74 
FISHES 
among the higher vertebrates, like the dog, horse, and 
man. 
Fishes which eat minute animals and plants have many 
sharp pointed projections on the inside of the gill arches 
which act as strainers and gather quantities of this small 
food as the water passes over, the gills. These projections 
are called gill-rakers (Figure 64). Their development 
seems to vary in proportion as they are needed for service. 
Fishes that feed on crayfish and on small fish have no use 
for gill rakers or strainers and accordingly their gill rakers 
are undeveloped. 
The food captured by the teeth of the fish or caught in 
the gill strainers passes at once into a short esophagus 
which expands into the thick-walled stomach. Here the 
various plants and animals that have been swallowed undergo 
partial digestion, the remainder of the process being com¬ 
pleted in the intestines. The dissolved foods are absorbed 
through the walls of the stomach and intestines by osmosis 
and pass into the blood. The main parts of the digestive 
system and their adaptation *to digestion are the same as 
in all the higher vertebrates. 
59. Respiration. — Water is taken in through the mouth 
and passes out through two openings, one on each side of 
the neck. In each opening four or five gills are found. 
The gills are made up of numerous, small, very short, fleshy 
threads or filaments (Figure 64). Into each filament a blood 
vessel penetrates and here the blood throws off carbon 
dioxide and takes oxygen from the water by osmosis just 
as the blood of the crayfish does. The thin-walled gill 
filaments are adapted to respiration in the water. The 
water is drawn into the mouth and forced out over the 
gills in much the same way as water is pumped from a well. 
When a fish opens its mouth, the water rushes in. As the 
mouth is closed, the floor of the mouth and throat is raised 
slightly, pushing the water against the side of the neck and 
