86 THE GOLDFISH AND ITS CULTURE. 
The water in which the fish lives is very nearly as heavy as the 
fish itself, the latter then requiring comparatively little strength to 
move about. The motion necessitating the greatest expenditure of 
power is that of propulsion forward, and is accomplished by the 
action of the caudal fin. The pectoral fins are used to change the 
water in the neighborhood of the gills, thus serving as adjunct 
respiratory organs. 
The ventral fins are mainly useful as a brake when the fish wishes 
to come to a sudden stop when in motion, or for a backward move- 
ment; the dorsal and anal fins serve the purpose of balancing the body. 
The gills, which are organs of respiration, are situated on each 
side of the head, protected by ‘‘opercles” or gill-covers., (fig. f.) 
The gills themselves consist of bony arches which are covered with 
a tissue containing a large number of blood-vessels. As a rule there 
are four of these arches on each side of the head. The life-giving 
principle that supports animal life is a gas called oxygen, and as this 
exists in the water, the function of the gills is to extract it. When 
the water passes through them, as it does when taken in by the 
mouth of the fish, and is pressed through the gills, it comes in con- 
tact with the blood-vessels, the oxygen being absorbed into the blood 
for the nourishment of that fluid and the body generally. 
It will thus be seen that it is not the water that the fish breathes, 
as many suppose, but the air contained in it, as can be proved by 
placing fish in water from which the air has been taken, by pro- 
longed boiling or otherwise. 
Fish that are kept in a vessel will come up to the surface where 
the air can be mixed with the water when that in the water of the 
vessel has become exhausted. It would seem from the fact that as 
the fish breathes only the air and not the water, that it could just as 
well live in the open air, where it could get plenty. Such is not the 
case however, for the gills are so constituted by nature that they 
