REACTIONS TO LIGHT 247 



I . Reactions to Shadows — Protective 



Many animals respond only to shadows, i.e., to a sudden 

 decrease in light intensity, not to a gradual decrease or to 

 an increase, no matter how sudden or how great. Such 

 reactions to shadows are widely distributed. They are 

 highly protective in all instances, and consequently vary 

 somewhat in accord with the different habits of the different 

 animals. The holothurian Thyone briareus contracts when 

 the light intensity is suddenly reduced, frequently to such 

 an extent that any portion protruding above the sand and 

 mud in which the animal lives is entirely withdrawn (Pearse, 

 1908, p. 277). Several different sea urchins, according to 

 von Uexkiill (1897), turn the spines toward the shaded part, 

 apparently to ward off an approaching enemy. Various 

 tubicolous annelids have been observed to contract violently 

 and jerk back into their tubes when an object passes be- 

 tween them and the source of light, e.g., Amphitrite bombyx 

 (Dalyell, 1853), Branchiomma koUikeri (Claparede, 1868), 

 Serpula (Ryder, 1883), Hydroides dianthus (Andrews, 

 1891, pp. 285, 296), Serpula uncinata (Loeb, 1893, p. 103), 

 Spirographis spallanzani (Nagel, 1896, p. 76), Potamilla 

 oculifera, Sabella microphthalmia and Protula intestimum 

 (Hargitt, 1906, p. 310), and Bispira voluticornis (Hesse, 

 I899). Shadows cause Pecten (Rawitz, 1888), Avicula, 

 Area, and Cardium (Patten, 1886) to close their valves rap- 

 idly. Nagel (1896, pp. 18-77) observed similar reactions in 

 23 species of lamellibranchs without eyes, 4 species of gastro- 

 pods with the eyes removed, and several blind arthropods. 

 I have also frequently seen the short-neck clam Maya ara- 

 naria close its siphon and contract, and Littorina littorea 

 retract rapidly into its shell. The hermit crab Pagurus 

 also darts back into its stolen home when a shadow is cast 

 on it. Mosquito larvae, ordinarily found at the surface of 

 the water, scurry to the bottom at the approachof a shadow, 

 and the killifish Fundulus responds much in the same way. 

 Barnacles, according to the observation of Pouchet and 



