278 THE VITAL FUNCTIONS. 



Eels are enabled to carry on respiration when out 

 of water, for a certain period, in consequence of 

 the narrowness of the aperture for the exit of the 

 water from the branchial cavity, which enables it 

 to be closed, and the water to be retained in that 

 cavity.* 



1 have already stated that in all aquatic animals, 

 the water which is breathed is merely the vehicle 

 by which the air it contains is brought into contact 

 with the organs of respiration. This air is con- 

 stantly vitiated by the respiration of these animals, 

 and requires to be renewed by the absorption of a 

 fresh portion, which can only take place when the 

 water freely communicates with the atmosphere ; 

 and if this renewal be by any means prevented, the 

 water is no longer capable of sustaining life. Fishes 

 are killed in a very few hours, if confined in a li- 

 mited portion of water, which has no access to fresh 

 air. When many fishes are enclosed in a narrow 

 vessel, they all struggle for the uppermost place, 

 (where the atmospheric air is first absorbed), like 

 the unfortunate men imprisoned in the black-hole 

 at Calcutta. When a small fish-pond is frozen over, 

 the fishes soon perish, unless holes be broken in the 

 ice, in order to admit air : they may be seen flock- 

 ing towards these holes, in order to receive the 

 benefit of the fresh air as it is absorbed by the 

 water ; and so great is their eagerness on these occa- 

 sions, that they often allow themselves to be caught 

 by the hand. Water, from which all air has been 



* Dr. Hancock states that the Doras costatus, {Silurus cosiatus, 

 Linn.) or Hassar, in very dry seasons, is sometimes seen, in great 

 numbers, making long marches over land, in search of water. Edin. 

 Phil. Journal, xx. 396. 



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