84 THE BIOLOGY OF THE ANIMAL 



small dot on the surface of the glass jar so as 

 to cover the point of attachment of each of sev- 

 eral hydras. Leave the jar undisturbed for a 

 time, then look to see if any of the animals 

 have changed or are changing position. If not, 

 quietly turn the jar a little, so that the light 

 may strike it from a slightly different direc- 

 tion, and examine again after a time. Have 

 any of the hydras moved? If so, in what di- 

 rection 1 Do all move ? Do they migrate at 

 the same time ? At the same rate ? To the 

 same distance from the original position ? In- 

 dicate the new position by a new set of marks, 

 as by circles or dashes, endeavoring to trace 

 the path of each animal. Turn the jar again 

 and repeat the examination. If the student 

 has time and inclination, let him watch to see 

 the manner in which the hydra moves from 

 place to place. Does it use the foot alone? 

 How rapid is its progression ? 



If the above experiment prove successful, 

 what would you say has caused the hydra to 

 change place ? 



The following method may also be tried to 

 induce the hydras to move : With a fine thread 

 suspend a small piece (about one-eighth of an 

 inch square) of raw beef against the inside of 

 the jar of water in which the hydras are kept. 

 If possible, hang the meat in the midst of a 

 group of the animals in such a position that 

 the meat will be an inch or more distant from 

 each hydra. Does the meat prove attractive? 

 If so, which animals move first, those above, 

 below, or at one side of the meat ? 



