MECHANICS OF RESPIRATION 



it through the gill-slits. This incessant renewing is 

 the process by which it breathes. By means of these 

 successive operations, it continually distributes fresh 

 water to the respiratory organs whose business it is 

 to absorb oxygen and throw out carbon dioxide. 



The goldfish and most of its fellows exhibit these 

 phenomena on a relatively small scale. The yawning 

 is not very extensive; the beating of the gill-covers 

 is relatively weak. Other species, with large heads 

 and an active respiration, show them more clearly. 

 The hog-fish or scorpion-fish (genus Scorpcend) are 



Fig. 45. — Respiratory movements in the Scorpion-fish or Hog-fish. 

 Right, inspiration : the mouth partly open and cheeks expanding. 

 Left, expiration : the mouth closed and cheeks contracting. 



such fish. A marine aquarium tank which contains a 

 certain number of them has a good deal to teach us. 

 The hog-fish usually lie stretched out motionless on 

 the bottom. From time to time one of them rises, 

 swims around a while, but soon falls back and resumes 

 its condition of inertia. Their large, heavy heads, 

 bristling with spines, bearing fine strips of skin which 

 wave on their cheeks and under their chins, lighted 

 by two large bulging eyes, are monstrous heads with an 

 evil expression, yet the fish repose with a calmness 

 which the movements of the other fish about them do 

 not appear to disturb in the slightest. Despite their 

 ugly appearance, and the venom of their spines, the 

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