CHAPTER 1 



Characteristics of Fishes 



Dayton L. Alverson 



In the systematic arrangement of animals, fish are grouped in the 

 phylum Chordata which encompasses all vertebrates including lampreys, 

 sharks, bony fishes, amphibians, reptiles, birds, and mammals. The fishes 

 are generally subdivided by taxonomists into two major classes: Selachii 

 and Pisces. The Selachii, which include the sharks, rays, and chimaeras, 

 differ considerably in external shape, but have a fundamental similarity 

 in that all have cartilaginous skeletons with primitive ribs and a com- 

 plete brain case (condocranium). 



In Pisces, or the true fishes, the skeleton is normally bony and com- 

 prised of a vertebral column, ribs, and a skull having many distinct 

 bones. Through general use, both shellfish (mollusks and crustaceans) 

 and the whales and porpoises (cetaceans) are at times included with the 

 fishes. However, these animals are structurally far removed from ''fin 

 fishes." 



Form 



Fishes possess a wide variety of external shapes or forms. The most 

 common fish shape is that normally referred to as fusiform, that is, 

 roundish and somewhat compressed and tapering towards the head and 

 tail. The fusiform is considered to offer minimum resistance in water. 

 The Pacific salmon is a good example of this shape (Figure 1.1a). 



In some fishes the bodies are greatly compressed such as in the bramids 

 (Figure Lib) and sunfishes. Those fishes that are thin and flat are referred 

 to as depressed and include such fishes as the skates and rays. Flatfishes 



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