FISHES. 



FISHES. 



IN the FISHES, the last class of vertebrated animals, the chief and most obvious dis- 

 tinction lies in their adaptation to a sub-aqueous existence, 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 whale and the whole of the cetacean 

 tribe, an account which may be found in vol. i. page 521. But these creatures are gener- 

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

 those of the mammalia, and they are forced to breathe atmospheric air at the surface of 

 the waves. And though they would die if left upon land, their death would occur from 

 hunger and inability to move about in search of food, and in almost every case 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 existence : and the 

 beautiful respiratory organs, which we know by the popular term of "gills," are so 

 constructed that they can supply sufficient oxygen for the aeration of the blood. They 

 have not the power, as is sometimes imagined, of separating the oxygen, which, in its 

 combination with certain proportions of hydrogen, composes the element in which they 

 live, but are able to take advantage of the atmospheric air which is contained in the 

 water. 



Any reader who happens to possess a globe with gold- Fish can prove, and doubt- 

 lessly has proved, the truth of this assertion. It often happens that when the supply of 

 water is insufficient, or the mouth of the vessel too small to permit the air to be absorbed 

 by the water in sufficient volume, the Fish come gasping to the surface, and there swim 

 with gaping mouths, sucking in the air with audible gulps. But if a little water be taken 

 up in a cup or spoon, and dashed back from a little height, so as to cause a sharp 

 splash, or, better still, if a syringe be employed for the same purpose, so as to drive a 

 quantity of atmospheric air into the water ; the Fish soon become contented, their 

 anxious restlessness abates, and they quietly swim backward and forward without dis- 

 playing any more signs of uneasiness. 



The reason that Fishes die when removed from the water, is not because 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 Fish whose gills can longest retain moisture will live the 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 sufficiently 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 entire shape of these creatures, subjected though it be to manifold variations, is 

 always subservient to the great object of passing rapidly through the ponderous liquid in 

 which they swim, so as to enable them to secure their prey or avoid their enemies. Even 

 in creatures of such different shapes as the sharks, the eels, the salmon tribe, and the flat 

 fish, the capacity for speed is really wonderful, and is in all effected by simple and 

 beautiful modifications of one mechanical principle, that of the inclined plane or screw. 



In all Fishes, the power of progression lies in the wonderfully muscular tail with its 

 appended fin, and the creature drives itself forward by repeating strokes of this organ in 

 exactly the same manner that a sailor urges a boat through the water by the backward 

 and forward movements of a single oar in the stern. 



To show the power of this principle, I will mention that being on one occasion left 



