210 A HISTORY OF FISHES 



effect of such markings is twofold : they give the fish a more 

 perfect resemblance to the ground on which it lies or the rocks 

 and weeds among which it lurks; or, by their separate and 

 conflicting patterns, they tend to obliterate the visibility of the 

 form, and to break up the outline of the body against either a 

 pale or dark background, as do the stripes on the body of a 

 Zebra. The beautiful Angel-fish (Pterophyllum) of South 

 America, a great favourite with aquarium lovers, provides an 

 excellent example of the value of markings in concealment, its 

 very thin, almost circular body being crossed by several deep 

 black bars, which are continued on to the long filamentous 

 fins (Fig. 8c). These markings harmonise very closely with 

 the stems of the water plants among which the fish remains 

 suspended almost motionless for hours on end, while the slowly 

 waving fins help to perfect the deception. 



With certain exceptions, of which the small Sun-fishes [Centrar- 

 chidae) of the rivers of North America, and some of the Cichlids 

 {Cichlidae) of Africa and South America may be mentioned, 

 fresh-water fishes are more or less soberly coloured, and even 

 the bright hues of these forms mentioned above, so obvious 

 when the fishes are viewed through the side of an aquarium, 

 are largely obscured in life by the olivaceous markings and 

 dark blotches of the upper parts. Dark spots, blotches, stripes 

 and bars of all descriptions are profusely developed, especially 

 in those species which habitually dwell at or near the bottom. 

 In the few fishes living in caves, wells, and subterranean 

 streams, where dark colours would be unnecessary and even 

 disadvantageous, no pigments are developed, the prevailing 

 colour being white or pale pink. 



In the sea the same general principles apply. Fishes habitu- 

 ally swimming at or near the surface, such as the Herring 

 (Clupea), Blue Shark (Carcharimis) , Mackerel [Scomber), or Tunny 

 (Tkynnus) are coloured silvery or white on the belly and sides, 

 and the back parts are dark green, black, or steely blue, 

 sometimes ornamented with black, spots or streaks, but as a 

 rule more or less uniform (Fig. 82A). The water in the sea 

 being generally bluer and clearer than that of the rivers, the 

 olivaceous hues of the fresh-water fishes give place to these 

 metallic shades, and seen from above against a background of 

 dark water, or from below against a light sky, the fish is in- 

 conspicuous to its enemies, whether they be birds or other 

 fishes. Larval fishes, swimming for the most part at or near the 

 surface, obtain similar protection by the absence of pigments, 



