io8 



THE DARWINIAN THEORY 



Fig. 17. 



Evolution of eyes in Mollusca. 

 i%, Layer of nerve tissue ; 0, Optic nerve ; /, Lens. 



A. — Eye of Solen, consisting of sensitive pigmented cells at the bottom 

 of pit-like depressions in the skin. 



B. — Eye of Patella. The pit is more developed, and the sensory cells 

 less exposed. 



C. — Eye of Haliotis. The mouth of the pit is narrow, and the sensitive 

 cells still more protected. A well-developed optic nerve is present, and a 

 layer of nerve tissue outside the sensitive cells of the retina. 



D. — Eye of t'issurella. The mouth of the pit is closed, and a lens is 

 developed. 



