448 Dynamic Theory. 



all directions, even backward over the shoulder, and enable the animal 



to see behind without turning his body. 



Among the animal forms below the articulates we find pigment spots 

 with nerve attachments as the organs of vision. Some of the Star 

 fishes have such eyes, similar to those of the Nereis, on the points of 

 their rays. They are also found in the Sea Urchins, at the margins of 

 many Medusae, and in some Polypi ; and they have even been shown to 

 exist in a large number of infusoria (Agassiz.) 



Fro. 180. Euglena Viridis, a Flagellated Infusorian. 

 S. Red spot sensitive to light; an elementary eye. 



Plants and many minute animals destitute 

 visible organs are observed to move with 

 FlG 180 reference to light, generally trying to get 



more of it. In both plants and the little animals, certain parts are more 

 or less differentiated to a degree of greater sensibility to the influence of 

 light. The Euglena Viridis (figure 180) an infusorian which is about 



FIG. 181. Diagrammatic Section of the Eye of the Sepia (Cuttle Fish). 



C Cornea. Ci. -Ciliary Body. P. Pigment Layer of Retina. 



Yi.-Lens. If. Cartilage of Optic Bulb. IF.- White Body. 



Jk. Iris Cartilage. V. Vitreous Humor. K K. Cephalic Cartilage. 



^.Epithelial Cover. Re Outer Layer of Rods. Go Optic Ganglia. 



Ae. External Argentine Ri. Inner Layer of Rods. Opt. Optic Nerve. 



[Layer. (After Hensen.) 



as much vegetable as animal, is sensitive to light in its anterior, flagel- 

 lated end. And there it possesses a red pigment spot. Even the moving 

 zoospores of certain Algae have such a red spot, which Lubbock supposes 

 to have reference to light. Most of the Mollusks have eyes or eye spots at 



