VERTEBRATE ANIMALS 149 
of air will be seen to rise through the liquid. The 
air has been dissolved in the water and is driven off when 
the liquid is heated. It is for this reason that the water in 
which Gold-fish are kept requires to be constantly renewed 
—and why, moreover, a more frequent change of water is 
necessary in a small vessel than in a large one. The fish 
has special breathing organs, called gills, to enable it to 
breathe under water. If the gill-cover be raised or removed, 
the gills will be exposed ; these consist of feathery fringes 
of red flesh growing from the gill arches. The fringes 
themselves are crowded with blood vessels covered with a 
moist skin so thin that we can see the red colour of the 
blood within. If a fish be watched, it will be seen to be 
constantly opening and closing its mouth, as if it were 
drinking. The water however is not swallowed but is 
passed backwards over the gills and out again through 
the gill-openings at the sides of its head. In its passage 
over the gills the dissolved air diffuses through the 
thin moist walls and enters the blood vessels, whilst 
the carbon-dioxide carried by the blood diffuses into 
the water. The gills in fact are to fish what lungs are 
to the other vertebrate animals. Fish living in rivers 
or in currents, point their heads up stream, as in this 
position a flow of water over the gills is more easily 
obtained. Anglers are able to drown fish by leading 
them down stream and so . preventing a sufficient 
supply of water from entering the gills. If a fish be 
removed from the water it quickly dies, not in conse- 
quence of an insufficient supply of air but because the 
gills become dry, which retards the circulation of the blood, 
and hence prevents the diffusion of oxygen into the blood 
vessels. Some fish—notably Eels—are able to live for a long 
time out of waver because of an arrangement which keeps the 
gills moist. Jn the case of the Eel, the gill-opening is small 
