FISHES. 209 
349. The Fish has not lungs, for these are organs which 
are fitted to introduce air alone to the blood. But it has 
gills, which are fitted to have the blood in them acted 
upon by the air that is mingled with water. These gills 
are frmges which are made up of very minute blood-ves- 
sels. There are commonly four of them, fixed to some 
arches of bone; and they are covered on the outside by 
a lid, called the operculum. In order to have the air in 
the water act on the blood in them, the water is taken 
into the mouth, and then passes out through these fringes. 
If you watch fishes in an aquarium, you will see the mouth 
constantly opened to take in the water, and the opercu- 
lum as constantly raised to let it out. 
350. When a fish is taken out of the water it really 
dies for want of air, although it is in the midst of a plenty 
of it. The explanation is this: the fringes of the gills 
are kept apart by the water while the fish is in its native 
element; but, when’ taken out of it, the fringes fall to- 
gether, and soon become dry. When they are in this 
condition the blood will not circulate freely in them, and 
what blood is there is not acted upon by the air. In 
agreement with this explanation we find that those fishes 
which live the shortest time out of the water have their 
gills most exposed, while those that live a longer time 
have their gill-openings narrow, thus tending to keep the ~ 
fringes moist. In some there is an especial arrangement 
for moistening them, and in such a case the fish can live 
in air quite a long time. Dr. Carpenter states that some 
fishes having this arrangement are accustomed to leave 
the water and crawl about in the grass or on the ground. 
351. The plan of the circulation in the Fish is peculiar. 
In the Mammals and Birds there is a double heart, as 
illustrated in Hig.155. In Reptiles the heart is double 
only so far ag the auricles are concerned, as illustrated in 
Vig. 156. In Fishes the heart is single, having but one 
auricle and one ventricle. The blood passes from the 
ventricle to the gills. Here it becomes arterial blood, as 
