HYDRA 139 



165. Behavior. — Hydras may be stationary for a time if conditions 

 remain uniform and food is plentiful, but with changing conditions in 

 the environment and the necessity of searching for food they usually 

 exhibit considerable locomotor activity. When hungry the hydra will 

 extend its body and tentacles, the latter being ready to grasp any food 

 which comes in contact with them. If after a time no food is encountered, 

 the animal moves to another location. 



Locomotion may be accomplished in several ways: (1) One method is 

 by a gliding movement, the basal disc sliding slowly over the substratum 

 to which the animal is attached and at no time being free. (2) The 

 animal may reach over with its tentacles and after attaching them may 

 release the basal disc, bring it up close to the tentacles, and attach it 

 once more, raising itself to an erect position in the new location. If 

 this is repeated it represents a type of locomotion similar to that of a 

 measuring worm. (3) Another way has been described as a modification 

 of this method. The disc is released, carried clear over, and attached 

 again beyond the tentacles, which causes the animal to turn a sort of 

 handspring. (4) Finally, if the animal is dislodged, it may drop to the 

 bottom and use its tentacles as if they were legs. When at the bottom 

 of a pool it may form a gas bubble on its basal disc and by means of this 

 rise to the surface. 



Hydras respond to several conditions in the environment. To strong 

 stimuli of any kind negative responses are given. To a nonlocalized 

 stimulus, which is one that affects the animal as a whole, it responds by 

 withdrawing its tentacles and contracting its body. To a localized 

 stimulation, such as the contact of any moderate stimulus with a single 

 tentacle or one particular point on the body, it responds by contracting 

 the area affected, which may cause the withdrawal of that single tentacle 

 or the bending of the body. But if the localized stimulus is a powerful 

 one, the other tentacles and the rest of the body will be involved, and the 

 reaction is then the same as that to a nonlocalized stimulus. 



These animals respond to an optimum of light, which varies v/ith 

 different species. The green hydra possesses an optimum at a high 

 degree of illumination, while the other species possess optima at a much 

 lower light intensity. Hydras also possess a temperature optimum which 

 is relatively low — that is, they flourish in cool water. They are found in 

 abundance under the ice in winter but perish at a temperature which 

 may be reached by a shallow pool exposed to the full warmth of the 

 summer sun. 



The response to chemicals depends upon the nature of the chemical. 

 The animal avoids injurious chemicals and responds positively to those 

 which indicate the presence of food. Chemotropism and thigmotropism 

 both figure in the food-taking reaction, the feeding movements being 

 much more vigorous if to the respon.se due to contact with the struggling 



