388 



COLLEGE ZOOLOGY 



The diphycercal tail has long been con- 

 sidered the primitive type from which the 

 others were derived, but this is not true. 

 In most cases diphycercal tails are shown by 

 fossil history to have been derived from 

 heterocercal ones, and the homocercal type 

 is also of heterocercal origin. 



Scales 



The scales of fishes form a protecting 

 exoskeleton. They are of three principal 

 types: (1) ganoid, (2) cycloid, and (3) 

 ctenoid (Fig. 261). Ganoid scales are usu- 

 ally rhomboid- or diamond-shaped. They 

 have layers of ganoin deposited on a layer 



of bone. Ganoid scales occur in gars, pikes, 

 sturgeons, and their allies; these are often 

 called ganoid fishes. Cycloid and ctenoid 

 scales are arranged in overlapping rows as 

 described for the perch. Cycloid scales are 

 nearly circular with concentric rings about 

 a central point; they are characteristic of the 

 more primitive teleosts. Ctenoid scales are 

 similar to cycloid scales, but the part which 

 extends out from under the neighboring 

 scales bears small spines; these are generally 

 found in the higher teleosts. In many fishes 

 the scales develop into large spines or fuse to 

 form bony plates which are protective. Some 

 fishes such as the catfishes are scaleless. 



Ganoid (gar pike) 



Ctenoid (perch) 



Cycloid (northern pike) 



Figure 261. The different types of scales on bony fishes. Ctenoid scale shows winter growth 

 rings (numbered), which are used to determine age in years. The winter rings show slower 

 growth periods which result from low food supply. (Photomicrographs of cycloid and ctenoid 

 scales courtesy of Institute for Fisheries Research, Michigan Department of Conservation.) 



Color 



The general impression is that fishes are 

 not brightly colored, but many, especially 

 those in tropical waters, are exceedingly bril- 

 liant. The colors are due to pigments within 

 special dermal cells called chromatophores 

 or to reflection and iridescence resulting 

 from the physical structure of the scales 

 which contain cr\'stals of guanine. The pig- 

 ments are red, orange, yellow, or black, but 

 other colors may be produced by a combina- 

 tion of chromatophores; for example, yellow 

 and black when blended give brown. Usu- 



ally the colors are arranged in a definite pat- 

 tern consisting of transverse or longitudinal 

 stripes and spots of various sizes. Coral-reef 

 fishes have long been famous for their bril- 

 liant colors, and many fresh-water fishes of 

 the temperate zone exhibit bright hues dis- 

 tributed so as to form striking and intricate 

 patterns (e.g., the rainbow darter). 



The dispersion or concentration of the 

 pigment in the chromatophores of certain 

 fishes results in changes in coloration. These 

 changes are due to incident light reflected 

 from surrounding surfaces and act through 

 the eye and nervous system on the differ- 



