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THE LIVING WORLD. 
WONDERFUL ORGANISM OF THE FISH. 
The term fish is applied to animals that .breathe through the medium of 
water, extracting the air therefrom by means of gills, and which are specially 
adapted to an aquatic existence. It is impossible to give an exact definition 
without describing characteristics prominent alike in fishes and amphibians, 
for even the feature most peculiar to fishes, viz., gills, is possessed by crea¬ 
tures belonging to other and distinct orders, such as the proteus, and other 
animals that are supplied with gills while in the larval state. We must 
therefore be guided in a measure by the knowledge which every one possesses 
in a degree, which, with the descriptions to be given, will enable us to dis¬ 
tinguish the several orders, so far as classification can be trusted. 
The breathing organs of fishes are not in the lungs, for these are wanting, but 
in the gills, which are found on each side of the head, and covered by articulated 
arches called the opercula. These constitute the most sensitive parts of the 
fish, because they correspond in purpose to the heart, being composed of laminae 
permeated by blood-vessels so near the surface as to be in contact with the 
water, and extract the oxygen therefrom. The process of such extraction is by 
receiving water into the mouth and expelling it at the gill orifices, by which 
action the air is absorbed and the blood accelerated, similar to the effect pro¬ 
duced by the breathing of atmospheric air by land animals. 
When a fish is deprived of water his gills become dry, and this is followed by 
suffocation because he can no longer absorb the air. This is proved by the fact that 
the constant pouring of water over the gills, even though the fish be removed 
from its element, will serve to sustain its life for a considerable time. That the 
fish absorbs air in the act of drawing water through its mouth and dispensing it 
over the gills, is also proved by the well-known fact that the creature cannot 
long exist in a small body of water unless it be frequently renewed, because the 
air with which the water is impregnated soon becomes exhausted, and is then to 
the fish what carbonic gas is to land animals. Any artificial means that will 
force air into the water will again revive the fish, just as fresh air will relieve 
suffocation arising from carbonic gas in land creatures. 
The wonderful adaptation thus seen in fishes, which fits them for life in the 
water, is reinforced by their peculiar structure, which qualifies them for movements 
in their element. A fish is the very embodiment of grace, and exhibits truly 
marvellous design in its structure. The body is formed so as to pass through its 
medium with the least possible resistance, but to this advantage is added scales 
or skin covered with an oily secretion, which prevents friction, and facilitates its 
motion to such an extent that inertia may be said to be practically the only 
retarding influence it has to overcome. The fins are not always the same in 
all fishes, some having more and others fewer, but generally they are supplied 
with one on either side near the gills, which are called pectoral fins, two under 
the thorax, called ventral fins, one on the back, which projects from the spine, 
called the dorsal fin, one, and sometimes two, near the vent, called anal fins, 
and the tail, or caudal fin. Besides these organs of locomotion, many fishes are 
supplied with an air bladder, which is a rudimentary lung, by the inflation or 
contraction of which it is able to rise or sink at pleasure. 
Scales are not always present, but when wanting their place is supplied by 
an excellent substitute in the form of a strong and oily epidermis very difficult 
