REACTIONS TO HEAT AND COLD. 1 7 



backward, circling at the same time toward the right or aboral side, as 

 shown in Fig. 7. This lasts but a moment ; then the animal swims 

 forward, at the same time turning to the right or aboral side. That is, 

 the individuals give the typical motor reaction, as described in the fifth 

 of my studies (Jennings, 1900). This reaction is repeated many 

 times, as long indeed as the animal remains in the heated region. But 

 of course this movement scatters the animals rapidly. Those that 

 strike against the end or sides of the trough repeat the reaction above 

 described, backing, turning to the right, then going forward (Fig. 7 at 

 8, 9, 10, n). They thus become directed in some other way. Those 

 that are directed away from the heated region pass into cooler water 

 and hence no longer give the reaction, but continue their course (Fig. 

 7 at 13, 14). The result is that the individuals which swim away from 

 the heated end continue their course, while those starting in any other 

 direction are stopped and turned (through the motor reaction), until 

 they too get started away from the heated region. Thus after a time 

 there is a steady stream of organisms swimming or creeping away from 

 the heated end, while there is no regular movement in any other 

 direction. In this manner arises the orientation of the animals, with 

 anterior ends directed away from the heated region. 



The movements of the individuals are exactly as above described 

 even when the heat is applied some distance from the region where 

 the animal is found and gradually approaches it from one side. The 

 animal by no means turns directly away from the heated region, but 

 repeatedly gives the backing and turning reaction till it is finally mov- 

 ing in a direction which takes it out of the heated region. 



How is this continued backing and turning to be accounted for on 

 the theory of direct action on the locomotor organs of the two sides as 

 maintained by Mendelssohn? This author speaks in the case of Para- 

 mecium merely of "disordered" movements when the reaction first 



animal reacts by turning to the right and backing (i, 2, 3), turning again (3-4), 

 swimming forward (4-5), backing (5-6), turning again to the right (6-7), etc., 

 till it comes against the wall of the trough (8). It then reacts as before, by 

 backing (8-9), turning to the right (9-10). This type of reaction continues as 

 long as the Oxytricha is in the heated region, or as long as its movements carry 

 it either against the wall or into the heated region. When it finally becomes 

 directed away from the heated region (13), as it must in time if it continues its 

 reactions, it swims forward, and since it is no longer stimulated, it no longer 

 reacts. When large numbers of animals react in this way, in the course of 

 time nearly all become pointed in the same direction, as at 13 or 14, so that a 

 marked "orientation" is produced. Thus orientation is produced by "ex- 

 clusion," due to the fact that the organism is prevented, either by the heat or 

 the walls of the trough, from swimming in any other direction. 



