HYDRA AND CCELENTERATES IN GENERAL 127 



DIGESTION. Immediately after the ingestion of food the gland 

 cells in the entoderm show signs of great activity; their nuclei 

 enlarge and become granular. This is due probably to the forma- 

 tion of enzymes which are discharged into the gastrovascular 

 cavity and begin at once the dissolution of the food. The action 

 of the digestive juices is made more effective by the churning of 

 the food as the animal expands and contracts. The cilia extend- 

 ing out into the central cavity also aid in the dissolution of the 

 food by creating currents. This method of digestion differs from 

 that of Ameba and Paramecium in being carried on outside of the 

 cell; i.e. extracellular. There is evidence that intracellular 

 digestion also takes place in Hydra; the pseudopodia thrust out 

 by the entoderm cells seize and engulf particles of food which are 

 dissolved within the cells. However, most of the food is di- 

 gested in the gastrovascular cavity. The digested food is 

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

 globules, is passed over to the ectoderm, where it is stored. 



EGESTION. All insoluble material is egested from the mouth. 

 This is accomplished by " a very sudden squirt " which throws 

 the debris to some distance (123). 



Behavior. Hydra mridis gives a more prompt and decisive 

 response when stimulated than any other species of Hydra, and 

 for this reason its behavior has be n studied more thoroughly 

 than that of the others. The following paragraphs have been 

 compiled largely from experiments upon green Hydras, although 

 enough work has been done with other forms to prove that their 

 reactions are practically the same, only more sluggish. 



NORMAL POSITION OF HYDRA. Hydras maybe found attached 

 to the sides or bottom of an aquarium, to parts of water plants, 

 or hanging from the surface film. Usually they are near the top 

 where more oxygen can be obtained from the water than at greater 

 depths. If attached to the bottom, the body is usually held 

 upright; if to the sides, the body is in most cases horizontal, 

 the hypostome generally being lower than the foot; and if to 

 the surface film, the body is allowed to hang directly downward. 



