FISHES DIAGRAM 5. 95 



CLASS OF FISHES. 



[ DIAGRAM 5. ] 



FISH are cold-blooded animals like reptiles, but they always 

 Hive in water, and breathe by means of gills. These are shaped 

 'like combs, and are of a fine red colour, as may be seen on raising 

 the gill-covers. Fish breathe by taking in water by the mouth, 

 .and discharging it through the gills. On touching the gills, the 

 air which is contained in the water, parts with its oxygen, and 

 takes up in exchange carbonic acid from the blood, so that the 

 respiration of fishes does not differ essentially from that of 

 mammals and birds ; only it is effected by means of the air con- 

 tained in the water, instead of atmospheric air. 



The limbs of fish are replaced by fins, but they swim in the 

 water, especially when they wish to move quickly, by the motion 

 of the tail alone. Many have a bladder in the body, which is 

 completely closed and filled with air to assist them to float in the 

 water. Most fish are very voracious, and swallow their prey at a 

 single gulp. 



They lay a great many eggs, but often very small ones ; and 

 -when the animal is full of eggs, it is said to be full of roe, which 

 is the name generally applied to fishes' eggs. It has been esti- 

 mated that a salmon may lay 27,000 eggs ; a pike 500,000 ; a 

 sole, 100,000 ; a mackerel, 500,000'; and a cod-fish from 3 to 9 

 millions of eggs. 



