146 PHYSIOLOGY 



a hydroid. This hydroid forms a colony which in 

 course of time gives rise to jellyfish, and so the process 

 repeats itself (Fig. 82). This process is called alterna- 

 tion of generations. 



Summary. — The coelenterata are symmetrical ani- 

 mals characterized by the absence of a distinct body 

 cavity, by the presence of a blind digestive tract, and 

 by the presence of only two layers in the body wall. 

 The third layer sometimes present is rudimentary and 

 may be jelly-like. 



Movement of the organism as a whole is produced 

 by the activity of contractile tissue similar to the mus- 

 cles of higher forms. In fixed forms this results in a 

 change of form and position; in free forms, of move- 

 ment from place to place. An especially irritable cell, 

 called the nettle cell, throws out a tube which contains 

 poison. This protects the animal from its enemies and 

 aids it in capturing food. 



Assimilation takes place in a tube with but one open- 

 ing to the outside. The oesophagus may project outward 

 as in the hydra, or it may be turned inward and be 

 held in place by septa as in the sea anemone. 



Reproduction is both sexual and non-sexual, and in 

 some species the sexual and non-sexual forms may alter- 

 nate in succeeding generations. 



In some forms a skeleton is present made by the 

 deposit of calcareous salts in the tissues. 



