138 METAZOAN PHYLA 



164. Metabolism. — The food of the hydra consists of any animal 

 sufficiently small and weak that it may be held by the tentacles, anaes- 

 thetized by the poison of the nematocysts, and brought to the mouth 

 into which it is passed. Small insect larvae and crustaceans form the bulk 

 of the food. After being ingested the food is digested in the enteron by 

 means of enzymes formed by the gland cells of the entoderm. This type 

 of digestion, which is met here for the first time, is termed extracellular 

 digestion. The hydra also possesses intracellular digestion, particles of 

 food being taken into the entoderm cells by means of pseudopodia and 





Fig. 57. — Nerve net of Hydra oligactis Pallas. (From Rogers, "Text-hook of Comparative 

 Physiology," by the courtesy of McGraw-Hill Book Company, Inc.) Highly magnified. 



digested within food vacuoles. During the process of extracellular diges- 

 tion the food is carried about in the enteron and mixed with the digestive 

 juices; this circulation is due both to the movements of the body and to 

 the currents formed by entodermal flagella. After digestion the food is 

 absorbed into the entodermal cells and circulated by being passed from 

 cell to cell. Secretion is performed by certain cells in both the ectoderm 

 and the entoderm. Excretion is carried on by each cell for itself; and 

 since the body consists essentially of two layers of cells, elimination and 

 excretion become one process. Respiration, also, is carried on by each 

 cell individually. Egestion takes place through the mouth, which thus 

 functions also as an anal opening. 



