294 FISHES 



frozen condition by a dog was thawed by the heat of the 

 stomach and vomited alive. In the sea in Arctic regions many 

 fishes live at or near the freezing-point and from this lower limit 

 fishes are found at all temperatures up to the greatest heat of 

 tropical swamps where Protopterus and Lepidosiren occur. In 

 the ocean abysses the temperature is not far from freezing-point, 

 varying from 28 in the Atlantic to 35 in the Pacific. Fishes 

 have also accommodated themselves to all intensities of light, 

 from that of the surface of the sea in the tropics to the absolute 

 darkness of the Kentucky caves. The ocean abysses of the 

 sea are evidently not dark, although the sunlight never pene- 

 trates to them, for most of the bathybial fishes have large eyes 

 and pigmented skins ; only a few species are blind, these being 

 probably burrowing forms. The light in these depths is 

 derived from the luminous organs of the fishes themselves and 

 other animals, and it may be limited to the immediate neigh- 

 bourhood of the animals which produce it, but in any case it is 

 enough to enable the eyes to perform their function, while 

 Amblyopsis in the caves of Kentucky, and the Cuban cave- 

 fishes Stygicola and Lucifuga, have lost their eyes altogether, 

 and also the pigment of the skin. 



In the fresh waters the quantity of soluble salts is at a 

 minimum, and the density is not appreciably different from 

 that of pure water ; fresh-water fishes, therefore, live in a 

 medium less dense than their own blood. In the sea the 

 density of the open ocean at the surface is 1*027, at the 

 bottom, 1 "029 ; this is exceeded in the Red Sea, where the 

 density is I'o^o. Between these extremes all intermediate 

 densities occur and in all of them fishes live ; in the Black Sea, 

 for example, in consequence of the large rivers which flow into 

 it, the density is only 1*025 an< ^ in the Baltic it is less, while in 

 every estuary there is a gradual transition from the salt water 

 of the sea to the fresh water of the river. The density of i'027 

 in Atlantic water corresponds to the presence of 3*5 parts of 

 dry salts per cent in solution, and this is far above the propor- 

 tion present in the blood or flesh of the fish ; it is an obvious 

 fact that the flesh of marine fish is not so salt as the water in 

 which they live, and therefore the living cells of the skin have 

 the power of preventing the passage of the salt from the sea- 

 water into the body of the fish. This power is due to an ad- 



