l6o LIGHT AND THE BEHAVIOR OF ORGANISMS 



raised slightly from time to time during the process of 

 locomotion. As the organism proceeds on its course it 

 secretes a mucous substance in the form of a delicate fiber 

 which can be readily detected by pushing a needle across 

 the path a short distance from the posterior end. The 

 planula and the needle usually adhere so firmly to the 

 mucous fiber that the former can be lifted to the surface of 

 the water. 



Hargitt (1904, p. 272) referring to the reactions of Euden- 

 drium planulae says, " At the height of the breeding season 

 their numbers were large and they promptly swam directly 

 toward the strongest light with great uniformity. By inter- 

 posing a dark screen between this source of light and allow- 

 ing another from the opposite side to operate upon the 

 aquarium, there w^as an almost instantaneous response, the 

 entire number almost without exception facing directly 

 about, like a body of soldiers at command, and moving 

 without deviation in the opposite direction, that is, toward 

 the second source of light." When studied en masse in 

 light from a window^ they do appear to orient very accu- 

 rately; but if attention is focused on individuals it soon 

 becomes evident that there is considerable variation in the 

 direction of motion. This becomes still more evident if the 

 reactions of individuals are studied in light from a single 

 compact source. Thus several active specimens which ap- 

 peared to be moving directly toward the window were 

 selected and exposed one at a time in light of about the 

 same intensity from a Nernst glower so arranged as to 

 eliminate practically all refraction and reflection from the 

 different substances in the aquarium, th€ glass walls, 

 the surface of the water, and particles in suspension in the 

 water. The angle between their direction of motion and the 

 direction of the rays was frequently measured, and it was 

 found that it varied all the way from o to 10° and even 

 more in a few individuals. If a number of specimens are 

 put into the aquarium at the same time and at the same 

 point, it is found that as they proceed toward the source 



