CCELENTERATA 141 



Through the contraction of the tentacles the food is 

 forced into the cavity. Partial digestion takes place 

 there. Non-nutritive substances pass off through the 

 opening; the nutritive portions are engulfed by the 

 entodermal cells, which have the power of sending out 

 ameboid branches. Complete digestion follows in the 

 entodermal cells, and the products pass from cell to 

 cell by osmosis. (Fig. 75.) 



As the digestive tract extends to all parts of the body 

 food is distributed without the intervention of a special 

 circulatory apparatus. It therefore performs the office 

 usually performed by blood vessels. 



Breathing takes place through the outer cell walls. 

 Oxygen dissolved in the water passes by osmosis into the 

 cells and carbon dioxide formed in the cells passes into 

 the surrounding water. 



Irritability. — In addition to the ameboid motion of 

 the entodermal cells and the spring-like motion of the 

 nettle cells, the cells of both ectoderm and entoderm 

 send out contractile branches which foreshadow the 

 muscle cells in higher forms. The simultaneous con- 

 traction of these branches forces the animal to contract 

 as a whole or allows a curious form of locomotion from 

 place to place. 



Some forms move about freely, some are fixed firmly, 

 and some are able to hold fast to a support by suction. 

 The jellyfish moves by the rhythmical contraction of 

 its swimming bell. The bell relaxes and the hollow 

 becomes filled with water, it contracts and the water is 

 forced out. As this occurs rhythmically the animal 

 moves jerkily through the water. The sea anemone at- 



