44 THE ORIGIN OF THE EYE. 



Eyes of invertebrates. Great range of structure. 

 What constitutes an eye. Simplest eye. Pigment 

 Bpot with nerve fibers. Eye of Nais. 



Eyes of mollusks showing series of increasing com- 

 plexity ; of the limpet ; of the abalone ; of the snail ; 

 of the squid. Development of eyes of mollusks. The 

 more highly developed passes through the stages repre- 

 sented in the lower forms. 



Eyes of insects. Simple. Compound. 



All eyes alike in that they consist, in the essential 

 parts (retina and nervous elements), in a number of 

 epidermal cells with long internal processes the one 

 specialized for being affected by light, the other for 

 the conduction of impulses thus generated, and further 

 in having other epidermal cells developed into groups, 

 in intimate relation with these, the visual centers. 

 The meaning of accessory parts. The properties of 

 protoplasm. The physiological division of labor illus- 

 trated in the organs of vision. 



