CHAPTER VI. 

 Fishes. 



FISHES belong to the last or lowest class of the Vertebrates which we have 

 been considering. They are, as is well known, entirely aquatic animals, and 

 are naturally specially adapted for their life in the water. 



Notice, first of all, their characteristic shape, which enables them to move 

 with ease and swiftness beneath the surface. The fins, arranged in pairs, 

 correspond to the four limbs of all back-boned animals. They are used, not 

 for swimming, but for steering and directing purposes, as well as keeping the 

 body upright. The fish swims by means of powerful strokes of its tail, and 

 can swim as easily against as with the current, whilst, owing to its peculiar 

 structure and shape, it is not affected by the swiftness of a stream. Fishes 

 are cold-blooded animals, of approximately the same temperature as the 

 water. Their method of obtaining air is unique amongst animals. The 

 water, which they continually swallow, instead of passing into the stomach, 

 is discharged through the gill-covers, the wide slits at the back of the head 

 familiar to all who have handled fish. In these slits are the gills proper organs 

 of fringed or feathered membranes (generally four on each side of the head) 

 of a deep red colour, being abundantly supplied with blood-vessels. 



Here the water is only separated from the blood by a very thin membrane, 

 through which the air in the water is easily absorbed into the blood. But 

 the gills cannot perform their functions unless kept damp, for which reason 

 a fish speedily dies of suffocation when out of the water. 



Most fishes have an air bladder, by which they can rise or sink. To rise 

 to the surface, the fish fills its air bladder with the quantity of air required ; 

 to go lower or to sink, it presses out the air by muscular contraction. 



Some species which live at the bottom do not possess this organ, but all 

 surface-swimming species have it. 



Fishes lay enormous numbers of eggs, known as spawn, deposited in various 

 places, sometimes at the bottom of a stream, on sand or gravel, sometimes on 

 seaweed or water plants. Some species for example, salmon ascend the 

 rivers to spawn ; others, like the eel, go down to the sea for that purpose. 

 The roe of a single fish may contain over a million eggs. The cod, for example, 

 lays several million a year, and if it were not the case that the majority of these 

 eggs are devoured by enemies, our rivers and seas would literally be blocked 

 by solid masses of fish. 



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