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TEXTBOOK OF ZOOLOGY 



There are three principal types of scales which cover and protect 

 the body of most true fish (a notable exception is the catfish to be 

 described later). These are: ganoid, cycloid, and ctenoid. The first 

 are usually rhombic or oval in shape and are covered by a dentinelike 

 substance called ganoin. Such fish as gar pikes and bowfins possess 

 this type. Cycloid scales are rather disc-shaped with conspicuous 

 concentric lines. They are usually imbricated on the skin, like a 

 shingle roof. The third type is similar to the cycloid except that 

 the free edge of the scale bears some spiny projections or cteni. 



Fig. 242. — The different types of fish scales. 1, cycloid; 2, ctenoid; 5, ganoid; 

 i, placoid. (From Krecker, General Zoology, published by Henry Holt and Company, 

 after Her twig.) 



Cycloid scales are found on the carp while the ctenoid are charac- 

 teristic of the perch and sunfishes. The age of many fish can be 

 determined by the distribution of the concentric lines on the scales. 

 The lines formed during nongrowing periods fuse closely together, 

 thus indicating seasonal periods on the scale. 



The skeleton includes, besides the paired iins and girdles already 

 mentioned, the amphicoelous (concave in both ends) vertebrae and 

 bony cranium, which is complete and independent of the visceral 

 skeleton. This latter portion consists of seven arches, the jaw 

 structures, and five gill arches. The bones of the operculum arise 

 as a part of this division. 



The digestive tract is in the usual form of a canal with out- 

 growths. Food ranging from vegetation, insect larvae, Crustacea, 

 clams, and snails to small fish and amphibia is utilized. It passes 



