BEHAVIOR OF CCELENTERATA 



191 



different ways. In the commonest method the animal places its free 

 end against the substratum, releases its foot, draws the latter forward, 

 reattaches it, and repeats the process, thus looping along like a measur- 

 ing worm (Fig. 117). In other cases it attaches itself by its ten- 

 tacles, releases its foot, and 

 uses the tentacles like legs. 

 A still different form of loco- 

 motion has been described, 

 in which the animal is said 

 to glide along on its foot; 

 how this is brought about is 

 not known. 



In sea anemones, rhyth- 

 mical contractions of the un- 

 disturbed animal have ap- 

 parently not been described. 

 But Loeb (1891, p. 59) finds 

 that Cerianthus if not fed 

 will after a time leave its 

 place in the sand and creep 

 about, finally establishing 

 itself in a new place. The 

 common sea anemone Me- 

 tridium moves about fre- 

 quently from place to place 

 on the sides or bottom of the 

 aquarium, and so far as can 

 be observed, this seems often 

 due simply to hunger or other 



intprnnl rnndiHrmt;- it nrrurs ^'°- "7-— Hydra looping along like a leech, 



mternal COnaitlOnS, it occurs ^^^^ Wagner (1903). 1-6, Successive positions. 



under apparently uniform 



external conditions. A common method of movement in sea anemones 

 is to glide about on the foot, — the lower surface of the foot sending 

 out extensions and moving in a manner similar to that of the foot of 

 mollusks. There are doubtless other methods of locomotion. 



The spontaneous contraction and change of position which plays a 

 subordinate part in fixed forms has become the rule in medusae. They 

 are commonly found swimming about by means of rhythmical contrac- 

 tions. Since there are no corresponding changes in external condi- 

 tions, these contractions must be due to internal changes. The 

 internal changes need not of course be themselves of a rhythmical 

 character. They may take place steadily, inducing a contraction only 



