REACTIONS TO LIGHT- 249 



Muschel regungslos, der Schatten geniert sle in keiner 

 Weise. 



'' Noch auffallender ist die Erscheinung bei der Auster 

 und unserer Malermuschel. Wenn diese einmal durch einen 

 Schatten erschreckt worden sind, dann bleibt jede weitere 

 Beschattung ohne Erfolg. Erst wenn eine Stunde oder 

 mehr seit dem ersten Versuche ohne Storung verflossen ist, 

 sind die Muscheln wieder fiir den Schattenreiz empfang- 

 lich." I have frequently noticed that the hermit crab 

 (Pagurus), mosquito larvae and tubicolous worms, especially 

 Hydroides, soon fail to respond to ordinary shadows if they 

 are kept in a place where the shadows frequently occur, 

 but under such conditions they still respond to reduction 

 of intensity greater than they ordinarily experience. The 

 experimental results of Mrs. Yerkes (1906) and Hargitt 

 (1906, 1909) on Hydroides dianthus lead to the same con- 

 clusion. Hargitt's observations (1909, p. 158) are of especial 

 interest in showing the relation between the reaction and 

 the habitat. He found that specimens taken from a depth 

 of about twenty fathoms did not respond to shadows which 

 caused very definite reactions in specimens taken in shallow 

 water. Whatever the immediate physiological cause of all 

 these reactions may be, it is evident that they are admirably 

 adapted to protect the organism against the attack of 

 enemies. There are however animals in which similar re- 

 actions are found which serve quite a different purpose. 

 These are included in the following group. 



2 . Reactions to Shadoivs — Procuring Food 



Whitman observed that even a very faint shadow causes 

 the leech, Clepsine, to become restless and stretch up and 

 sway from side to side, apparently in search of something 

 to seize. Bateson records similar reactions in shrimps and 

 prawns. When a shadow passes near these animals they 

 raise their antennae and swing them about. The primary 

 cause of reaction in these animals is probably the same as 



