PINEAL EYE ‘ 53 
admitted as its possible use. Its close genetic relationship 
with the hearing organ suggests the kindred function of 
determining waves of vibration. These are transmitted in 
so favourable a way in the aquatic living medium, that from 
the side of theory a system of hyper-sensitive end organs 
may well have been specialized. The sensory tracts along 
the sides of the body are certainly well situated to deter- 
mine the direction of the approach of friend, enemy or 
prey. 
The Pineal Eye 
_ The presence or absence in fishes of the pzneal end organ, 
the “unpaired median eye of chordates,” may finally be 
noted, since the condition of the epzphysis and its associ- 
ated structures in fishes has an important bearing on 
general vertebrate morphology. 
It is well known that in many forms of reptiles there 
_ exists, at the distal end of the epiphysis, a well-defined 
sensory capsule, whose structure shows unquestionably its 
optic function. It has seemed to many, therefore, that 
throughout the chordates the epiphysis has been primi- 
tively associated with a median eye, which has degenerated 
as the paired eyes became better evolved. That it has 
been retained in an almost perfect condition in reptiles 
has accordingly been looked upon as an outcome of a 
life habit which concealed the animal in sand or mud, 
and allowed the forehead surface alone to protrude: — 
the median eye thus preserving its ancestral value in 
enabling the animal to look directly upward and backward. 
If this view as to the presence of a parietal eye in the - 
ancestral vertebrate is to be generally accepted, one would 
_ fhaturally suggest that the organ should be present, at all 
_ €vents to a recognizable degree, in some of the varied forms 
