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 
cuided 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 
