Photoreception 



385 



iris. Secondary and tertiary neurones are located in an optic ganglion im- 

 mediately behind the retina, rather than on the front of the retina, as in 

 vertebrates. Functionally as well as structurally, the eye of Se^pia is similar to 

 that of vertebrates, except that the retinal elements are not inverted. 



The sixth abdominal ganglion in the crayfish is photosensitive, but the na- 

 ture of the photoreceptor cells is unknown. ^''^''* 



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Fig. 102. Orientation of swimming Englena to light. From 1 to 2, light from direction 

 of top of page; 2, direction of light reversed. The animal swerves to the dorsal side each 

 time the receptor is shielded from the light by the eye spot. (Jennings, 1906.) 



Function in Orientation. The photoreceptor of Englena and related forms 

 is a directional receptor by virtue of the pigment of the stigma. This orange- 

 red pigment mass serves to shade the swelling of the base of the flagellum 

 when illumination is on the "dorsal" surface and does not do so when illum- 

 ination is from the opposite direction. When exposed to a single light source 



