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 Polycheeta, and 
in the family of the Glyceride 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 branchie, 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 
quicker vibrations constituting sound. He concludes that surface 
wave-movements, whether produced by air moving on the water or 
