THE LIFE PROCESS IN VERTEBRATES. 



261 



gram. Their relation to the digestive, respiratory, and 

 renal organs is shown very diagrammatically. 



3. Respiration is effected by means of gills or of lungs. 

 In those vertebrates which have naked, moist skins, it is 

 probable that considerable air is absorbed directly through 

 the skin. 



When the blood has been replenished and aerated, ma- 

 terials have been supplied for all the chemical changes 



Fish or Tadpole. 



Adult Frog. 



Eabbit. 



COMPARATIVE DIAGRAM OF CIRCULATION, SHOWING IN A GENERAL 



THE COURSE AND DISTRIBUTION OF THE BLOOD : V, ventricle ; G, gills ; 

 P, lungs and pulmonary vessels ; L, liver ; S, stomach ; /, intestine ; 

 K, kidney. Black vessels carry venous blood ; light ones, arterial blood. 

 Arrows indicate the course of circulation. 



(metabolism) which take place in the tissues. The con- 

 structive processes (anabolism), by which the food be- 

 comes tissue ; .and the destructive processes (katabolism), 

 by which the tissue becomes waste water, carbonic acid, 

 etc., are, so far as we know, the same in all animals; 

 and the end of this building-up and tumbling-down of 

 organic molecules, in "continual round of change," is the 

 production of heat, motion, nerve force, etc., necessary to 

 the animal's life and activities. 



