The Structure of Game Fishes 7 
the water in a cloud of minute bubbles, and the 
fish is immediately at ease. The deep-sea forms 
obtain the limited oxygen which they need from 
slow-moving currents which sweep along the bot- 
tom of the ocean at the greatest depths, passing 
from the surface to the abysmal regions laden with 
oxygen. The gills are permeated with blood-ves- 
sels, which give them their rich color, and their 
office is to take oxygen from the water, the opera- 
tion being as follows. Watching a fish in con- 
finement, a continued movement of the mouth and 
gills is observed, an endless opening and shutting ; 
indeed, the fish’s mouth is never closed. It is 
taking in water which passes over the gills, bath- 
ing them in an endless stream. As it passes, the 
gills take up the oxygen contained in the fluid ; 
this is absorbed by the blood, which is being 
pumped by the heart to the gills for this purpose, 
as blood in man is pumped to the lungs to receive 
the oxygen taken in at the nostrils. The water 
so used is thrown out at the gill openings by the 
depression of the operculum, or door, or cover of 
the gill chamber. The passage of blood through 
man is rapid, and he is warm-blooded; in fishes 
it moves very slowly, and the temperature of the 
blood is lower than that of the surrounding 
