II 



SIGHT— GENERAL 



Quite apart from the anatomical and physiological aspects 

 of the senses which will be dealt with fully in Part II, it will 

 be helpful to the field naturalist to have a broad idea of the 

 subject under the heading of each sense. It seems right 

 to commence with sight, but before going into the visual 

 abilities of the different groups of animals it may be as 

 well to consider the limits within which we can study this 

 sense. 



When dealing with the higher animals it is comparatively 

 easy to convey what one means in talking of "sight" ; but it 

 must be kept in mind that if the word is taken literally it 

 implies the reception of some more or less definite image. 

 However, among lower forms of life there are light receptive 

 organs which produce positive or negative reactions in the 

 creature concerned, but these cannot by any means be 

 termed "eyes". 



Some brief reference to these organs must be made if the 

 complicated eyes of animals which are more highly developed 

 are to be properly appreciated. Fieldwork and the collection 

 of these lower organisms will be made easier if a student 

 knows where best to find a given type in relation to its 

 toleration or otherwise of light. 



In the Protozoa, for instance, there are many kinds to be 

 found in areas of water which receive a considerable amount 

 of light ; on the other hand, there are some simple aquatic 

 forms which seek the dark. The various kinds of water-fleas 

 (which are really crustaceans) can also be caught in well- 

 lighted areas; but on land earthworms will return to their 

 burrows if a light is flashed on them. 



All these creatures have a means of perceiving light, and 

 water-fleas have "eyes" which can be clearly seen under a 

 low-power microscope — yet these "eyes" do not present 



