic regions. 
Throughout this book the evidence which I have put forward 
has in all cases pointed to the same conclusion, viz. that the verte- 
brate arose by way of the Cephalaspide from some arthropod, either 
belonging to, or closely allied to, the group called Palsostraca, of 
which the only living representative is Limulus. If, then, my argu- 
ment so far is sound, the appendages of Limulus, both prosomatic 
and mesosomatic, ought to possess special sense-organs which are 
concerned in equilibration or the appreciation of the depth of the 
water, or in some modification of such function, and among these 
we might expect to find that somewhere at the junction of the pro- 
soma and mesosoma such sense-organs were specially developed to 
form the beginning of the auditory organ. 
Now, it is a striking fact that the appendages of Limulus do 
possess special sense-organs of a remarkable character, which are 
clearly not simply tactile. Thus Gegenbaur, as already stated, 
has drawn attention to the remarkable branchial sense-organs of 
Limulus ; and Patten has pointed out that'special organs, which he 
-considers to be gustatory in function, are present on the mandibles 
of the prosomatic appendages. I myself, as mentioned in my address 
to the British Association at Liverpool in 1896, searched for some 
special sense-organ at the junction of the prosoma and mesosoma, 
and was rewarded by finding that that extraordinary adjunct to the 
