PHYSIOLOGICAL 249 



After a certain stage was reached, probably among the worms, 

 the evolution of eyes proceeded along three distinct lines. There 

 was the line which finds its cHmax in birds and mammals. But quite 

 diiferent in detail and in development is a type of eye found among 

 molluscs, and reaching a climax in the extraordinarily effective 

 eyes of octopuses and their relatives. Entirely different again, and 

 occupying a place by itself, is the compound eye of insects and 

 higher crustaceans, where there are hundreds of lenses and hundreds 

 of percipient retinules, the image formed being a mosaic built up 

 of numerous minute contributions. This type of eye seems better 

 adapted to the detection of movements than to the perception of 

 form. 



The story of the evolution of vision should take account of the 

 movements of the eyes, the binocular apparatus, the discrimination 

 of colour, and the general improvement in picture-forming. The last 

 depends mainly on the size of the instrument, the number of sensitive 

 receptors in the retina, and the number of nerve-fibres passing from 

 the receptors to the brain. Nor can we forget the evolution of the 

 brain behind the eye ; for it is the brain, with its mind, that changes 

 vital photography into intelligent scrutiny, and sublimes sight into 

 vision. The eye sees what it brings with it, the power of seeing, as 

 we all prove every day, both pleasurably and painfully. 



Photoptic Sense in Earthworms. — How does an earthworm 

 know when it is time to come out of its burrow? — for it has no eyes. 

 In the strict sense, perhaps, it does not know anything; therefore 

 our question becomes : how does it sense the difference between light 

 and darkness? The earthworm is constitutionally a light-shy 

 animal, as technical people say, negatively heliotropic. Those earth- 

 worms that we see above-ground during the day are out of health, 

 or drowned out of their burrows, or parasitised by some fly. But let 

 us keep to our question: how is the earthworm made aware of 

 differences of light and shade ? It is a familiar fact that there are 

 numerous sensory nerve-cells in the earthworm's skin which send 

 tidings of various kinds from the outer world to the nerve- cord. 

 But Dr. Walter N. Hess has found a special kind of superficial cell 

 which is sensitive to light. It does not seem to have been noticed 

 before, though hundreds of zoological students are studying micro- 

 scopic sections of earthworms every year. Each of these special 

 cells, according to Dr. Hess, has a central glassy body like a 

 miniature lens, and this is surrounded by a network of nerve fibrils 

 which will act like a little retina. These cells are most abundant 

 in those parts of the earthworm's skin that are most sensitive to 

 hght. They are absent from the under surface of the rings except at 

 the two ends of the body. Similar "photo-receptors" are well known 

 in leeches, which are exquisitely sensitive to light and shade- 



