THE HYDRA AS A SIMPLE METAZOAN 263 



tact with the surface, release the foot, draw it to a new point of 

 attachment, and extend the tentacles once more, repeating this 

 process after the manner of caterpillars that move by "looping 

 along." Again, the hydra may attach the tentacles, release the 

 base, and move by slow somersaults. Under an increasing stimu- 

 lus of hunger, these forms of locomotion appear in the order named. 

 In some cases, the animal crawls clumsily upon its tentacles with 

 the body free and contracted. Sometimes hydras float about and 

 may be collected from the open water by a fine net in pond or 

 aquarium. 



Failing to find food after a long search, or under other unfavor- 

 able conditions, hydras may become contracted and remain with- 

 out movement in one place. If this condition continues, the ten- 

 tacles maj be resorbed and later regenerated unless the unfavor- 

 able conditions result in death. Hungry h3'dras may go to the 

 bottom of an aquarium and gorge themselves with ooze. This 

 seems to be an alternative method of feeding not commonly 

 recognized. 



Feeding. — In feeding, the activities of green and brown hydras 

 differ markedly. If a green hydra that has not fed for a time is 

 carefully observed, it will be seen to extend its body and tentacles 

 and remain in one position for only a short period. Failing 

 to secure food, the body is contracted and reextended at a new 

 angle. This process is repeated until the area within reach of any 

 one point of attachment has been irregularly " covered." The 

 almost imperceptible ghding on the base may occur during these 

 activities and thus new territory may be reached. If no food is 

 captured by these lesser activities, the hydra begins the looping 

 movements (Fig. 120), and if still unsuccessful it begins to somer- 

 sault, finally becoming quiescent if its quest is useless. In con- 

 trast with this, the brown and gray species of hydra, which have 

 longer tentacles, remain motionless for considerable periods with 

 all the tentacles widely extended, much as a party of fishermen 

 might set out a number of lines in all directions from a boat and 

 wait for a catch. If no food is secured the}^ begin to move clumsily 

 after the same general manner as the green hydras. 



The function of the nematocysts in paralyzing the prey has 

 been described. When a water-flea or some such organism has 

 thus been caught by a tentacle, the remaining tentacles usually 

 " go into action " in a coordinated fashion, although it is possible 



