THE SIMPLEST MANY-CELLED ANIMALS 339 



place rapidly when there are no living coral-animals to keep 

 adding to the coral rock ; and so the central rock might be 

 worn away, leaving a ring of living animals continually 

 adding new coral rock to the outside. 



292. Economic Relation of Coelenterates. There are 

 probably more than three thousand species of ccelenterates, 

 but with the exception of coral-animals, none of them are 

 of direct economic importance. Some of them serve as foods 

 for animals of the sea, e.g., small fishes may be seen biting oft 7 

 the branches of hydroid colonies; but the large amount of 

 water in the larger forms makes them of little food value 

 to other animals. However, the ccelenterates prove that 

 human interest is not limited simply to those things which 

 have a money value or are good to eat, for the various forms 

 of hydroids, jelly-fishes, comb-jellies, and sea-anemones have 

 always been great favorites with both amateur and profes- 

 sional naturalists. The great importance of the coral- 

 animals is, of course, that of forming islands and reefs which 

 protect the shores of other islands. Also, there is the red 

 coral, already mentioned as valuable for ornaments. 



Classes of Ccelenterata 



Hydrozoa Hydra, hydroids, small medusa. 

 Actinozoa sea-anemone, coral-animals. 

 Scyphozoa large medusae (jelly-fishes). 

 Ctenophora ctenophores (comb-jellies). 



