126 AN INTRODUCTION TO ZOOLOGY 



tentacle, which has captured the prey, bends toward the mouth 

 with its load of food. The other tentacles not only assist in this, 

 but may use their nematocysts in quieting the victim. The 

 mouth often begins to open before the food has reached it. The 

 edges of the mouth gradually inclose the organism and force it 

 into the gastrovascular cavity. The body wall contracts behind 

 the food and forces it down until it reaches the basal end of the 

 body. Here it remains during the process of digestion. Fre- 

 quently organisms many times the size of the Hydra are success- 

 fully ingested. 



REACTIONS TO FOOD. It is not uncommon to find Hydras that 

 will not react to food when it is presented to them. This is due 

 to the fact that these animals will eat only when a certain interval 

 of time has elapsed since their last meal. The physiological 

 condition of Hydra, therefore, determines its response to the food 

 stimulus. The collision of an aquatic organism with the tentacle 

 of Hydra is not sufficient to cause the food-taking reaction, since 

 it has been found that not only a mechanical stimulus, but also a 

 chemical stimulus must be present. A very hungry Hydra will 

 even go through the characteristic movements when it is excited 

 by the chemical stimulus alone. This has been shown by the 

 following experiment. When the tentacles and hypostome of a 

 moderately hungry Hydra are brought into contact with a piece 

 of filter paper, which has been soaked for a time in the same cul- 

 ture medium, there is no response. If the filter paper is then 

 soaked in beef juice and offered to the Hydra, the usual food 

 reactions are given. 



Beef juice alone calls forth no response in a moderately hungry 

 animal; but does inaugurate the normal reflex, if a very hungry 

 specimen is selected for the experiment. The conclusion reached 

 is that well-fed Hydras will not respond to either mechanical or 

 chemical stimuli when acting alone or in combination; that 

 moderately hungry animals will react to a combination of the 

 two, and that hungry animals will exhibit food-taking movements 

 even if a chemical stimulus alone is employed (123). 



