REACTIONS TO LIGHT 263 



(2) Contraction of naked protoplasm due to sudden 

 changes In the Hght Intensity, i.e., changes In the amount 

 of hght energy received, resulting In orientation in some 

 instances — Amoeba. 



(3) Fixed responses (avoiding reactions) caused by sud- 

 den changes of intensity, the nature of the response depend- 

 ent upon the structure of the organism. No orientation, 

 but aggregation at the optimum — Bacteria. 



(4) Reactions similar to those under (3), but more defi- 

 nitely circumscribed by the structure of the body, especially 

 the localization of tissue sensitive to light. Definite orien- 

 tation and movement directly toward the optimum — Sten- 

 tor, etc. 



(5) Reactions to a sign. The change in illumination 

 which causes the response is of no consequence to the organ- 

 nism, but the illumination which would follow If it did not 

 respond may be — Euglena, Volvox, etc. 



(6) Reactions to a sign. The change of intensity 

 (shadow) which causes the response represents objects 

 which may be beneficial or injurious, food or enemies — 

 Clepsine Hydroides, etc. 



(7) Reactions to a sign. The light condition or con- 

 figuration which causes the response represents objects, not 

 by means of shadows cast by them, but by means of the 

 light reflected from them expressing size, form or color — 

 Animals with image-forming eyes. 



