FISHES. 



In the Fishes, the last class of vertebrated animals, the chief and 

 most obvious distinction lies in their adaptation to a sub-aqueous exist- 

 ence and their unfitness for life upon dry land. 



There are many vertebrate animals which pass the whole of their 

 lives in the water, and would die if transferred to the land, such as the 

 whales and the whole of the cetacean tribe, an account of which may 

 be found on page 156. But these creatures are generally incapable of 

 passing their life beneath the waters, as their lungs are formed like 

 tliose of the mammalia, and they are forced to breathe atmospheric air 

 at the surface of the waves. And, though they w^ould die if left upon 

 land, their death would occur from inability to move about in search 

 of food, and in almost every case a submersion of two continuous hours 

 would drown the longest-breathed whale that swims the seas. 



The Fishes, on the contrary, are expressly formed for aquatic exist- 

 ence, and the beautiful respiratory organs which we know by the pop- 

 ular term of " gills " are so constructed that they can supply sufficient 

 oxygen for the aeration of the blood. 



The reason that Fishes die when removed from the water is not be- 

 cause the air is poisonous to them, as some seem to fancy, but because 

 the delicate gill-membranes become dry and collapse against each other, 

 so that the circulation of the blood is stopped and the oxygen of the 

 atmosphere can no longer act upon it. It necessarily follows that those 

 Fishes whose gills can longest retain moisture will live longest on dry 

 land, and that those whose gills dry most rapidly will die the soonest. 

 The herring, for example, where the delicate membranes are not suf- 

 ficiently guarded from the effects of heat and evaporation, dies almost 

 immediately it is taken out of the water; whereas the carp, a fish whose 

 gill-covers can retain much moisture, will survive for an astonishingly 

 long time upon dry land, and the anabas, or climbing perch, is actually 

 able to travel from one pool to another, ascending the banks, and even 

 traversing hot and dusty roads. 



The power by which the Fishes propel themselves through the water 

 is obtained almost entirely by the lateral movement of the tail. The 

 fins are scarcely employed at all in progression, but are usually used as 

 balancers, and occasionally to check an onw^ard movement. 



Before proceeding further, I may mention that all the fins of a Fish 

 are distinguished by appropriate names. As they are extremely im- 

 portant in determining the species, and even the genus, of the individ- 



531 



