318 ESSENTIALS OF ZOOLOGY 



other cells of this layer. The general morphology of the adult 

 animal is very similar to the gastrula stage of the developing embryo 

 of more complex metazoans. 



Metabolism 



The food of hydra consists of small insect larvae, minute worms, 

 small bits of organic matter in the water, water fleas, and other 

 small Crustacea. Ingestion of the food has been described already. 

 Upon entering the mouth the morsel of food is moved some distance 

 down in the cavity by successive wavelike contractions of the column 

 progressing from distal to proximal. Such serial contractions are 

 usually called peristaltic contractions. Here in the upper half of the 

 enteron digestion takes place. The wall possesses many more of the 

 gland cells in the endoderm, and the food material disintegrates into 

 smaller particles here in this region. The digestion which occurs 

 here is spoken of as intercellular digestion and is brought about by 

 enzymes produced by the secreting cells of the endoderm. The dis- 

 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 adsorbed 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. 

 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. 



Many of the animals used as food have hard skeletal parts that 

 will not digest. These indigestible portions are ejected from the 

 cavity through the mouth by reverse peristalsis, and the process is 



