238 



THE BEHAVIOR OF LOWER ORGANISMS. 



This method of trial and error is especially well developed in free- 

 swimming single-cell organisms — the flagellate and ciliate infusoria — 

 and in higher animals living under similar conditions, as in the Roti- 

 "^i^.. fera. In these creatures the structure and the 

 method of locomotion and reaction are such 

 as to seem cunningly devised for permitting 

 behavior on the plan of trial and error in the 

 simplest and yet most effective way. 



These organisms, as they swim through the 

 water, typically revolve on the long axis, and at 

 the same time swerve toward one side, which 

 is structurally marked. This side we will call 

 Jl. Thus the path becomes a spiral. The or- 

 ganism is, therefore, even in its usual course, 

 successively directed toward many different 

 points in space. It has opportunity to try suc- 

 cessively many directions though still progress- 

 ing along a definite line which forms the axis 

 of the spiral (see Fig. 79). At the same time 

 the motion of the cilia by which it swims is 

 pulling toward the head or mouth a little of the 

 water from a slight distance in advance (Fig. 

 79). The organism is, as it were, continually 

 taking "samples" of the water in front of it. 

 This is easily seen when a cloud of India ink 

 is added to the water containing many such 

 organisms. 



At times the sample of water thus obtained is 

 of such a nature as to act as a stimulus for a 

 motor reaction. It is hotter or colder than 

 usual, or contains some strong chemical in solu- 

 tion, perhaps. Thereupon the organism reacts 

 •^ in a very definite way. At first it usually 

 stops or swims backward a short distance, then 

 it swings its anterior end farther than usual 

 toward the same side X to which it is al- 

 ready swerving. Thus its path is changed. 

 After this it begins to swim forward again. The amount of backing 

 and of swerving toward the side X is greater when the stimulus is more 

 intense. 



Fig. 79. 



*FiG. 79. — Spiral path in the ordinary swimming of Paramecium, showing 

 how the anterior end is pointed successively in different directions, and how a 

 sample of water is drawn to the anterior end and mouth from each of these 

 directions. 



