250 LIFE : OUTLINES OF GENERAL BIOLOGY 



It Is important to keep in mind the interesting fact that 

 before the eye became in the course of ages a seeing eye, able to 

 form an image, it was a structure for distinguishing between light 

 and darkness; and that is the level represented by these scattered 

 "photo-receptors" of the earthworm. Similarly, before the ear 

 became in the course of ages a hearing ear, it was a balancing organ, 

 as it continues to be. And before the brain became a thinking organ, 

 it was the organ for controlling muscular movements. 



Sense of Light without Eyes. — Earthworms have no eyes, 

 yet they are very sensitive to differences of light and shade. Various 

 blind or blinded insects, both adult and larval, are known to react 

 to light. Some investigators have spoken of a "dermatoptic sense", 

 meaning that the general surface of the insect's body is sensitive 

 to light. In this connection some striking experiments on entirely 

 blinded minnows have been made by Scharrer, proving reactivity 

 to light after the complete removal of both eyes. They put on dark 

 colour when illumined; they assumed a light colour in darkness. In 

 an aquarium constantly but faintly lighted, a stronger light was 

 turned on shortly before and during each meal, and the blinded 

 minnows established a conditioned reflex in a few days, and when the 

 light was turned on would snap and jump about in search of the 

 food even when there was none. This reactivity was found to have 

 its seat in a region of the head corresponding to the optic thalami 

 of the brain — the region from which the paired eyes grow out. This 

 region also gives origin to the parietal organ, which has an eye-like 

 structure in some reptiles, notably in the New Zealand hzard or 

 Sphenodon. The parietal organ is in a very primitive state in bony 

 fishes, and its extirpation does not affect the blinded minnow's 

 reactions to light. It is the whole region of the optic thalami that is 

 important as a seat of sensitiveness to light, and a remarkable 

 feature is that it responds to very slight illumination. 



Eyes that Shine in the Dark. — Why do a cat's eyes shine in 

 the dark ?> 



Part of the answer is that the question is not rightly put. When 

 we see a cat at night with .shining eyes there is always some illu- 

 mination, such as a candle or a street lamp. There must be some 

 rays from somewhere, for there is no production of light in the 

 eyes. The real puzzle is: Why do the cat's eyes give forth a light 

 much more brilliant than that of a candle or a lamp? 



The next step is to notice that the cat is by no means the only 

 animal that has eyes which shine in the dark. If we open the door 

 of a dark stable in which there is a sheep we see the same shining 

 eyes. The peculiarity is .shown by some other mammals, and the 

 eyes of various moths, like the death's-head, shine in the dark. So 

 do those of many beetles. When the light of a lamp falls sideways 

 on the eyes of the big rhinoceros beetle of Ceylon, its eyes shine out 



