in the uniform whiteness of the flat underside — which serves the usual function 

 of shadow-neutralizing when the fish swims about. The upper side, as is 

 well known, is not only exactly like the sand or pebbles in general tint, but 

 is finely peppered with lighter and darker, grayer and browner, flecks, in 

 exquisite imitation of the surface of the sand bed; or it is marked with va- 

 riegated pebble-patterns, or with the two kinds in combination, or with sea- 

 weed colors and patterns, — as the case may be * The sand-picturing patterns 

 of these flat-fish are almost exactly duplicated by those of certain beach- 

 haunting amphibious crabs. Such for instance is the exquisitely dainty and 

 elusive little " Spirit Crab " of the American tropical and subtropical coasts. 

 This little crab dwells in myriads on the flat sandy beaches of the West In- 

 dian Islands, etc. ; but its obliterative shading, sand-color, and sand-picturing 

 pattern are so potent that it is next to impossible to see it until it moves. Even 

 then it is inconspicuous enough, though it runs with great speed. Aquatic 

 crabs which haunt submerged rocks are marked much like the fishes inhab- 

 iting like situations. Those of northern seas are as a rule not gaudily colored, 

 although often richly marbled and spotted and streaked with accurate rock- 

 picturing patterns, founded always on full obliterative shading. Some of 

 these rock-patterns, particularly those of fishes, are almost as minutely accu- 

 rate as those worn by certain warm-blooded land creatures, such as the 

 Nighthawk (Chapter V, Figs. 30-31). As the Nighthawk's pattern contains 

 an admixture of the picturing of lichens and other small and delicate dry- 

 land vegetable forms, so the rock-surface patterns worn by fishes contain 

 generalized pictures of seaweed, minute mollusca, etc., — as it were the rock 

 lichens of the water. But whereas in cold and temperate seas this superficial 

 growth is comparatively scanty, often allowing the actual surface of the rock 

 to show, in tropical waters it tends toward a completely masking luxuriance 

 and ornateness. Particularly is this the case in coral regions. The fishes of 

 tropical coral-shores and reefs are almost as famous as tropical butterflies 



* The rays (Raiidce) have much the same general system of coloration as the flat-fishes, though 

 usually wanting the minute adaptive markings. 



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