6o 



BEHAVIOR OF THE LOWER ORGANISMS 



On the other hand, as we have seen in the preceding chapter, the 

 direction in which the animal turns in the avoiding reaction does not 

 depend on the side of the body stimulated. The animal turns toward 

 the aboral side as well when that side is touched, as when the oral side 

 receives the stimulus. 



The reactions which we have thus far described have the effect of 

 removing the animal from the object with which it comes in contact, so 

 that they may be called negative reactions. But under 

 certain conditions, not very precisely definable, Paramecium 

 does not avoid the object which it strikes against. On the 

 contrary it stops and remains in contact with the object. 

 This seems most likely to happen when the animal is swim- 

 ming slowly, so that it does not strike the object violently. 

 But this does not explain all cases ; many individuals seem 

 much inclined to come to rest against solids, while others do 

 not. Often all the individuals in a culture are thus inclined 

 to come to rest, while in another culture all remain free 

 swimming, and give the avoiding reaction whenever they 

 come in contact with a solid. Observing a single swim- 

 ming specimen, it is often seen to react as follows. When 

 it first strikes against an object it responds with a weak 

 avoiding reaction, swimming backward a short distance, 

 turning a little toward the aboral side, then swimming for- 

 ward again. Its path carries it against the object again, 

 whereupon it stops and comes to rest against the surface. 



The objects against which Paramecium strikes under normal con- 

 ditions are usually pieces of decaying vegetable matter or bits of bacte- 

 rial zooglcea. Remaining in contact 

 with these helps it to obtain food. The 

 cilia that come in contact with the 

 solid cease moving, and become stiff 

 and set, seeming to hold the Parame- 

 cium against the object (Fig. 45). 

 Often it is only the cilia of the anterior 

 end that are thus in contact and im- 

 movable; in other cases cilia of the 

 general surface of the body show the 

 same condition. Meanwhile, the cilia 

 of the oral groove continue in active 

 motion, so that a rapid current passes 

 from the anterior end down the groove 

 to the mouth (Fig. 46). This cur- 



FIG. 45. 

 Parame- 

 cium at rest 

 against a 

 cotton fibre, 

 showing the 

 motionless 

 cilia in con- 

 tact with the 

 fibre. 



FIG. 46. Paramecium at rest with 

 anterior end against a mass of bacte- 

 rial zooglcea (a), showing the currents 

 produced by the cilia. 



