or nematocysts. These are able to shoot out micro- 

 scopic poison darts which paralyse the prey. The 

 tentacles then turn towards the mouth and the food is 

 passed inside. In some cases feeding is helped by the 

 beating of small hairs or cilia which cover the upper 

 surface of the polyp. The food is rapidly digested by 

 juices formed by the mesenteries. 



All reef-building corals contain within their tissues 

 a large number of very small single-celled plants known 

 as Algae. Since plants take in carbon dioxide from 

 their surroundings and give off oxygen the coral bene- 

 fits by their presence by obtaining a supply of oxygen 

 in its tissues for breathing. In the same manner the 

 Algae are able to use the carbon dioxide which is given 

 off continually in the breathing of all animal tissues. 

 The waste products of the coral are also useful to the 

 Algae. Thus the strange association of a plant living 

 inside an animal is mutually beneficial. 



The rate at which corals grow has been the subject 

 of a considerable amount of experiment. Results ob- 

 tained by Vaughan show that on the Florida and Ba- 

 hamas reef the increase in diameter varies from about 

 one half inch to nearly two inches per year. In twenty- 

 three years a brain coral measured in the Pacific by 

 Dr. Mayor had grown from thirty inches to seventy- 

 six inches. It is easy to appreciate, in the light of these 

 figures, that corals may, over a period of years, grow 

 into dangerous obstructions in channels once considered 

 safe and that they have built up considerable masses of 

 land during the long period of geological history. 



47 



