FORMATION OF NEW COLONIES OF MEGALOTROCH A. 189 



up or at some distance from it. Sometimes it is formed at the 

 end of a branch, sometimes at its base, or even on the main stalk ; 

 at other times in intermediate positions. If the swimming balls are 

 allowed to remain in a dish containing no water plants soon after 

 dark they begin to form a colony on the sides or bottom of the 

 dish. Apparently when the stimulus from the light is removed 

 the colony-forming instinct becomes dominant. When the young 

 balls are placed in artificial darkness, colony-formation begins in 

 from fifteen to thirty minutes and is usually completed within an 

 hour or an hour and a half. Most of these colonies are found in 

 well-lighted places. This doubtless arises from the fact that the 

 swimming balls remain on the lighted side of the vessel and tend 

 to form a permanent colony soon after this stimulus is removed. 

 This is of advantage to the animals for they feed partly on diatoms 

 and small protozoa which are more abundant in favorably lighted 

 places. 



Thus the young rotifers tend to remain attached in the swim- 

 ming ball during their natatory life, but at the proper time they 

 are able to sever this connection and to leave the ball one by 

 one. Possibly the stimulus due to the contact of their posterior 

 ends is sufficient to keep them together, until some stronger 

 stimulus, such as food, overcomes the contact stimulus and 

 induces them to separate. Hunger satisfied, they again respond 

 to the stimulus of mutual contact and assemble anew, this time 

 to form a permanent colony. 



With the formation of this permanent colony changes take 

 place both in the behavior and in the structure of the animals. 

 Up to this time the young rotifers have behaved in a very nervous 

 manner, constantly contracting and extending the body. But as 

 soon as the animals settle down in the permanent colony they 

 lose much of this irritability and remain with body and trochal 

 disc expanded for long periods. Yet even in the adult colony 

 one individual or another is frequently seen to suddenly fold the 

 trochal disc and strongly contract the body, then at once begin 

 to expand more slowly. Sometimes this contraction is evidently 

 a reaction to some floating particle that has touched the trochal 

 disc. More frequently there is no visible cause for the contrac- 

 tion. The animals in the permanent colony have another method 



