ORGANIC EVOLUTION 213 



There are more or less definite channels {lymph vessels) 

 developed in all the higher vertebrates, for the circulation 

 of the body fluids aside from the blood vessels. These, in 

 our foregoing hasty survey, we have left out of account. 

 But there is one such vessel {the thoracic duct) of very great 

 importance in mammals; for by it the greater part of the 

 food enters the general circulation, in the manner diagram- 

 matically indicated in figure 136. 



Aquatic and aerial respiration. — In water, the supply of 

 free oxygen is that contained in the air which the water has 

 absorbed. The simpler organisms, being small, readily 

 obtain a supply by direct absorption through the surface of 

 the body. Increase of size, however, disturbs the ratio 

 between volume and surface in the body. As compensation 

 for the excessive increase of volume, absorbing surfaces are 

 increased by the outgro\\i:h of gills : and then mechanical 

 arrangemxcnts for bringing more water into contact with the 

 gills follow. The gills are lodged in respiratory chambers 

 through which a constant stream of fresh water is main- 

 tained, but still the amount of oxygen available is much 

 more limited than in free air. There are no warm blooded 

 animals except air breathers. 



In the open air, the oxygen supply is inexhaustible : but 

 air absorbing surfaces, such as are adequate for aquatic 

 respiration, cannot endure exposure to dry air. Some land 

 animals like the earthworm, living in moist places, are able 

 to breathe through the skin, by keeping it moistened with 

 mucus secretion; but if a worm be exposed to a dry 

 atmosphere it quickly dies of evaporation. 



The respiratory process, being essentially aquatic, requires 

 moist thin-skinned surfaces for the intake of oxygen, and 

 in organisms that live in dry atmosphere these can only be 

 maintained inside the body; hence, the lungs, reached by 

 long tortuous mucus-moistened passageways and main^ 



