254 



SCANDINAVIAN FISHES. 



brilliancy. The pretty network is much less distinct; 

 the central collections of" pigment which were previously 

 [ninctiform, have now increased in extent. But on the 

 l)odv, which is thus almost dark, the emerald spots on 

 the middle of the sides still glittei- with undiminished, 

 or, if |)ossible, still greater brilliancy. We now see 

 tliat tliese spots are composed of nothing more than 

 chromatophores exactly like those which form the selli- 

 f'oi-m spots, but subjected to extraordinarily strong 

 contraction. It is the immense number of glittering 

 disks that are liere collected within a comparatively 

 small space, that gives these spots their iridescent lustre, 

 which, when the chromatophores are expanded and the 

 glittering disks distributed over a larger surface, passes 

 into the dull gleam of the selliform spots, the top of 

 the head and the belly. 



"If Ave now place our darkened Goby on a light 

 bottom of straw or sulphur yellow, for example, after 

 a,bout ail hour the fish is as light as it was dark before, 

 and the selliform spots are invisible. Under the mi- 

 croscope we observe the following peculiarities. The 

 black chromatophores are contracted, the yellow ex- 

 panded. As the former are far more numerous than 

 the latter, ^ve may sometimes see the yellow bottom of 

 the vessel shine through at certain parts of the body, 

 Avhere only contracted chromatophores occur, especially 

 at the tail. This phenomenon is due to the thinness 

 of the body. In the selliform spots there now appear 

 immerous, yellow, expanded pigment-cells; but in addi- 

 tion to these the network of jiale, lustrous colour is 

 sharply marked. 



"Let us now move the fish, ^vhich is partly of a 

 yellow colour and partly transparent, showing the yellow 

 bottom, over some red substance. Instantaneously it 

 assumes a transparent, reddish tint, and the selliform 

 spots, which were invisible on the yellow l)ottom, be- 

 come distinct for a, moment. There is no other change in 

 the degree of expansion of tlic chromatophores than that 

 the yellow, as well as the black, are contracted, though 

 not so strongly; Avhile the few red chromatophores on 

 the upper surface of the body, which previously looked 

 like dots varying from brownish red to black, now begin 

 to form pretty projections and large processes of a red- 

 dish brown or cherry colotii-. The rest of the body is as 

 transparent as before. The selliform spots are visible, as 

 they contain no chromatophores, and therefore stand 

 out on the i-ed colour which shines through from the 

 bottom on which we have placed the fish. 



"If we now change the substratum from red to 

 blue and from blue to light green, we see that each 

 time we change the colour, the tish for a moment 

 assumes the corresponding tint, on account of its trans- 

 parency. Exactly the same results may be obtained by 

 setting a dark tish for a somewhat longer period, on 

 a red, ])]ue or green bottom. The adaptation seems to 

 take place most slowly on a green bottom. If a spe- 

 cimen is kept for some time on a light (white or light- 

 coloured) bottom, even the handsome, lustrous, lateral 

 spots grow pale, evidently on account of the expansion 

 of the chromatophores. Last of all the black caudal 

 spot disappears, the pectoral spot having already van- 

 ished. The handsome stripes on the fins gradually fade. 

 The male becomes exactly like the female, and in this 

 dress it is quite impossible to distinguish either of them 

 by the colour from a young dohius niger of the same 

 size that has l>een kept on a light bottom. But if we 

 take the two species out of their narrow glass vessels 

 and again put them among the plants in a large aqua- 

 rium, all the characteristic dift'erences of colour betAveen 

 them soon reappear. Gobius nigcr never possesses the 

 selliform spots or either of the caudal and pectoral spots 

 that distinguish Gobius flavcscens. The faintly lustrous 

 chromatophores, which are also present in the former, 

 are there irregularly sprinkled in small groups over 

 the surface of the body, and, when contracted, form 

 irregularly scattered, small, bright dots, but are never 

 arranged Avith the same I'egularity as in Gobius Jiavescens. 



"In short, Gobius Jiavescens possesses in a high 

 degree the power of adapting its own colour to that 

 of the bottom with comparative rapidity. If the colour 

 is such that the fish possesses chromatophores with the 

 corresponding pigment, these are expanded. If there 

 are no cells containing the corresponding pigment, all 

 the chromatophores are contracted, and the consequent 

 transparency of the ])ody brings about the same result. 

 The metallic lustre is due to extremely fine, crystalline 

 disks, which are either spread on thfe exterior of a 

 pigment-cell or completely fill it. These cells perform 

 their function in a manner quite the reverse of that 

 of the monochromatic cells; the stronger the contrac- 

 tion, the brighter is their lustre, and the greater their 

 expansion, the duller they appear. 



"The same variations of colour as we have no\v 

 produced artificially, so to speak, also proceed in an 

 aquarium, if its colour-relations correspond to those of 

 the natural home of the fish. The only difference is 



