PISHES. 



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 whales and the whole of the cetacean 

 tribe, an account of which may be found in vol. i. page 521. Biit these creatures are generally 

 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 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 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 doubtlessly 

 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 displaying 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 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 repeated 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 



