[Illustration: FIG. 111.--Headfish (adult), _Mola mola_ (Linnæus). Virginia.] =Tenacity of Life.=--Fishes differ greatly in tenacity of life. In general, fishes of the deep seas die at once if brought near the surface. This is due to the reduction of external pressure. The internal pressure forces the stomach out through the mouth and may burst the air-bladder and the large blood-vessels. Marine fishes usually die very soon after being drawn out from the sea. [Illustration: FIG. 112.--_Albula vulpes_ (Linnæus). Transformation of the Ladyfish, from the translucent, loosely compacted larva to the smaller, firm-bodied young. Gulf of California. (After Gilbert.)] [Illustration: FIG. 113.--Development of the Horsehead-fish, _Selene vomer_ (Linnæus). Family _Carangidæ_. (After Lütken.)] Some fresh-water fishes are very fragile, dying soon in the air, often with injured air-bladder or blood-vessels. They will die even sooner in foul water. Other fishes are extremely tenacious of life. The mud-minnow (_Umbra_) is sometimes ploughed up in the half-dried mud of Wisconsin prairies. The related Alaskan blackfish (_Dallia_) has been fed frozen to dogs, escaping alive from their stomachs after being thawed out. Many of the catfishes (_Siluridæ_) will live after lying half-dried in the dust for hours. The Dipnoan, _Lepidosiren_, lives in a ball of half-dried mud during the arid season, and certain fishes, mostly Asiatic, belonging to the group _Labyrinthici_, with accessory breathing organ can long maintain themselves out of water. Among these is the China-fish (_Ophiocephalus_), often kept alive in the Chinese settlements in California and Hawaii. Some fishes can readily endure prolonged hunger, while others succumb as readily as a bird or a mammal. [Illustration: FIG. 114.--Ice-fish, _Salanx hyalocranius_ Abbott. Family _Salangidæ_. Tientsin, China.] [Illustration: FIG. 115.--Alaska Blackfish, _Dallia pectoralis_ (Bean). St. Michaels, Alaska.] =The Effects of Temperature on Fish.=--The limits of distribution of many fishes are marked by changes in temperature. Few marine fishes can endure any sudden or great change in this regard, although fresh-water fishes adapt themselves to the seasons. I have seen the cutlass-fish (_Trichiurus_) benumbed with cold off the coast of Florida while the temperature was still above the frost-line. Those fishes which are tenacious of life and little sensitive to changes in climate and food are most successfully acclimatized or domesticated. The Chinese carp (_Cyprinus carpio_) and the Japanese goldfish (_Carassius auratus_) have been naturalized in almost all temperate and tropical river basins. Within the limits of clear, cold waters most of the salmon and trout are readily transplanted. But some similar fishes (as the grayling) are very sensitive to the least change in conditions. Most of the catfish (_Siluridæ_) will thrive in almost any fresh waters except those which are very cold.