44 NATHAN FASTEN 



but simpler conditions. On the trackless, snow-covered prairie, 

 the traveler wanders in circles, try hard as he may to maintain 

 a straight course, though it is possible to err only to the right 

 or left, not up or down as in the water. Paramecium meets 

 this difficulty by revolution on the axis of progression, so that 

 the wandering from the course in any given direction is exactly 

 compensated by an equal wandering in the opposite direction. 

 Rotation on the long axis is a device which we find very gen- 

 erally among the smaller water organisms for enabling an un- 

 symmetrical animal to follow a straight course. The device is 

 marvellously effective, since it compensates with absolute pre- 

 cision for any tendency or combination of tendencies to deviate 

 from a straight course in any direction whatsoever." 



'The normal movements of Paramecium are adaptive in 

 another respect. The same movements of the cilia, which carry 

 the animal through the water, also bring it its food. Thus 

 Paramecium is continually receiving samples of water in front 

 of it. Since in its spiral course the organism is successively 

 pointed in many different directions, the samples of water it 

 receives likewise come successively from many directions. The 

 animal is given opportunity to try the various different condi- 

 tions supplied by the neighboring environment." 



In the case of Lernaeopoda, we meet with a similar condi- 

 tion. The organism, like Paramecium and the other Infuso- 

 rians, circles through the water in its characteristic way, "try- 

 ing" the water, so to speak, in every direction. Of course, 

 the animal being a parasite throughout most of its life, must 

 come in contact with the proper host in order to carry on its 

 further development. In other words, the copepod, like Para- 

 mecium, must meet its food in order to exist. 



It is questionable whether the larva perceives objects through 

 its ocellus. Even if it did, however, the visual range would, 

 in all probability, be so short that the organism could not see 

 its host in the water. It thus becomes a question of the copepod 

 searching out its host by random movements, or perishing. 

 Chance plays the greatest part in its ever meeting the host. 

 The movements of the animal, therefore, are apparently adap- 

 tive. The copepod darts up and down, circles in this direction 

 and in that. Its movements bring it into as many different 

 localities as possible. If one path happens to bring it in con- 



