BEHAVIOR OF CCELENTERATA 219 



then they are readily swallowed. A chemical stimulation is a necessary 

 factor in producing the reaction. But under usual conditions the chemi- 

 cal alone — the meat juice — will not produce the food reaction. There 

 must be a combination of chemical stimuli (of the proper character) and 

 of mechanical stimuli before the reaction is induced. 



But when the Hydra is very hungry — when it has starved for a long 

 time — then a suitable chemical stimulus acting alone will produce the 

 food reaction. Placed in a solution of extract of beef the very hungry 

 Hydra opens its mouth widely and takes in the fluid. What seems 

 very remarkable is that a solution of quinine produces this effect as well 

 as does extract of beef (Wagner, 1905). 



Thus the food reaction is throughout dependent upon the physio- 

 logical condition of the Hydra. Hydras that are not hungry will not eat 

 at all ; moderately hungry specimens will take the solid food (chemical 

 and mechanical stimuli) ; very hungry ones take liquid food (chemical 

 stimulus alone). Hungry Hydras show still further modifications in 

 their behavior, compared with those that are not hungry. As we have 

 previously seen, they frequently contract and change to a new position 

 and even move about from place to place. Wilson (1891) records a 

 remarkable cycle of behavior in hungry yellow Hydras. Hydras usually 

 remain, as we have seen, in the upper layers of the water, on account 

 of the oxygen there found. But when the Crustacea on which the ani- 

 mals feed have become very scarce, so that little food is obtained, Hydra 

 detaches itself, and with tentacles outspread sinks slowly to the bottom. 

 Here it feeds upon the debris composed of dead organic matter which 

 collects at the bottom, often gorging itself with this material. It then 

 moves toward the light, and at the lighted side again upward to the 

 surface. Here it remains for a time, then sinks again and feeds upon 

 the material at the bottom. This cycle may be repeated indefinitely, 

 requiring usually some days for its completion. 



B. Food Reactions in Medusa 



The food reactions have been studied most carefully in Gonione- 

 mus. In this animal, as we have seen, there is a definite set of " fish- 

 ing" movements, having the function of obtaining food. These move- 

 ments are of course not direct reactions to food, but are, so far 'as food 

 is concerned, spontaneous movements of the animal. If food is brought 

 near a resting medusa, this sets the animal to moving. If a piece of 

 fish is placed at one side of the medusa, it does not move directly toward 

 the food, according to Yerkes (1902 a). After a few seconds the ten- 

 tacles nearest the food begin to move about irregularly, and this gives 



