Animal Life and Intelligence. 



This is the remnant of a median eye. It has been found in 

 varying states of degeneration in many reptiles (Fig. 34), and 

 in a yet more vestigial form in some fishes and amphibia. It 



is connected with a curious struc- 

 ture, associated with the brain of 

 all vertebrates, and called the 

 pineal gland. Descartes thought 

 that this was the seat of the soul ; 

 but modern investigation shows 

 it to be a structure which has 

 resulted from the degeneration of 

 that part of the brain which was 

 connected with the median eye. 

 Fig. 34.— Pineal eye. There is some reason to suppose 

 Yaf^^^t^n^^V'tl^'^Z"^ t^at, in ancient life-forms, like 

 ^P^""^^""-) the Ichthyosaurus, and Plesio- 



saurus, and the Labyrinthodont amphibians, it was large 

 and functional. In any case, there is a large hole in the 

 skull (Fig. 35) through which the nervous connection with the 

 brain may have been established. The structure of the eye 



is not similar to that of the 

 lateral eye, but more like that 

 of some of the invertebrates. 

 To these invertebrates we 

 must now turn. 



Insects have eyes of two 

 kinds. If we examine with a 

 lens the head of a bee, we 

 shall see, on either side, the 

 large compound or facetted 

 eye ; but in addition to these 

 there is on the forehead or 

 vertex a triangle of three 

 Fig. 35.— Skull of Meianerpeton. small, bright, simple eyes, or 

 m^t''o?S^^?t^tTtS^^'TtlZ ocelli. These ocelli, or eye- 



the parietal foramen. j^^g^ ^^^^^^ ^^ different iu- 



sects, as to the details of their structure ; but in general 



