PHOTOTAXIS 41 



tion. Loeb early distinguished phototaxis or heliotropism 

 from mere reaction to differences in the intensity of light 

 (Unterschiedsempfindlichkeit) where there are no definitely 

 directed movements and subsequently applied the term 

 photokinesis to the latter phenomenon. The tube-dwelling 

 annelid Serpula may be made to suddenly draw back if 

 a shadow is thrown upon its expanded gills. If the light 

 be suddenly increased no reaction occurs. In the bivalve 

 Psammobia, Nagel found that sudden increase of light caused 

 a retraction of the extended siphons; while many other 

 species (Cardium, Mactra, Solen) would give a similar 

 reaction to shadows. In this connection may be mentioned 

 the reactions to shadows of the large leech Clepsine which 

 is parasitic on turtles. If a shadow is thrown upon a lot 

 of hungry leeches in a dish of water they will raise up and 

 extend the anterior part of the body and sway it about in 

 various directions. The function of this response is to enable 

 the leeches to attach themselves to any passing turtle and 

 thereby secure their food. 



Animals may form collections in shaded localities not 

 because they are negatively oriented by the rays of light, 

 but because light stimulates them to general activity, and 

 when they happen to come into a place where they are less 

 stimulated they become relatively quiet. Loeb found that 

 if fresh water planarians are placed in a round dish in front 

 of a window they collect at the sides of the dish which are 

 more or less shaded instead of at the side farthest away from 

 the window where they would naturally assemble if they were 

 guided solely by negative phototaxis. In bright light these 

 animals are active and when they wander into a shaded 

 spot they move more slowly and thus tend to collect there. 

 Parker, however, has shown that Planaria maculata is 

 oriented to a certain extent by the rays of light, and Walter's 



