THE STRUCTURE OF .FISHES 55 



enamel. Scales of this type located on the rim of the jaw have become 

 enlarged and have developed into teeth. The teeth of all vertebrates 

 have been derived from this early tooth-scale. 



A second type of scale is found in a few primitive bony fishes, such as 

 the gar. Scales of this type, known as ganoid scales, consist of rhom- 

 boid plates covered with a hard, enamellike substance known as ganoin. 

 Ganoin differs from enamel in its origin, being derived from the under 

 layer of the skin {dermis) rather than from the upper layer {ej)i- 

 dermis) . 



A third type of scale is the cycloid scale, which is supposed to be the 

 more primitive of the two kinds of scales found on most modern bony 

 fishes. It is the type found on trout, whitefishes, tullibees, and some 

 other fishes. This type is more or less circular in outline, although in- 

 dentations may occur on the inner margin. These scales are bony ■ 

 without any enamel or ganoin. 



A fourth type of scale is the ctenoid scale, which is found on the ma- 

 jority of fresh-water fishes. This type is bony and serrated, i.e., has 

 many saw-toothed processes, on the free or outside margin. A few fishes, 

 such as catfishes, have lost all or most of their scales. Others, such as 

 the brook trout and the burbot, have small, almost imperceptible 

 scales. 



All cycloid and ctenoid scales are fully formed during the first few 

 weeks of the fish's life. No new scales are added as the fish grows, but 

 as growth proceeds the diameters of the scales increase. The growth of 

 the scale is accomplished by the periodic addition of marginal units 

 known as circidi. Growth is not as fast during the winter and through 

 the spawning season as in the summer. Consequently the circuli may 

 be crowded and incomplete during the winter. The resumption of 

 rapid growth in the spring causes new and complete circuli to be laid 

 down outside the incomplete circuli and forms a rather distinct mark 

 known as the annuhis. The age of the fish can be determined by count- 

 ing the annuli in much the same manner as the rings on a tree trunk 

 are counted. Any scales lost by accident at any time in the fish's life 

 can be regenerated or replaced by new scales. Regenerated scales can- 

 not be used to determine age because they do not show growth rings 

 previous to regeneration. 



Bony plates imbedded in the skin and covered by a thin epidermis 

 are found in many primitive fishes, such as sturgeons, gars, and dog- 

 fishes. In the higher bony fishes many of the bony plates of the head 

 have become part of the skull and no longer form part of the skin cover- 

 ing. Some of the bony fishes, such as minnows and suckers, have the 

 head naked; that is, covered by smooth skin only. Others may have the 

 head partly covered by scales. 



The size and number of scales in various dimensions of a fish are 



