142 MARINE PRODUCTS OF COMMERCE 



thin, fleshy, reddish rind, termed the coenenchyma, in which the polyps are em- 

 bedded. They are easily distinguished by their white hue, and secrete a lime 

 which serves to build up the structxare of the branch. These branches do not seek 

 the light as plants do, but spread horizontally along the bases to which they are 

 firmly attached. The fleshy rind covers the entire coral up to the ends of the 

 branches, which are therefore soft and pliable and can be severed with a sharp 

 knife. If a living coral-branch is allowed to dry in the air, the skin, or brick-red 

 rind, hardens, and little protuberances, containing the polyps, appear. These 

 protuberances have a round aperture in the center from which 8 short ridges ray 

 out. 



The greater part of the coral-producing organisms belong to the actinozoan 

 division of the Coelenterata, with the exception of the hydrozoan staghorn coral 

 (Millepora alcicornis) and its congeners. There are two divisions of the coral- 

 producing actinozoa: namely, the Octocoralla, in which the animal is 8-rayed, 

 and the Hexacoralla, or 6-rayed animal. The first-named class comprehends the 

 "sea fans" and "sea whips", Gorgonia and Muricea, of the southern coast of the 

 United States and the precious coral of the Mediterranean region. 



The Coral of Commerce. The specific gravity of coral, ranging from 2.6 to 2.7, 

 closely approaches that of calcite, but its hardness, 3.75, is a little greater owing 

 to slightly different composition. This low hardness facilitates the carver's or 

 turner's task. Although coral does not take a fine polish, its beauty of hue com- 

 pensates for it; although not very hard, the material has toughness and tenacity 

 and wears well. As it is slightly opaque, scratches do not show. Since it is com- 

 posed chiefly of calcium and magnesium carbonates, acidulous perspiration affects 

 its surface. 



The composition of red and black coral is indicated in Table 23. 



Table 23. Composition of Precious Coral 



Calcium carbonate 



Magnesium carbonate 



Calcium sulfate 



Ferrous oxide 



Organic matter 



Water 



Phosphoric acid (PoOg), Silica (SiOa), etc 



100.000 100.000 



From these analyses it appears that black coral differs from red chiefly in 

 organic content. Various observers have concluded that the color of the black 

 coral is due to the presence of organic matter. It has been assumed that the red 

 color of precious coral is caused by its iron content; however, the latter conclu- 

 sion does not seem probable as the red color is destroyed on ignition. The red 

 coloring matter is presumably an organic compound formed by the polyps. 



Location of Fisheries 



Mediterranean Fisheries. The most important and profitable coral banks in the 

 Mediterranean are on the Algerian and Tunisian coasts. The former extend east- 

 ward from Cape Ferro (Cap de Fer) toward Cape Bon, and in a southerly direc- 



