FISHES. Ill 



small and they are numerous, they will die from the 

 corruption of the water. — If a string be tied round 

 a fish in such manner that the free play of his gills 

 is obstructed, the animal will become immediately 

 convulsed, and will -not survive more than a few 

 minutes. 



Fishes are nearly of the same specific gravity with 

 water, and swim by means of their fins and tail. 

 The muscular force of the latter is very great. 

 Their direct motion is obtained by moving the tail 

 from one side to the other, with a vibrating motion. 

 When about to move itself, the fish turns the end 

 obliquely to the water, and moves it through it in 

 that position. The water re-acts obliquely against 

 the tail, and moves him partly forward, and partly 

 laterally. The lateral motion is corrected by th? 

 next stroke the contrary way, while the progressive 

 motion is continued. Assisted by their tail, they 

 turn sideways : striking strongly with it on that 

 side, and keeping it bent, it acts like the rudder ot 

 a ship. The fins of a fish keep it upright, especially 

 the belly fins, which act like two feet: without 

 these he would swim with his belly upwards, as the 

 centre of gravity lies near the back. By contract- 

 ing or expanding the fins, these also assist him in 

 ascending and descending : by inclining his tail ob- 

 liquely, and turning it a little from an erect position 

 to one side, it helps him to rise and fall. 



In addition to the fins and tail the air-bladder is 

 of material assistance to the fish in swimming, as it is 

 by means of this that they increase or diminish the 



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