Jennings, Behavior of Paramecinni. 475 



tion with anterior end upward. The strong swerving then 

 ceases ; the animals swim upward in the usual spiral path. 



Thus, observation shows that the reaction is not brought 

 about in accordance with the tropism schema, as was supposed 

 by Jensen. The animal does not turn directly into orientation, 

 as that theory requires, but the turning is throughout toward the 

 aboral side, and the orientation is attained by the "method of 

 trial and error." 



What is the cause of the reaction ? Jensen's theory that 

 it is the difference in pressure on the two sides of the animal 

 loses whatever plausibility it may have 'had, when the nature of 

 the reaction itself is known. As we have seen, the turning in 

 the reaction is not due to differential action on upper and lower 

 sides, but to swerving toward a side that is structurally defined 

 — the aboral side — whatever the position of the latter with 

 reference to gravity. Thus the difference in pressure certainly 

 does not act in the direct way supposed by Jensen. 



Furthermore, as we have seen above (p. 467), in no other 

 reactions of Paramecium is the difference in intensity of a grad- 

 uated stimulus on the two sides or ends of the animal known 

 to be the determining factor in the reaction. 



On many other grounds it is highl}Mmprobable that this dif- 

 ference in pressure is the effective agent. The difference in 

 pressure between the two sides is so excessively minute in pro- 

 tion to the total pressare acting on the animal, that it is almost 

 inconceivable that this difference should be perceived. The in- 

 fusorians are of course under atmospheric pressure ; this is 

 equal to the pressure of a little more than 10,000 millimeters 

 of water. As Jexse.x shows, the difference in pressure between 

 the two sides of certain of the infusoria which show the reac- 

 tion to gravity is only that of o.oi mm. of water. Hence the 

 difference in pressure between the two sides of the organism is 

 o^^ly i,oou HUB <^f ^^^^ pressure acting everywhere on the surface. 

 Furthermore, Jensen showed that the reaction still occurs when 

 the atmospheric pressure is more than doubled ; the effective 

 difference in pressure would then be less than 3 ,^^,, ,„jq the general 



