Chap. 33 VERTEBRATES LOWER CHORDATES AND FISHES 669 



another kind distributed through salt and fresh waters everywhere (Fig. 33.8). 

 They are the main food fishes. Seagoing fishermen catch more than 10,000,- 

 000,000 herrings annually to be salted, smoked, and packed. The 1947-1948 

 catch of sardines off the coast of Calfornia was 10,237 tons. Haddock, mack- 

 erel, flounders, and salmon are standards of the market among many other 



Efferent 

 branchial artery 



Gill 

 slit 



Ventrol aorta 



Afferent 

 branchial artery 



Fig. 33.7. The arrangement of the internal organs of a dogfish shark is near to 

 being a living diagram of a generalized vertebrate. 



fishes including those of fresh waters. Fishes are even more important as food 

 for other fishes than for man. From greatest to least, larger fishes eat smaller 

 ones. The great sport fishes are bony fishes — mackerel, tuna, and swordfishes; 

 in clear streams, the golden trout of the west, the rainbow trout, the eastern 

 brook trout and the hardier brown one (Fig. 33.9). 



Skin and Scales. In bony fishes, the outermost layer of the skin is a living 

 layer. Except for a coating of slime, the skin is constantly in contact with a 

 world of water. Fishes have no eyelids and no tears, but water is always wash- 

 ing their eyes. 



The skin secretes the first defense of the body, a slimy covering that per- 

 mits the fish to slide more easily through the water and protects the cells 

 against fungus and bacteria. With the skin, kidneys, and gills this helps to keep 

 an excess of water from passing in or out of the body. In the ocean, such 

 structures hinder the weaker salt solution of the body fluid from passing into 

 the stronger salt solution of sea water, thus shrinking the body. In lakes and 

 streams, they likewise hinder the fresh water from passing into the weak salti- 

 ness of the blood, thus bloating the body. 



The skin produces scales, the second defense of the body, by the division of 

 dermal cells in its inner layer. Scales, like fingernails, are composed mainly 



