192 BEHAVIOR OF THE LOWER ORGANISMS 



when a result of a certain intensity has been reached (see Loeb, 1900, 

 p. 21). 



In the small medusa Gonionemus there is under natural conditions a 

 cycle of activity that is of great interest ; it has been well described by 

 Yerkes (1902, a and b, 1903, 1904) and Perkins (1903). At times the 

 animal is found attached by certain adhesive pads on its tentacles to 



FIG. 1 18. Young Gonionemus resting on the bottom, with the opening of the bell upward. 

 After Perkins (1903). 



the vegetation of the bottom or to other surfaces (Fig. 118). Leaving 

 its attachment, it swims upward to the upper surface of the water, the 

 convex surface of the bell being upward, and the tentacles contracted 

 (Fig. 119). Reaching the upper surface it turns over 

 " and floats downward with bell relaxed and inverted, 

 and tentacles extending far out horizontally in a wide 

 snare of stinging threads which carries certain destruc- 

 tion to creatures even larger than the jellyfish itself 

 (Fig. 120) " (Perkins, 1903, p. 753). Reaching the 

 FIG. IIQ. bottom, it swims again to the top and repeats the 

 P*ess. * may thus continue this process of fish- 

 contracted tentacles, ing," as Perkins calls it, all day long. It is chiefly in 



(I9 3) " this way that it captures its food. 



This cycle of spontaneous activities is in some respects similar to 

 that of the green Hydra described above, though much more complex. 

 Both illustrate the fact that complex movements and changes of move- 

 ment may occur from internal causes, without any change in the environ- 

 ment. 



2. CONDITIONS REQUIRED FOR RETAINING A GIVEN POSITION; 

 RIGHTING REACTIONS, ETC. 



Hydra and the sea anemones tend to retain a certain position; we 

 usually find them at rest with foot attached and head free. This usual 

 position is often said to be due to a reaction to gravity or to contact, or 



