46 BEHAVIOR OF THE LOWER ORGANISMS 



similar but simpler conditions. On the trackless snow-covered prairie 

 the traveller wanders in circles, try hard as he may to maintain a straight 

 course, — though it is possible to err only to the right or left, not up or 

 down, as in the water. Paramecium meets this difficulty most effec- 

 tively by revolution on the axis of progression, so that the wandering 

 from the course in any given direction is exactly compensated by an 

 equal wandering in the opposite direction. Rotation on the long axis 

 is a device which we find very generally among the smaller water or- 

 ganisms for enabling an unsymmetrical animal to follow a straight 

 course. The device is marvellously effective, since it compensates with 

 absolute precision for any tendency or combination of tendencies to 

 deviate from a straight course in any direction whatsoever. 



The normal movements of Paramecium are adaptive in another 

 respect. The same movements of the cilia which carry the animal 

 through the water also bring it its food. The oral cilia cause a current 

 of water to flow rapidly along the oral groove (Fig. 33). In the water 

 are the bacteria upon which Paramecium feeds ; they are carried by 

 this current directly to the mouth. In the gullet is a vibrating membrane 

 which carries particles inward; the bacteria which reach the mouth 

 are thus carried through the gullet to the endosarc, where they form 

 food vacuoles and are digested. 



Not only food, but also other substances, may be brought to Para- 

 mecium by the currents due to the movements of the cilia. It is im- 

 portant for understand- 

 •V ;":'•'.;;•., ing the behavior of this 



;£&•&$& animal to realize that 

 ••:^:v^. not only does it move 

 J? ."'^ forward to meet the en- 



£•&&$& vironment, but the en- 

 ';.. :;'••; '.'•': '.... ''''■}&&''$ vironment, so far as that 

 fc;.' . -•' V.-'' : i s possible, also streams 



. . - ..'•. £$0 backward to meet it. 



" , ' .'' If there is a chemical 



'&&0&" diffusing in the water in 



Fig. 35. — Paramecium approaching a region contain- font Ot It, OT II tne water 

 ing India ink (shown by the dots). The India ink is drawn {§ warmer Or Colder, OF 

 out to the anterior end and oral groove of the animal. ..„ . , i 



differs in any other way, 

 a sample of this differentiated region is pulled backward in the form 

 of a cone, and as a result of the stronger beating of the oral cilia, 

 passes as a stream down the oral groove to the mouth (Fig. 33). This 

 may best be seen by bringing near the anterior end of a resting Parame- 

 cium, by means of a capillary pipette, some colored solution, such as 



