1 68 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



(2) The next step is a slight saucerlike depression of 

 the pigmented spot with an increased pigmentation and 

 elongation of the cells. This step is found in the sword 

 shell (So/en), in which the eye spots are strung all along 

 the edge of the mantle as the only exposed part, for 

 these animals are headless (Fig. 107, b). 



(3) In the next step the depression becomes deep, 

 cuplike. Evidently here there is a stronger impression 

 of light by reverberation in the hollow and consequent- 



Fig. 107.— Diagram representing the different stages in the evolution of the 

 invertebrate eye : a b c, eye-spots, no i'lia^e ; d, pin-hole imacre ; e, sim- 

 ple lens ima^e ; /, compound lens image ; r, retina ; on, optic nerve ; 

 v, vitreous humor ; /, lens ; cor, cornea. 



ly a greater specialization of cells for response. This 

 step is found in the limpet or Patella, and the organ is 

 situated in the head, for this is a gastropod. Already 

 we begin to see in the pigmentary layer a choroid and 

 in the elongated rodlike cells a bacillary layer of a retina 

 (Fig. 107, c). 



Thus far we have only eye-spots, not an eye proper; 

 only a specialized layer of nerve terminals, not an im- 



