130 ORGANS OF VISION IN THE LOWER ANIMALS. 



secondary modification, and considers that it had 

 previously served some other function. 



However this may be, it cannot be doubted that the 

 pineal gland in Mammalia is the representative of the 

 cerebral lobe which supplies the rudimentary pineal 

 eye of Keptilia, and this itself is probably the degenerate 

 descendant of an organ which in former ages performed 

 the functions of a true organ of vision. 



The Organs of Vision in the Lower Animals. 



Mere sensibility to light is possible without any 

 optical apparatus. Even plants, as we know, can well 

 distinguish between light and darkness; and though 

 it seems that in our own case the general surface of 

 the skin has lost its sensitiveness to light, still, in many 

 of the lower animals, light seems to act generally and 

 directly on the tissues. 



Some microscopic vegetable forms even, as, for in- 

 stance, Englena (Fig. 83), possess a red spot,* which 



appears to be specially sensi- 

 tive to light. 



The lower animals are, in 



a great many cases, very 



transparent. Light passes 



Yig. 83.-Engienaviridis. ©asily through them, and, 



e. Eye-spot. ^^^^^^ -^ ^^ £^^, ^^ -^ -^ ^^^ 



sorbed, can hardly be supposed to produce any effect. 

 The most rudimentary form of a light-organ, then, may 

 be considered to be a coloured spot. 



In the first chapter I have endeavoured to show how 



* The moving zoospores of certain algse also possess a red spot, 

 ■which may perhaps have special reference to light. 



