68 



BEHAVIOR OF THE LOWER ORGANISMS 



Now, the Paramecia gather in the solution of carbon dioxide pro- 

 duced by themselves, just as in that due to other causes. In this way 

 dense spontaneous groups are formed, in which the phenomena seen 

 in the collections about bubbles of carbon dioxide are reproduced. If 

 a large number of Paramecia are mounted in water on a slide, they do 

 not remain scattered, but soon gather in one or more regions (Fig. 52). 

 Within such groups the individuals move about in all directions. On 

 coming to an invisible outer boundary, they give the avoiding reaction 

 in a mild form, so that they do not leave the group. The area covered 

 by the group does not remain of the original size, but slowly enlarges, 

 as shown in Fig. 52. It continues thus to increase in size until it covers 

 the whole preparation. 



By the use of proper indicators it can be shown that such spontane- 

 ous groups contain an acid, and this is beyond doubt due to the carbon 



dioxide known to be produced in res- 

 piration. The groups are formed in 

 the following way. Two or three Para- 

 mecia by chance strike against some 

 small, loose object, a roughening of the 

 surface of the glass, or the like, and 

 come to rest, in the way described in 

 our account of the reaction to mechan- 

 ical stimuli. They of course produce 

 carbon dioxide, which diffuses into the 

 surrounding water. Other Paramecia 

 that swim by chance across this area of 

 carbon dioxide of course stop and re- 

 main. They too produce carbon diox- 

 ide, so that the area grows in size ; more 

 Paramecia enter it, and finally a large 

 and dense collection is formed. The 

 area occupied by such a collection con- 

 tinually increases in size, because the 



FIG. 52. Spontaneous groups formed Paramecia continue to produce carbon 



by Paramecia. A, B, C, successive stages ,. ., , ,. . ,.~. 



in the spreading out of such groups. dlOXlde, and this Continues to dltlUSC 



through the water. 



The tendency of Paramecia to gather in regions containing carbon 

 dioxide plays a large part in their life under natural conditions, and 

 this, together with the fact that they themselves produce carbon dioxide, 

 explains many peculiar phenomena in their behavior. When placed in 

 tubes or vessels of any kind, Paramecia usually show a tendency to 

 collect into groups or clouds, having a definite boundary (Fig. 53). 



