238 ANIMAL LIFE 
The simplest eyes, if we may call them eyes, are not 
capable of forming an image or picture of external objects. 
They only make the animal better capable of distinguish- 
ing between light and darkness or shadow. Many lowly 
organized animals, as some polyps, and worms, have certain 
cells of the skin specially provided with pigment. These 
cells grouped together form what is called a pigment fleck, 
which can, because of the presence of the pigment, absorb 
more light than the skin cells, and are more sensitive to 
the light. By such pigment-flecks, or eye-spots, the animal 
can detect, by their shadows, the passing near them of mov- 
ing bodies, and thus be in some measure informed of the 
approach of enemies or of prey. Some of these eye-flecks 
are provided, not simply with pigment, but with a simple 
sort of lens that serves to concentrate rays of light and 
make this simplest 
sort of eye even 
more sensitive to 
changes in the in- 
tensity of light 
(Fig. 150). 
Most of the 
many-celled ani- 
mals possess eyes 
by means of which 
a picture of exter- 
nal objects more or less nearly complete and perfect can 
be formed. There is great variety in the finer structure 
of these picture-forming eyes, but each consists essentially 
of an inner delicate or sensitive nervous surface called the 
retina, which is stimulated by light, and is connected with 
the brain by a large optic nerve, and of a transparent light- 
refracting lens lying outside of the retina and exposed to 
the light. These are the constant essential parts of an 
image -forming and image-perceiving eye. In most cyes 
there are other accessory parts which may make the whole 
VW i, Lym | 
Fig. 150.—The simple eye of a jelly-fish (Lizzia 
koellikeri).—After O. and R. HERTWwIe. 
