REPRODUCTION 109 



rakers or strainers and accordingly their gill rakers are 

 undeveloped. 



108. Special Senses. — The eye is well developed. It is 

 globular and projecting, and is believed to be near-sighted. 

 The organs of smell are usually located in the nasal cavity. 

 In the bull-head, they are found in the feelers, on the head, 

 and even in the skin of the tail. The ear is under the skin, 

 and there is no external opening. As water conducts 

 sound vibrations more readily than air, no device for 

 gathering sound waves is necessary. 



109. Circulation. — The blood of fishes is carried in well- 

 defined blood vessels and a heart of two chambers. The 

 blood is sent from the heart to the gills, where it is 

 purified of carbon dioxide and receives oxygen. It is then 

 carried by means of arteries to other parts of the body, 

 where the oxygen in turn is given up and carbon dioxide 

 is received. The blood from the gills and other parts of 

 the body is returned to the heart through veins. Because 

 the blood of fishes is at a lower temperature than the blood 

 of man, they are called cold-blooded animals. 



110. Reproduction. — The sexes of fish are distinct. At 

 certain seasons many fish migrate upstream to lay their 

 eggs (to "spawn"). Eggs are laid in large numbers by 

 the females, and in the same locality sperm cells are dis- 

 charged into the water by the males. The sperms unite 

 with the eggs. The fertilized eggs hatch after thirty or 

 forty days, or longer, depending on the kind of fish and 

 the temperature of the water. The yolk of the eggs is 

 attached to the young fishes 'for many days after they arc 

 able to swim, and they need no other food than that sup- 

 plied by this yolk (Figure 111). 



The spawning habits of fish must be understood thor- 

 oughly if they are to be raised artificially, as is done in the 

 many fish hatcheries. Most states have scientific game 



