272 True Fishes 



skeleton is composed mostly of cartilage. However, some 

 true bone is always present. 



2. There is only one gill opening on each side of the 

 head. 



3. The fins are supported by bony rays. 



General Information: Although water covers about 71 per 

 cent of the earth's surface to an average depth of over 

 2 miles, not all of it is teeming with fish. Most of the fish 

 are concentrated in the relatively shoal waters adjacent to 

 the continental land masses, on the Continental Shelf and 

 along the edges of the Shelf as it dips into the depths of 

 the ocean, the Continental Slope. In these areas two major 

 categories of fish exist: the demersal or bottom-dwelling 

 forms such as the flounder, cod, and haddock, and the 

 pelagic or midwater and surface forms such as the herrings 

 and bluefish. 



Beyond the edge of the Continental Slope on the ocean 

 floor live the abyssal or deep-sea forms, frequently gro- 

 tesque in appearance and possessing special light organs. 

 Over these great depths are found the pelagic oceanic spe- 

 cies, such as the ocean sunfish and tunas, some of which 

 commonly enter the waters over the Continental Shelf and 

 Slope. 



Just as fish are not equally abundant from the coastal 

 areas to the ocean depths, they are also unequally dis- 

 tributed from the equator to the poles. In terms of num- 

 bers the most fish are found in the subpolar and temperate 

 zones. Here the great commercial fisheries of the world are 

 concentrated. But it is in the subtropical and tropical zones 

 that the greatest number of different species occur. 



Within the expanse of water extending from the shore to 

 the ocean depths and from the North to the South Pole 

 there exists as wide a variety of environments as on land. 

 The land beneath the sea has its hills and mountains, val- 

 leys, broad fertile plains, and desert-like regions. Rivers 

 are replaced by ocean currents. In the fertile coastal plain 

 of the sea, land vegetation finds its counterpart in the sea- 

 weeds and even more in the stationary plant-like animal 



