190 



BEHAVIOR OF THE LOWER ORGANISMS 



the movements are due to internal changes of some sort, like those of 

 Vorticella. The same behavior may be produced, as we shall see later, 



by external stimuli. 

 In the yellow Hy- 

 dra such move- 

 ments do not occur 

 — at least not with 

 such frequency. 



Fig. 114. — Spontaneous changes of positions in an undis- 

 turbed Hydra. Side view. Tlie extended animal (i) contracts 

 (2), bends to a new position (3), and then extends (4). 



Fig. 115. — Dia- 

 gram of different posi- 

 tions taken by Hydra, as 

 seen from above. After 

 Wagner. 



This is apparently correlated with the fact that the yellow Hydra has 

 very long tentacles, which he in coils all about it, so that exploratory 

 movements are not necessary in order to reach such food as may be 



found in the neighborhood. 



If a green Hydra is left for long 

 periods undisturbed, it does not 

 remain attached in the same posi- 

 tion, but moves about from place 

 to place. The movements often 

 take place in random directions, — 

 the animal starting first in one direc- 

 tion, then in another. Figure ii6 

 shows the movements of a green 

 Hydra, which was left alone for some 

 days in the bottom of a large, clean 

 glass dish, the light coming from a 

 window at the right. This move- 

 ment is probably brought about by 

 Path followed by a green hunger — the animals taking a new 



Hydra that was left for some days undisturbed position when food bcCOmCS SCarce. 

 on the bottom of a clean glass dish. After „ 1 1 • 1 



Wagner (1905). Hydra may move about in several 



