APPLIED BIOLOGY 



secrete skeletons. The final result of oft-repeated budding 

 is a complicated calcareous mass, in tree-like or hemispheri- 

 cal form, with numerous cups representing the number of 

 polyps which took part in the formation. 



The sea-fans and sea-plumes are coral skeletons formed 

 in another way. Their outer surfaces are calcareous, and 

 their central axis is composed of a horny material. This is 



black in many species, but red 

 in the case of the precious coral 

 from the Mediterranean, which 

 is used in jewelry. 



One species of coral-animal is 

 found in Long Island Sound, but 

 most of them require warmer 

 waters. They can live at any 

 depth down to about 300 feet, 

 but are seldom found below 120 

 feet. They must have clean 

 and undiluted sea-water. Most 

 commonly the young animals 

 developed from eggs attach 

 themselves to favorable sea- 

 bottom near land, and form 

 fringing reefs of coral rock near 

 the shore, or barrier reefs when 

 a navigable channel is eroded between the reef and the 

 shore. The formation of an atoll, a peculiar coral island 

 in ring form and inclosing a body of water, may have 

 begun as a fringing or barrier reef around a volcanic 

 island which has later subsided or been eroded away while 

 the deposits of coral rock have been accumulating. Or a 

 small group of corals may have been established on sea- 

 bottom at a favorable depth, and as the colony grew out- 

 wards, the animals in the center may have been killed by 

 coral-sand washed over them by the waves. Erosion takes 



FIG. 106. A colony of coral-ani- 

 mals whose supporting "skele- 

 ton " (black in figure) contairite 

 the red coral of commerce. 

 (After Lacaze-Duthiers.) 



