HOW INSECTS BREATHE. 151 



which the breathing is effected in four familiar but very 

 different animals. 



Firstly, the whale, though often called a fish, is in reality a 

 warm-blooded air-breathing mammal, and must, therefore, 

 come to the top of the water at short intervals. For this 

 purpose the tail is flattened horizontally, which enables the 

 animal to rise and sink very quickly. If it breathed chiefly 

 through the mouth, the action of the tail would tend to drive 

 it out of the water, and much time would be lost in de- 

 scending, as we know to be the case if we try to dive when 

 swimming. In order to avoid this the nostrils are placed 

 in the highest part of the body, and are made to serve the 

 most important part in respiration. 



The fish, on the contrary, is a water-breather, that is to 

 say, it is provided with an apparatus in which the blood 

 makes use of air contained in the Water. This apparatus we 

 call the gills ; the water enters the mouth, and, instead of being 

 swallowed, comes out again behind the gill-plates. The tail 

 is flattened vertically, and it is by lashing this powerful organ 

 first to one side and then to the other, that the fish darts for- 

 ward with great velocity, but it cannot rise or sink so quickly 

 as the whale, which is not requisite, as it is no longer under 

 the necessity of coming to the surface to breathe. 



In some parts of Scotland it is believed that salmon dart 

 along tail forwards : it is almost unnecessary to say, that this 

 is an error, for not only is it contradicted by the mechanical 

 structure of the animal, but if it were so the fish would be 

 suffocated, for the rushing water would press the gill-plates 

 firmly to the sides of the fish, and no water therefore could 

 reach the gills. 



It is remarkable how few know why a fish dies when it 

 is taken out of water, and yet the reason is very simple. 

 "When in the water the laminae of the gills all stand apart, 



