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TEXTBOOK OF ZOOLOGY 



solution of the food by the enzymes is augmented by the churning 

 effect of the contractions of the body. The flagella present on the 

 nutritive-muscular cells create currents of water which also hurry 

 the process. The dissolved material is presumably absorhed by the 

 cells of the endoderm, and by diffusion the nutrient solution reaches 

 the ectoderm cells just outside. Small particles of the partially di- 

 gested substance are engulfed by the free ends of many of the 

 nutritive-muscular cells by virtue of their amoeboid activity. These 

 particles are taken in food vacuoles, and the digestion is completed 

 there just as it is in an amoeba or Paramecium. This illustrates 

 something of the primitive organization of hydra as a metazoan. 







Fig. 73. — Hydra with body turned inside out in attempting: to Ingest a piece 

 of meat. (From Curtis and Guthrie, Textbook of General Zoology, published by 

 John Wiley and Sons, Inc.) 



As will be remembered, this process of converting the digested food 

 into an integral part of the protoplasm is known as assimilation. 

 The food is distributed to all parts of the enteron, which extends 

 into the tentacles and buds, by the action of the flagella and by 

 bodily contractions. There is no separate system of transportation 

 or circulation of nutriment. This dissolved material reaches the 

 remote parts of the protoplasm by diffusion through the membranes 

 and protoplasm generally. The gastrovascular cavity has the dual 

 function of digestion and circulation. 



