240 A HISTORY OF FISHES 



seems to be normal, but they are capable of suspending their 

 respiratory movements for considerable periods. The external 

 gill-openings are generally much reduced in size, and this 

 modification enables the fish to retain a certain amount of 

 water in the gill-chambers during these periods of inactivity. 

 The relatively high amount of air dissolved in the water of 

 hill-streams, coupled with the low temperature of the water, 

 enabling the fish to exist with a small consumption of oxygen, 

 are other factors which help to make this temporary cessation 

 of breathing possible. In certain fishes the inner rays of the 

 pectoral fins are kept in constant motion, and are believed to 

 assist respiration by forcing water out of the gill-openings, or, 

 by driving away any excess of fluid, to assist adhesion. 



The modifications undergone by the eyes, air-bladder, etc., 

 must be briefly dismissed. With the flattening of the fish the 

 eyes tend to be pushed more and more towards the upper 

 surface, and in many species they lie close together on the upper 

 side of the head. They are generally reduced in size, 

 probably on account of the intense light encountered in clear 

 shallow water. In fishes living mainly on the bottom, upward 

 and downward movements are comparatively few, and in 

 rapidly flowing water solidity rather than buoyancy are required. 

 For this reason, the air-bladder is always much reduced, and 

 may be encased in a solid bony capsule. Finally, it may be 

 noticed that in hill-stream forms the tail is especially muscular 

 and capable of whip-like movements, by means of which the 

 fish is able to dart rapidly from one stone to another. 



The general eflfects of temperature on the life of a fish may 

 conveniently be considered here, for although extremes of heat 

 and cold do not appear to have led to marked structural 

 modifications, they play an important part in limiting the 

 wanderings of certain species. The range of temperature under 

 which different fishes live is very great, the frozen streams of 

 Alaska and Siberia and the hot springs and warm stagnant pools 

 of equatorial swamps all being inhabited by some forms of 

 fish life. 



As far as the fresh-waters are concerned, certain species have 

 pushed very far northwards, being held up only when the 

 water becomes actually frozen. Some, indeed, are able to 

 survive in regions where the water is only liquid for a few months 

 in the year. The little Black-fish {Dallia) of Alaska and Siberia 

 (Fig. 32h), a relative of the Pike (Esox), is renowned for its 

 extraordinary vitality, remaining frozen in solid ice for weeks 



