PLYEAL 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 \.}\Q pineal end organ, 

 the "unpaired median eye of chordates," may finally be 

 noted, since the condition of the epipJiysis 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 

 naturally suggest that the organ should be present, at all 

 events to a recognizable degree, in some of the varied forms 



