276 ANIMAL BIOLOGY 



the upper part of the chest and greatly diminish the supply 

 of air which the lungs require. Not only this, but the 

 abdominal organs are displaced thus giving rise to various 

 other harmful effects. 



Respiration is a function common to all organisms 

 without exception. In many of the primitive animals 

 respiration takes place through the whole surface of the 

 body. Only in higher forms are there specialized organs 

 of respiration and these commonly consist of structures 

 by means of which a relatively large surface can be 

 brought into contact with oxygen. In aquatic forms this 

 surface is usually in the form of outgrowths, such as 

 filaments or plates, the walls of which are very thin so as 

 to permit the free interchange of gases. Gills of various 

 forms are to be met with in many worms, mollusks, crus- 

 taceans, several aquatic insects, in all fishes and usually 

 in the young of amphibians. The oxygen which the gills 

 utilize is, as a rule, the free oxygen which is dissolved in 

 the water. If animals are placed in water from which 

 the oxygen has been driven off they will die of suffocation. 



In land animals the increase of surface for exchange of 

 gases is usua ly obtained not by outgrowths, such as gills, 

 but by ingrowths, such as the tracheal tubes of insects or 

 the lungs of higher vertebrates. As the thin membrane 

 that separates the blood from the oxygen must be kept 

 moist for the proper transfer of gases the exposure of 

 delicate gills to dry air, to say noth'ng of dust and dirt, 

 would be very disadvantageous. In all higher land 

 animals Nature has safely located the organs of respira- 

 tion within the body where their delicate surfaces are al- 

 ways moist and furnished with an abundant supply of 

 blood. All of the varied organs of respiration in the animal 

 kingdom are devices for securing essentially the same end, 

 whether they are gills, tracheal tubes, lungs, or simply the 

 general surface of the body as in the Hydra and earthworm. 



