CHAPTER XVI. 

 OKGANS 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 sub- 

 sequently 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 provided 

 with optic organs of greater or lesser 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 there- 

 fore no proof of genetic relationship between these eyes, but merely 

 of similarity of function. 



Enibryological data as to the development of the eye do not exist 

 except in the case of the Arthropoda, Mollusca and Chordata. From 

 such data as there are, combined with study of the adult structure of 

 the eye, it can be shewn that two types of development are found. 

 In one of these the percipient elements are formed from the central 

 nervous system, in the other from the epidermis. The former may 

 be called cerebral eyes. It is probable however that this distinc- 

 tion is not, in all cases at any rate, so fundamental as might be 



252 



