The Structure of Fishes 



Fishermen are often confused in their endeavors to distinguish cer- 

 tain species of fishes from others. Two closely related species may 

 resemble each other so closely that it is difficult to tell them apart. 

 Some of our common game fishes like the sauger and the walleye are 

 hard to distinguish. The black and the white crappies resemble each 

 other closely. Most people have difficulty distinguishing the several 

 species of sunfishes, the three species of bullheads, and the many species 

 of minnows, which resemble each other closely. A fisherman must 

 know numerous details of fish structure if he wishes to be able to 

 identify many species accurately. 



Many species of closely related fishes hybridize readily and the re- 

 sulting offspring usually offer a combination of the characters of each 

 parent. This has confused even expert ichthyologists, who have mis- 

 takenly described such forms as new species. 



Color and markings are often variable, unfortunately, and more 

 constant morphological characters should be sought in identifying 

 different species. The coloration of a fish often varies in two nearby 

 lakes, causing local residents to maintain stoutly that they have two 

 different species. Walleyes may be deep brown in one lake and pale 

 silver in another lake. Often the color and even the markings of a 

 species vary in the same lake. Color is often influenced by the color of 

 the water and the bottom. Some fishes like rock bass can change color 

 from almost black to silver in a few minutes, according to the color 

 of their background. 



Young fish often differ from adults in markings and color. This fre- 

 quently causes confusion and is partly responsible for the common 

 but erroneous assumption that the spotted and heavy-bodied adult 

 northern pike is a different species from the slender fingerling. which 

 is streaked with bars. 



Structurally modern fishes are sometimes considered to be of a simple 

 vertebrate type, but most of the fresh-water fishes are highly developed 

 modernized forms, truly specialized for efficient aquatic existence. In 

 this modern specialization some fishes have lost or modified many funda- 

 mental structures, such as fins, scales, and teeth. 



In the description of fishes certain general anatomical terms are used. 

 Anterior refers to the front or forward part of the body. Posterior refers 

 to the tail or hind part of the body. Dorsal refers to the upper side or 

 back, and ventral refers to the under or belly side of an animal. (See 

 Diagram 1.) 



A fish consists chiefly of two parts, the head and the trunk. There is 



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