THE BEHAVIOR OF INFUSORIA; PARAMECIUM 6l 



rent of course carries many of the bacteria found in the zoogloea or 

 on the decaying plant tissue ; these serve as food for the animal. The 

 cilia of the remainder of the body usually strike only weakly and 

 ineffectively, so that the currents about the Paramecium are almost all 

 due to the movements of the oral ciha. The body ciha directly behind 

 those in contact with the sohd are usually quite at rest. 



The function of this positive contact reaction is evidently, under 

 ordinary conditions, to procure food for the animal. But Paramecium 

 shows no precise discrimination, and often reacts in this way to objects 

 that cannot furnish food. Thus, if we place a bit of torn filter paper 

 in the water containing the animals, we often find that they come to 

 rest upon this, gathering in a dense group on its surface, just as they do 

 with bits of bacterial zoogloea (Fig. 47). The oral cilia drive a strong 

 current of water to the mouth, as usual, but this bears no food. To 

 bits of thread, ravellings of cloth, pieces of sponge, or masses of pow- 

 dered carmine, Paramecium may react 

 in the same way. In general it shows a 

 tendency to come to rest against loose or 

 fibrous material; in other words, it re- 

 acts thus to material with which it can 

 come in contact at two or more parts of 



the body at once. To smooth, hard Fig. 47. — Paramecia gathered 



materials, such as glass, it is much less '" ^ ''^'^ '"^^^ ^'^°"' ^ ""'^ °^ ^^^^' 



' ° ' paper. 



likely to react in this manner, so that it 



clearly shows a certain discrimination in this behavior. These hard 

 substances, it is evident, are less likely to furnish food than the soft 

 fibrous material to which Paramecium reacts readily. But under cer- 

 tain conditions Paramecium comes to rest even against a smooth glass 

 surface, or against the surface film of the water. Specimens are often 

 found at rest in this manner in the angle between the surface film of a 

 drop of water and the glass surface to which it is attached. 



Paramecia often behave in the manner just described with reference 

 to bodies of very minute size, — to small bits of bacterial zooglcea, or 

 to a single grain of carmine. Such objects are of course too small to 

 restrain the movements that naturally result from the activity of the oral 

 ciHa in the contact reaction. These cilia continue to beat in the same 

 manner as when the object is a large one, producing currents similar to 

 those shovm in Fig. 46. This ciKary motion of course tends to drive 

 the animal forward, and since all the active cilia are on the oral side, it 

 tends also to move the animal toward the aboral side. The resultant 

 of these two motions at right angles is movement in the circumference 

 of a circle. The animal moves in the lines of the water currents shown 



