chap, xii.] OPTIC ORGANS. 445 



generally, we observe a distinction between the pig- 

 mented and the sensory cells ; the latter are not mere 

 cylindrical bodies, but have their peripheral portion 

 converted into an elongated process, not unlike a small 

 rod, while they are continuous behind with ganglionic 

 cells. In the simplest cases there is no cornea or 

 lens, or organ to concentrate the rays of light \ in the 

 more complicated the investing cuticle becomes convex 

 in shape, and has, no doubt, some such function ; so 

 that we have now to observe an apparatus which is 

 composed of parts that are respectively refractive, 

 light-absorbing, and light-perceiving. These eyes lie 

 at the base of the tentacles, and have been proved by 

 direct experiment to be really sensitive to luminous 

 impressions ; specimens of Aurelia (the common jelly- 

 fish), which, when uninjured, were found to swim 

 towards a beam of light flashed upon the water in 

 which they were kept, were, when the eye spots were 

 removed, observed to exhibit no change of manner oil 

 the application of a similar stimulus. 



The earthworm is without any organs that can bo 

 called eyes, and, as a general rule, we find that bur- 

 rowing forms are always less well provided with optic 

 organs than their allies which live on the surface of 

 the land ; at the same time the worm is sensitive to 

 light, and ordinarily withdraws from it ; the sensitive- 

 ness is confined to the anterior region of the body. 

 This cannot but be regarded as a very striking phe- 

 nomenon, when correlated with the concentrated con- 

 dition of their nervous system, and the fact that in 

 Vermes with a more diffused arrangement of the 

 nervous system, eyes are found in various regions of 

 the body. 



In the lower worms, simple eye-spots are not un- 

 frequently present, and, as often happens with organs 

 in a simple or indifferent condition, they are present 

 in large numbers ; some Tiirtoellaria, for example, 



