108 FISHES 



Note the shape and relative position of the head, trunk, and tail region. 

 The gills are covered by a bony shield, the operculum. What is its size 

 and how attached ? Where are the eyes located ? Do they move ? Can 

 the eyes be closed ? How is the body covered ? Of what use is this 

 covering to the fish ? 



106. 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. 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 through the gill opening. The mouth is 

 thus emptied of water so that when it is opened again 

 more water flows in. 



107. Food Taking. — Fishes eat insects, worms, crayfish, 

 snails, and other fish. The teeth of fish serve to seize, tear, 

 and hold food. None of the fish have teeth which are 

 adapted to crushing or chewing the food, as is the case 

 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. 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 



