198 BRANCH CHORDATA 



generally "rhomboid and arranged like parquetry." They are 

 covered with a thick coating of vitrodentin which gives an 

 iridescent effect, and are often closely articulated into a coat of 

 armor. (3) The cycloid scales are closely related. They are 

 placed loosely in the pockets and arranged in rows. In over- 

 lapping, one scale covers parts of two scales posterior to it. The 

 middle part of the scale is surrounded by concentric lines from 

 which proceed radiating lines. (4) The ctenoid scales (see Fig. 

 174) have the posterior edges truncate and the free margin 

 toothed. The scales are often striated or polished, and this 

 gives rise to various colors, especially the iridescent gleam on 

 the sides of the fish. 



Color. The color in general harmonizes with its environment. 

 Most of the fresh-water fishes are dark colored (olive or greenish) 

 above and whitish below, so that to the enemies from above, 

 as fish-eating birds, the form appears indistinct in the water, and 

 to the enemies below they look white like the light. Many are 

 variously dotted or striped with lighter or darker colors, thus 

 simulating the lights and shadows among the weeds and grasses. 

 The scales reflect all the hues and tints of the rainbow, causing 

 the fishes to rival the birds in beauty. The males of some species 

 put on brighter colors at the spawning season. Some species 

 have the power of changing color at once to meet the surround- 

 ings, as the pipe-fishes, some sticklebacks, the plaice, and the little 

 Oligocottus snyderi, of Monterey Bay, California. Many others 

 change the colors more gradually. Recent experiments upon 

 fishes in aquaria have shown that if the light be thrown from 

 l>elow and cut off from above, the upper part grows light colored 

 and the lower part dark colored. This would seem to show that 

 the colors are due to the action of light, but while many fishes 

 in caves are colorless, it is said that those in the black depths 

 of the ocean may be either pearly white or black; so the question 

 is yet unsolved. Many deep-sea forms are phosphorescent. 



Some fishes have special protective resemblance, as the leaf- 

 finned sea-horse, the pipe-fish, and some angler-fishes, the pos- 

 terior fins of which are bedecked with fringes "that exactly 

 mimic seaweed." The mouse-fish, or Sargassum, is colored to 

 harmonize with the gulf weed, Sargassum, among which plants 

 it lives. 



