76 BEHAVIOR OF THE LOWER ORGANISMS 



move across it to the opposite side. Reaching this, they at first con- 

 tinue the course by swimming obliquely downward, to the point x. 

 Here the reaction occurs; the animals turn around and swim upward 

 again. Studying the movements of the Paramecia at this point, one ob- 

 serves that the forward motion becomes slower, while the spiral course 

 becomes wider. The animals swerve more strongly than usual toward 

 the aboral side, so that the anterior end swings about in a circle, as in 

 Figs. 37 and 38. Thus the animals are giving the avoiding reaction, 

 "trying" successively many different positions. This is continued or 

 repeated till after a time they come into a position with anterior end 

 upward. The strong swerving then ceases; the animals swim upward 

 in the usual spiral course. 



The position of individuals at rest against a solid is usually quite 

 independent of gravity. The body axis may be placed at any angle with 

 the pull of gravity, with either end higher. The contact reaction inter- 

 feres with the reaction to gravity, preventing it almost completely. Yet 

 there is a tendency, even when in contact with a solid, to take a position 

 with anterior end above. If Paramecia are placed in clean water in a 

 clean, upright glass tube, in the course of time many individuals come to 

 rest against the perpendicular walls. It will now be found, in some 

 cases, that a considerable portion of the animals, though by no means 

 all, are resting with the body axis nearly in line with gravity and with 

 anterior end upward. When a swimming individual places itself in con- 

 tact with the wall, it is often seen to make a sudden turn toward the 

 aboral side, just as it comes to rest, till the anterior end is upward ; then 

 it remains in that position. The proportion thus oriented with reference 

 to gravity is in some cultures sufficiently great, amounting perhaps to 

 half the individuals, to show that the position is not accidental. In 

 other cultures there may be almost no indication of any influence of 

 gravity on the position of the attached specimens. 



The precise nature of the determining factor in the reaction to gravity 

 is very obscure. Jensen (1893) held that the reaction is due to the dif- 

 ference in pressure between the upper and lower portions of the organism. 

 The cilia on the side where the pressure was greatest (the lower side) 

 were supposed to beat more rapidly, thus turning the animal directly 

 upward. But, as we have seen above, exact observation of the move- 

 ments of the individuals shows that the reaction does not take place in 

 this way. Moreover, the difference in pressure between the two sides 

 of the organism is in certain reacting infusoria only one millionth of the 

 total pressure, and this difference seems beyond question too slight to 

 act as an effective stimulus. 



Davenport (1897, p. 122) held that the reaction to gravity is due to 



