Chap. VI.] ORGANS OF EXTREME PEEFECTION. 225 



an unaltered or little altered condition. But the state 

 of the same organ in distinct classes may incidentally 

 throw light on the steps by which it has been perfected. 



The simplest organ which can be called an eye 

 consists of an optic nerve, surrounded by pigment-cells 

 and covered by translucent skin, but without any lens 

 or other refractive body. We may, however, according 

 to M. Jourdain, descend even a step lower and find 

 aggregates of pigment-cells, apparently serving as organs 

 of vision, without any nerves, and resting merely on 

 sarcodic tissue. Eyes of the above simple nature are 

 not capable of distinct vision, and serve only to dis- 

 tinouish light from darkness. In certain star-fishes, 

 small depressions in the layer of pigment which 

 surrounds the nerve are filled, as described by the 

 author just quoted, with transparent gelatinous matter, 

 projecting "with a convex surface, like the cornea in the 

 higher animals. He suggests that this serves not to 

 form an image, but only to concentrate the luminous 

 rays and render their perception more easy. In this 

 concentration of the rays we gain the first and by far 

 the most important step towards the formation of a 

 true, picture-forming eye; for we have only to place 

 the naked extremity of the optic nerve, which in some 

 of the lower animals lies deeply buried in the body, and 

 in some near the surface, at the right distance from the 

 concentrating apparatus, and an image will be formed 

 on it. 



In the great class of the Articulata, we may start 

 from an optic nerve simply coated with pigment, the 

 latter sometimes forming a sort of pupil, but destitute 

 of a lens or other optical contrivance. With insects it 

 is now known that the numerous facets on the cornea 



