REACTIONS OF INFUSORIA TO LIGHT AND TO GRAVITY 131 



animal may even turn toward the source of light. The essential point 

 is the swimming back into the shaded region, without reference to the 

 direction from which the light comes. 



Similar phenomena are observed if the side of the vessel next to the 

 source of light is shaded, the shadow of the screen reaching to the middle 



A 



y 



Fig. 88. — Reaction of Stentor to light when one half the vessel next the source of light is 

 shaded by a screen 5-5 (as indicated in Fig. 89). On reaching the line x-y, where it would pass 

 into the light, the animal responds as shown at c, 1-5. 



of the vessel, so that the side farthest from the source of light is illumi- 

 nated (Figs. 88 and 89). Under such circumstances the Stentors gather 

 in the shaded area, next to the window. A specimen in the shaded area 

 which swims toward the Ughted side is of course moving when it comes 



to the boundary line in the same direction 

 the rays of light. It nevertheless re- 

 sponds by the avoiding reaction, — stop- 

 turning toward the right aboral side, 

 and swimming back to the 

 shadow. This often happens 

 when the animal has com- 

 pletely passed the boundary 

 and is entirely within the 

 lighted area (Fig. 88, 6). In 

 passing back into the dark- 



s 



Fig. 89. — Side view of the conditions in the 

 experiment shown in Fig. 88. The arrows show the 

 direction of the rays of hght. 



ened area it now swims of course directly toward the source of light. 



All together, then, our experiments thus far have shown that the cause 

 of the avoiding reaction is the change from darkness to light. At every 

 such change, Stentor responds by the avoiding reaction ; that is, it tries 

 swimming in other directions until it is no longer subjected to the light. 



Let us now arrange the conditions in such a way that all parts of the 



