CHAPTER 26 



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Class Osteichthyes. 

 Bony Fishes 



378 



HE Osteichthyes (bony fishes) are the 

 true fishes. They range from the ordinary 

 fish, such as the yellow perch, to the unusual 

 lungfish. They are aquatic animals; one 

 adaptation to their habitat is the gills which 

 serve as respiratory organs. They are found 

 in all the waters of the world, from the sur- 

 faces to great ocean depths. Usually, they 

 have an exoskeleton of scales or bony plates 

 which furnishes protective covering for their 

 bodies. They swim by means of fins. Their 

 bodies are usually streamlined, but some are 

 grotesque in shape; and others possess amaz- 

 ing luminescent organs. 



PERCA FLAVESCENS- 

 A BONY FISH 



External anatomy 



The yellow perch, Perca flavescens (Fig. 

 253), inhabits the fresh-water streams and 

 lakes of the northeastern United States and 

 ranges west to the Mississippi Valley. Its 

 body is about a foot long and is divisible 

 into head, trunk, and tail. There are two 

 dorsal fins, a caudal fin, a single median 

 anal fin just posterior to the anus, two 

 lateral pelvic fins, and two lateral pectoral 

 fins. On each side of the body is a lateral 

 line. The head bears a mouth with well- 

 developed jaws armed with teeth, a pair of 

 lateral eyes, a pair of external nares in front 

 of each eye, and gill covers (opercula), be- 

 neath which are the gills. The skin is pro- 

 vided with a number of scales, which are 

 arranged like the shingles on the roof of a 

 house, and protect the fish from mechanical 

 injury. Mucous glands are abundant in the 

 skin, and they produce the "slime" (mucus) 

 which makes the fish slippery. 



Figure 252. Facing page, representatives of the 

 bony fishes. The lines suggest possible relationships. 

 The figures are not drawn to scale. (Based on a 

 diagram by Leonard P. Schultz, Curator of Fishes, 

 United States National Museum, Smithsonian Insti- 

 tution, and made expressly for this book.) 



