f 



58 THE ORIGIN OF VERTEBRATES 



solid bodies falling into the water, are accompanied by disturbance* 

 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 Cephalaspidse from some arthropod, either 

 belonging to, or closely allied to, the group called Palceostraca, 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 apjDreciation 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 auditorv 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 



