CHAPTER XVI. 



ORGANS OF VISION. 



IN the lowest forms of animal life the whole surface is sensitive 

 to light, and organs of vision have no doubt arisen in the first 

 instance from limited areas becoming especially sensitive to light 

 in conjunction with a deposit of pigment. Lens-like structures, 

 formed either as a thickening of the cuticle, or as a mass of cells, 

 were subsequently formed ; but their function was not, in the first 

 instance, to throw an image of external objects on the perceptive 

 part of the eye, but to concentrate the light on it. From such a 

 simple form of visual organ it is easy to pass by a series of steps 

 to an eye capable of true vision. 



There are but few groups of the Metazoa which are not pro- 

 vided with optic organs of greater or less complexity. 



In a large number of instances these organs are placed on the 

 anterior part of the head, and are innervated from the anterior 

 ganglia. It is possible that many of the eyes so situated may 

 be modifications of a common prototype. In other instances 

 organs of vision are situated in different regions of the body, and 

 it is clear that such eyes have been independently evolved in each 

 instance. 



The percipient elements of the eye would invariably appear 

 to be cells, one end of each of which is continuous with a 

 nerve, while the other terminates in a cuticular structure, or 

 indurated part of the cell forming what is known as the rod or 

 cone. 



The presence of such percipient elements in various eyes is 

 therefore no proof of genetic relationship between these eyes, 

 but merely of similarity of function. 



Embryological data as to the development of the eye do not 



