THE EVIDENCE OF THE AUDITORY APPARATUS 357 



found its strongest support in the similarity of two sets of segmental 

 organs found in annelids and vertebrates. On the one hand, great 

 stress was laid upon the similarity of the segmental excretory organs 

 in the two groups of animals, as will be discussed later ; on the other, 

 of the similarity of the segmentally arranged lateral sense-organs. 



These lateral sense-organs of the annelids have been specially de- 

 scribed by Eisig in the CapitellidEe, and, according to Lang, " there are 

 many reasons for considering these lateral organs to be homologous 

 with the dorsal cirri of the ventral parapodia of other Polychreta, and 

 in the family of the Glyeeridae we can follow, almost step by step, 

 the transformation of the cirri into lateral organs." Eisig describes 

 them in the thoracic prebranchial region as slightly different from 

 those in the abdominal branchial region ; in the latter region, the 

 ventral parapodia are gill-bearing, so that these lateral organs are 

 in the branchial region closely connected with the branchite, just 

 as is also the case in the vertebrates. It is but a small step from 

 the gill-bearing ventral parapodia of the annelid to the gill-bearing 

 appendages of the phyllopod-like protostracan ; so that if we assume 

 that this is the correct line along which to search for the origin of 

 the vertebrate auditory apparatus, then, on my theory of the origin 

 of the vertebrates from a group resembling the Protostraca, it follows 

 that special sense-organs must have existed either on or in close 

 connection with the branchial and prebranchial appendages of the 

 protostracan ancestor of the vertebrates, which would form an inter- 

 mediate link between the lateral organs of the annelids and the 

 lateral and auditory organs of the vertebrates. 



Further, these special sense-organs could not have been mere 

 tactile hairs, but must have possessed some special function, and 

 their structure must have been compatible with that function. Can 

 we obtain any clear conception of the original function of this whole 

 system of sense-organs ? 



A large amount of experimental work has been done to determine 

 the function of the lateral line organs in fishes, and they have been 

 thought at one time or another to be supplementary organs for 

 equilibration, organs for estimating pressure, etc. The latest experi- 

 mental work done by Parker points directly to their being organs 

 for estimating slow vibrations in water in contradistinction to the 

 cpuicker vibrations constituting sound. He concludes that surface 

 wave-movements, whether produced by air moving on the water or 



