THE BIONOMICS OF CONVOLUTA UOSCOFFENSTS. 405 



few hours of free existence they acquire this tropism which 

 becomes such a masterful factor in rletermiuiug their habits. 



5. Reactions to Two or more simultaneously 

 Applied Stimuli. 



Couvoluta offers admirable material for an investigation 

 into the nature of the reflex movement which occurs when 

 two or more stimuli are combined. As would be expected, 

 one stimulus often dominates another completely, so that the 

 response is that which would occur were the dominating 

 stimulus alone applied. This is illustrated in a striking 

 manner when Couvoluta is subjected to both heat and light 

 stimulation. To this end we place a shallow, oblong trough 

 with its long axis parallel with the direction of the light. 

 Couvoluta, placed with sea Avater in the trough, mass them- 

 selves up on the side toward the light. This end of the 

 trough is gradually heated, and though, under ordinary cir- 

 cumstances, Couvoluta is negatively thermotropic (at about 

 35° C), the animals remain in the position induced by light, 

 and die in dense masses as the temperature reaches the fatal 

 point (about 38° C). 



But, in the case of light- and gravitational stimuli acting 

 together, the resulting position shows that each has produced 

 an effect, though the action of the one stimulus modifies 

 that of the other. When, for example, Couvoluta is placed 

 in a tall glass cylinder with water, the animals rise to the 

 surface and mass themselves on the side toward the light.- 

 But if the light-conditions are modified so that the brightest 

 region is some distance below the surface — as, for instance, 

 by interposing a screen consisting of several plates of ground 

 glass or a black card between the source of light and the 

 top of the water, — Couvoluta at once releases its hold and 

 swims downward to take up its position just below the edge 

 of the glass screen, where the light is brightest. It sub- 

 ordinates its geotropic to its phototropic reaction. If, how- 

 ever, only one ground-glass plate is used, so that the difference 



