68 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



more than fifteen or twenty fathoms below the surface of the 

 water, so we find the reefs as fringing reefs lying attached to 

 some shore line, or as barrier reefs, lying a little further out and 

 separated from the land by an intervening lagoon. Or they 

 may be in the condition of atolls, which make up groups of small 

 coral islands, each surrounding a more or less circular or oval 

 lagoon. These coral islands are themselves often protected 

 by barrier reefs. The foundations upon which the coral 

 atolls rest are probably the summits of submarine mountains 

 which come to within 100 or 150 feet of the surface. On 

 such places coral polyps and many other kinds of marine 

 animals and plants grow, and accretions due to these and 

 other substances slowly raise the bank toward the surface 

 of the water, so that at low tide the tips of the growing 

 branches of the coral may be above the surface. As the 

 coral is broken and ground into fine bits by the action of the 

 waves and as other pieces are washed higher on top of these, 

 the island gradually rises above the level of the tides. The 

 waves continue to break up some of the coral into fine 

 particles, that, together with other debris, forms a little cal- 

 careous soil in which may germinate the seeds carried to it by 

 birds or ocean currents. With the growth and decay of vege- 

 table life the soil gradually becomes more fertile, until finally 

 the islands may become covered with a luxuriant plant growth 

 which in turn serves as the home of many insects and birds and 

 other animals. 



There are over 200 kinds of coral polyps known. Many 

 kinds are used for ornaments or decoration. The red coral, 

 which grows chiefly in the Mediterranean, is much used for 

 jewelry. The rose-pink coral is very valuable, some of the 

 finest kinds selling for hundreds of dollars an ounce. 



To the class Ctenophora (Gr. kteis, comb; phero, bear) 

 belong a few peculiar delicate, transparent, medusoid jelly- 

 fish, swimming by means of the rhythmical beating of several 

 rows of vibratile plates and not by means of the motion of 

 the bell or umbrella as in the case of other jelly-fishes. 



