The Senses of Animals. 293 



to be aware of the presence of an object until it is brought 

 within a quarter of an inch or less of the tentacle. In all 

 probability the eye does little more than enable the snail 

 to distinguish between light and dark. And the same may 

 be said of the eye of many of the molluscs. In some, 

 however, the cuttle-fishes and their allies, the eye is so 

 highly developed that it has been compared with that of 

 the vertebrate. There is an iris with a contractile pupil. 

 And the ganglion with which it is connected forms a large 

 part of the so-called brain. The powers of accurate vision 

 in these higher forms are probably considerable. 



It is interesting to note that whereas in the cuttle-fishes 

 and most molluscs, the rods of the retina are turned 

 towards the light, in Pecten, Onchidium (a kind of slug), 

 and some others, they are, as in vertebrates, turned from 

 the light. In Pecten the nerve to supply the retina bends 

 round its edge at one side. But in Onchidium it pierces 

 the retina as in vertebrates. 



In worms, eyes are sometimes present, sometimes 

 absent. In star-fishes and their allies they often occur. 

 In medusae (jelly-fish) they are sometimes found on the 

 margin of the umbrella. Eyen in lowly organisms, like 

 the infusoria, eye-spots not unfrequently occur. We must 

 remember, however, that, in these lower forms of life, the 

 organs spoken of as eyes or eye-spots merely enable the 

 possessor to distinguish light from darkness. 



Even when eyes or eye-spots are not developed, the 

 organism seems to be in some cases sensitive to light — 

 using the word "sensitive," once more, in its merely physical 

 acceptation. The earthworm, for example, though it has 

 no eyes, is distinctly sensitive to light ; and the same has 

 been shown to be the case with other eyeless organisms. 

 Graber holds that his experiments demonstrate that the 

 eyeless earthworm can distingush between different colours 

 — in other words, is differentially sensitive t® light-waves 

 of different vibration-period — preferring red to blue or 

 green, and green to blue. And the same observer has 

 shown that animals provided with eyes — the newt, for 



