80 ; VISION, 
preciation of a picture does not appear at once in 
the simpler forms of eyes met with in animals low 
in the scale ; and perhaps the most interesting way 
of looking at the structure of the human eye is to 
compare it with others of a less complex sort. 
The simplest form of indubitable eye is a nerve- 
termination with pigment on it, and more simple 
still there are spots of pigment in various animals 
in which nerves are not to be found; and those 
spots are possibly or in some instances even pro- 
bably eyes; that is to say sensitive to the rays 
which the pigment reflects. 
The star fishes furnish examples of a simple form 
of indubitable eyes. They have not all got eyes ; 
but those which possess them have them at the tips 
of their arms in groups which form a scarlet spot. 
We know that these organs are eyes because they 
have not only bright colour round them, but have a 
transparent structure at the extremity, which admits 
light to the nerve-extremities behind it. There can 
be no doubt, therefore, that’ light is the irritant 
which acts on these organs ; but it is equally certain 
that they are utterly incapable of displaying’ the 
forms of surrounding objects, for every ray which 
enters must fall on the red cup which surrounds the 
nerve terminations, and in consequence of this its 
red elements will be reflected from place to place in 
