484 



SENSATION IN THE LOWEE ANIMALS 



snail these simple eyes are two in number and are located 

 at the ends of the tentacles. In the scallop they are 

 numerous and are located on the edges of the mantle. 



The next step in advance is found in the insect. Here 

 thousands of oceUi are massed together side by side to 

 form what is called a compound eye. (See Fig. 226.) 



Fig. 226 — Compound eye of insect; A, section; a, optic ganglion; 6, secondary 

 nerves; c, retina; (J, pigment layer; f, optic nerves; B, group of ocelli; /, bulb of 

 optic nerve; g^ pigment layer; A, vitreous liumor; i, cornea. 



Each ocellus points at a different angle and hence while 

 the whole eye is fixed the range of vision is extensive. 

 In such an eye each ocellus forms its own image independ- 

 ently of its neighbours. These images are all collected 

 by the brain into a single perception much as are the 

 images of our two eyes. 



The nearest approach to the structure found in man and 

 in all other vertebrates among the invertebrate forms is the 



