EGG-LAYING HABITS OF BONY FISHES 



247 



heart to the gills, where it loses carbon dioxide and receives oxygen ; 

 it then passes on to other parts of the body, until it reaches very 

 tiny tubes called capillaries. From the capillaries the blood 

 returns, in veins of gradually increasing diameter, to the heart 

 again. During its course around the body some of the blood 

 passes through the kidneys and is there relieved of its nitrogenous 

 waste. Circulation of blood in the fish is rather slow. Since the 

 temperature of the blood is nearly that of the water in which the 

 fish lives, fishes are called cold-blooded animals. 



Nervous system. As in all other vertebrate animals, the 

 nervous system of the fish consists of the brain and spinal cord. 



hrai 



heart 



gall blacCcC^^si^E^^^ 



Diagram of a fish cut lengthwise to show the relative position of the internal organs. The 

 veins, arteries, and all smaller organs a e omitted. Where would the gills be with reference 

 to the heart? Why? The swim bladder is attached to the fool tube. What is the value 

 of this ? 



Nerve cells located near the outside of the body send messages 

 to the brain, where they are received as sensations. Cells of the 

 central nervous system, in turn, send out messages which result 

 in the movement of muscles. 



The egg-laying habits of the bony fishes (teleosteans) . The eggs 

 of most bony fishes are laid in great numbers. The number varies 

 from a few thousand in the trout to many hundreds of thousands 

 in the shad and several millions in the cod. The time of spawning 

 is usually spring or early summer. After the eggs are laid the 

 male usually deposits milt, consisting of millions of sperm cells, 

 in the water just over the eggs. The sperm cells move rapidly 

 through the water to the egg cells, and unite with them, thus 

 bringing about fertilization. Some fishes, as sticklebacks, sunfish, 



