312 ESSENTIALS OF ZOOLOGY 



from place to place. It keeps the tentacles extended, ready to grasp 

 any food which may come into its reach. Nematocysts or sting 

 bodies are discharged when the tentacle comes in contact with 

 potential food, and if it chances to be a small animal, it will likely 

 be paralyzed by the toxin which is injected by the nematocysts. 

 The prey is then carried to the mouth and tucked into it by the 

 tentacles. Frequentlj^ hydra is able to stretch its body over articles 

 of food which are actually larger than the hydra usually is in nor- 

 mal condition. Hydra will eat only when it is hungry and will not 

 react to food at other times. It is more sensible than many people 

 in this respect. On the other hand, it has been authentically re- 

 ported that a hungry hydra will perform the characteristic feeding 

 movements when only beef extract is in solution in the water. Thus 

 it responds to a chemical stimulus alone, but it will not respond to a 

 mechanical stimulus only. 



These animals show response to a number of environmental con- 

 ditions. Any sudden change is likely to bring about a negative 

 response. If the stimulus is of a general nature and of considerable 

 intensity, the animal will contract all of the tentacles, and the body 

 also. If the stimulus is restricted to one locality and is not too 

 strong, the animal will contract in the affected area, by the with- 

 drawal of one tentacle. 



The common tropismSj which have been described previously, are 

 present in hydras. They respond to light and will find an optimum 

 intensity which varies with the different species. Green hydras 

 react positively to sunlight and withstand moderate temperature; 

 hence they are adapted to the Southwest. They likewise possess 

 an optimum for temperature and prefer relatively cool water. As 

 pointed out previously above, both chemotropism and thigmotro- 

 pism are concerned in food-taking. Contact stimuli are of consider- 

 able significance in a sedentary animal like this. It remains at- 

 tached in contact with some solid body most of the time. Sudden 

 mechanical stimulation like stirring the water or jarring the attach- 

 ment of the animal will cause it to contract vigorously. 



Locomotion is accomplished in at least four ways. Gliding from 

 one point to another by partially releasing the basal disc and slipping 

 it to a new location is common. Or the animal may bend over and 

 cling to the substratum by the tentacles, release the basal disc, then 

 draw the body toward this point of the tentacles, where the basal disc 



