3 ORGANIC BEHAVIOUR 
therefore many are collected together they form a centre of 
the production of this substance ; and when other Paramecia 
come, in the course of their random movements, into such a 
Fic. 2.—Behaviour of Paramecia (after Jennings). 
centre they remain there and help to swell the numbers in the 
cluster. If Paramecia be placed in water to which a dis- 
tinctly reddish tinge is giveu by mixing it with asmall quantity 
