RESPIRATION. 353 



not one of those creatures that have it, can remain in 

 water unless they are suffered to come to the surface 

 and breathe. This holds in the case of the plunger 

 now under notice, as well as in all the insects and 

 larvae which are not, through the whole succession of 

 their changes, to be confined to the water ; and any 

 one who waits by the side of a stagnant pool, during 

 those warm months when all is activity and life, may 

 notice the incessant ascent of larvae and full-formed 

 insects to the surface, for the purpose of that aeration 

 which is essential to life. On the other hand, the 

 animals, be they large or small, which are furnished 

 with apparatus that can separate oxygen at once from 

 water, cannot live in the air, but must get to the water 

 in order to breathe ; and it is quite as correct to say 

 that a water animal is drowned in the air, as that a 

 land animal is drowned in the water. 



And whatever specific difference there may be in 

 their structure, there is a generic form of organs for 

 each class. The land animal, that which breathes 

 " free air," or air without the admixture of water, 

 whether it inhale the air by nostrils or by pores in 

 the skin, always receives it into cells, and after a 

 little time discharges it again; while those that breathe 

 air in conjunction with water, and have a double sepa- 

 ration to make, first the air from the water, and then 

 the oxygen of the air from the nitrogen, receive the 

 water in a passing current ; and perform the double 

 chemical operation by the delicate fringes of gills of 

 some description or other ; over the surface of which, 

 the minute vessels of the circulating system are rare- 

 fied. 



2 H 3 



