THE METABOLIC MACHINERY OF ANIMALS 3L3 



while the next centers around its transfer to the cells of the body. 

 An examination of the first stage shows the blood passing through the 

 capillaries which are bathed in lymph where the oxygen pressure is 

 very low. This condition brings about dissociation of the oxyhemo- 

 globin to such a degree that it loses over a third of its oxygen during 

 its brief passage through the capillaries. The lymph in turn loses oxy- 

 gen to the cells in the same way. While oxygen is being liberated 

 carbon dioxide is being returned to the blood stream in exactly the 

 same manner, for carbon dioxide is present in greater concentration 

 in the cells than in the lymph and in the blood stream respectively. 



External Respiration 



While the phenomenon of respiration is a common one yet it is 

 accomplished in manj' different ways. Small, single-celled, or rela- 

 tively simple organisms have no need of a complicated respiratory 

 system. However, it is well to remember that while the surface of a 

 body varies as the square, its volume varies as the cube of its diam- 

 eter. This means that as an object increases in size the ratio of its 

 surface to its volume becomes smaller. By transferring this thought 

 to biological fields we can readily appreciate that as animals increase 

 in size respiratory systems become a real necessity. 



A survey of the animal kingdom shows that organisms have met 

 this need in a great variety of complex and sometimes rather 

 peculiar ways. Four types of respiration are commonly found, 

 namely, respiration through the surface of the body, by means of 

 gills, tracheae, and lungs. Three other methods are less commonly 

 found, namely, by means of respiratory papillae, respiratory trees, and 

 lung-hooks. 



Respiratory Papillae. These occur as evaginations from the 

 dorsal surface of such forms as the starfishes, where they are known 

 as dermal branchiae. They are really outpocketings of the body 

 wall. 



Respiratory Pouches or Trees. These tubular and more or 

 less branched pouches occur in such groups as the sea urchins, holo- 

 thuroideans, and some starfish. In the first group the pouches are 

 outgrowths from the mouth, while in the holothuroidea they are 

 outpocketings from the rectal region (see figure, page 314). 



Lung-books. Such structures consist of a series of folds suggest- 

 ing the pages of a book. Each "leaf" is filled with blood spaces and 

 is exposed on two sides to the air. Respiratory devices of this type 



