148 PRINCIPLES OF ANIMAL BIOLOGY 



and lipolytic enzymes, that is, protein-, carbohydrate- and fat-splitting 

 enzymes respectively. How food reserves are built up into protoplasm 

 is not known but it is supposed that enzymes are concerned in the process. 



Respiration. — The oxygen requirement of living things is met by secur- 

 ing uncombined oxygen from the air or from solution in water. In most 

 animals of any size and complexity tracheae, gills, or lungs (described 

 in Chapter VI) serve as respiratory organs. These organs present a 

 surface large enough and of such a character that an ample supply of 

 oxygen enters the tissues. In animals not so highly differentiated, or 

 in some of those living in situations favorable to the maintenance of a 

 moist external surface, the highly vascularized skin itself may function 

 as a surface for gaseous exchange. This is particularly true of many 

 of the annelid worms in which there are no gills, and of the frog and other 

 amphibians in which the skin is an important organ of respiration. 



Respiratory Movements Not Respiration. — Any movements of the 

 body or of its parts which tend to bring fresh air or fresh oxygen-laden 

 water to these external or internal respiratory surfaces may be called 

 respiratory movements. These movements are ordinarily termed breath- 

 ing in terrestrial animals. Respiratory movements also include the 

 movements of the mouth and the gill covers of fish by which fresh water 

 is circulated over the gills, and the movements of portions of the bodies 

 of certain small aquatic annelid worms (Tubifex) which raise the posterior 

 third or half of the body above the mud and wave it about in order to 

 bring the skin in contact with more oxygen. Respiratory movements are 

 accessory to the act of respiration, but they do not constitute respiration. 



Mechanism of Oxygen Collection and Transportation. — In insects 

 the circulatory system has little or nothing to do with the transportation 

 of oxygen. Air enters the tracheal system through breathing pores 

 (spiracles) on the sides of the body and is carried into the ramifications 

 of the system by means of respiratory movements of the body. Oxygen 

 in the minute tracheal vessels diffuses through the delicate membranes 

 into the plasma with which the tissue cells are bathed and thence into 

 the tissue cells. In most higher invertebrates and in the vertebrates, 

 oxygen is transported by means of the blood contained in the circulatory 

 system from the respiratory surfaces to the tissues of the body. Gills 

 or lungs present a large surface for gaseous exchange. The internal 

 surface of the human lungs has been estimated at more than 100 square 

 yards, or many times the area of the body. Coming in intimate contact 

 with the delicate membrane of the lungs or gills is the blood which is 

 continually making a circuit through the circulatory system, as described 

 in Chapter VI. The efficiency of the mechanism for oxygen absorption 

 depends largely upon the properties of the protein hemoglobin which in 

 vertebrates resides in the red blood corpuscles but which in most of 

 the invertebrates having red blood is diffused throughout the blood 



