XIV 



SIGHT IN INVERTEBRATES 



In the last chapter we considered the structure of the eye 

 in the vertebrate animals and saw something of what is 

 known about its working. We, being vertebrates, have eyes 

 built to the basic design common to them all and conse- 

 quently we can, with due caution, interpret the sight of 

 others by our own subjective experience. But what of the 

 sense of sight in the invertebrates, whose kinds greatly out- 

 number those of the vertebrates? Their brains are so dif- 

 ferent from ours that it is difficult to appreciate how the 

 world may appear to them. Yet different as their brains may 

 be, the same principle is found in all organs of vision more 

 complex than simple light-sensitive spots of pigment — a 

 camera consisting of a box with a lens at one end and a 

 light-sensitive screen at the other. 



We have seen that the protoplasm of even the simplest 

 animals whose bodies are not divided up into separate cells 

 responds to light, but evidently early in the process of evolu- 

 tion it became an advantage to possess a particular part of 

 the body specialized for responding to the stimulus of light. 

 Hence even in the protozoa there are species which contain 

 a speck of pigment that, like the silver bromide in a photo- 

 graphic film, is very easily changed chemically when light 

 falls upon it. Eye-spots of this kind are found not only in the 

 protozoa but in many animals of more complex structure 

 whose bodies consist of great numbers of cells of different 

 sorts segregated into special tissues for different functions. 



The evolution of eyes from simple eye-spots consisting of 

 light-sensitive substances was almost inevitable. In a many- 

 celled animal the cells containing such pigment will generally 

 lie at the surface of the body, and their pigment will be at 

 the inner end of the cells, as close as possible to the under- 

 lying nerve fibres. The transparent protoplasm of the cell 



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