70 CCELEN TERA TES 



55. Economic Importance. — The corals alone of the 

 coelente rates are of economic importance ; they add to 

 many islands, protect others from being washed away, and 

 in some cases form entirely new islands. 



SUMMARY 



The hydra-like animals represent an advance in the 

 division of labor. The layers of their bodies are more 

 definite and do their work better than in the sponges. 

 Hydroids and the corals illustrate the formation of a 

 colony. In some of the colonies the division of labor is 

 more extensive than in others. The economic importance 

 of the corals has been, and continues to be, very great. 



QUESTIONS 



Explain fully how the hydra gets its food and how some of this food 

 finally nourishes the ectoderm cells. Compare the hydra and the hydroid. 

 In what are they alike ? In what are they different ? How does the 

 hydra reproduce ? How does the hydra get its oxygen ? Explain how 

 the coral animal has been able to form islands. 



REFERENCES 



Darwin, Structure and Distribution of Coral Reefs. 

 Hegner, Introduction to Zoology, Chapter VIII. 



