PHYLUM OELENTERATA 113 



from escaping. Food is carried to the mouth by the bending 

 over of the tentacle which captured it; other tentacles also assist. 

 The mouth opens and slowly moves around the food, which is 

 then forced down to the basal end of the gastrovascular cavity 

 by the contraction of the body-wall behind it. 



Hydras will not capture prey or respond to food stimuli when 

 they have recently been fed. Moderately hungry specimens 

 will exhibit the characteristic food-taking reactions if both 

 chemical and physical stimuli are applied at the same time, e.g., 

 a piece of filter paper soaked in beef juice. A hungry animal 

 will respond by making swallowing movements when a chemical 

 stimulus alone is applied. 



Digestion takes place in the gastrovascular cavity and probably 

 also within the entoderm cells. The gland cells of the entoderm 

 secrete a fluid into the gastrovascular cavity ; this fluid dissolves 

 the food. Digestion is aided by the currents set up by the 

 flagella of the entoderm cells and by the churning resulting from 

 the expansion and contraction of the body. Part of the food 

 is evidently engulfed by the pseudopodia of the entoderm cells 

 and undergoes intracellular digestion. The dissolved food is 

 absorbed by the entoderm cells ; part of it, especially the oil glob- 

 ules, is passed over to the ectoderm, where it is stored until 

 needed. 



BEHAVIOR. - - Hydras are usually found attached to the bot- 

 tom or sides of the aquarium, or to aquatic plants, or are sus- 

 pended from the surface film of the water. The position of rest, 

 with the body stretched out and the tentacles widely spread, 

 allows the animal to obtain food from a considerable area. At 

 intervals of several minutes an undisturbed Hydra, especially if 

 hungry, will contract rapidly and then slowly expand in a new 

 direction, as shown in Fig. 69. This brings it into a new part of 

 its surroundings, where more food may be present. Finally, 

 these spontaneous movements cease, and the animal moves to 

 another place. 



Locomotion is known to be effected in three ways. Usually 

 I 



