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TWO-LAYERED ANIMALS— PHYLUM COELENTERATA 



positive protection in the dull surroundings found in temperate waters 

 since it tends to blend the animal with the background and to make it less 

 easily seen and consequently less easily caught by some larger form. On 

 the other hand, such a dull colored animal placed among the brilliant 

 rock, coral, and seaweeds of the tropics would stand out like a black 

 sheep, so we find that the tropical forms have taken on brilliant hues 

 which cause them to be better adapted to their surroundings. 



There is another small polyp form in this class, not much larger than 

 hydra, yet it is an architect that has constructed some of the largest 

 structures that have been produced by any animals since time began, 



Photo by Winchester 



Fig. 9.9. A group of sea anemones with their spreading tentacles is a beautiful sight, 



resembling an underwater flower garden. 



including man. This is the little coral polyp, not much more than a half 

 an inch long yet building islands and reefs of tremendous size, beside 

 which the pyramids of Egypt are insignificant by comparison. The 

 great barrier reef which runs along the eastern shores of Australia is 

 more than 1,200 miles long, yet it was built entirely by the patient efforts 

 of these little animals. They naturally have soft bodies and secrete little 

 protective cups of limestone around themselves and, as they bud and 

 grow and secrete more limestone, they gradually build up a sizeable struc- 

 ture. Growth of a mass of coral may be no more than a half an inch a 

 year ; yet, in the eons of time in which they have been growing, the huge 



