THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 391 



exchange of the gases between the blood and the protoplasm of the 

 cells over the body. Much of the carbon dioxide given up by the 

 cells becomes carbonic acid and carbonates which may be trans- 

 ported by the plasma (fluid) of the blood. 



Respiration has been defined as the process involving the ex- 

 change of gases between the protoplasm of an organism and its 

 environment. All living protoplasm must be provided with a means 

 of receiving oxygen and giving up carbon dioxide. In protozoa and 

 simple metazoa, such as sponges, coelenterates, flatworms, round- 

 worms, and even some annelids, this gaseous exchange is made by 

 almost direct diffusion through the cell membranes to the surround- 

 ing medium. This movement of gas through the cell membranes de- 

 pends on the partial pressure of the particular gas on the two sides 

 of the membrane. Gas will flow in the direction toward the least 

 pressure. 



In the larger and more complex animals where the volume of 

 tissue is such that a more active interchange of gases is required 

 than the general body surface will permit, special organs or modi- 

 fications of the surface must be provided. Also the possibilities of 

 oxygen absorption are greatly increased by the development of 

 respiratory pigments like hemoglobin and hemocyanin, which are 

 carried in a blood vascular system all over the body. These pig- 

 ments readily unite with oxygen to form oxyhemoglobin in the 

 case of the former. Thus the blood is enabled to absorb far more 

 oxygen than an equal quantity of ordinary liquid. When the oxy- 

 gen pressure of the surrounding tissue is sufficiently low, the oxy- 

 hemoglobin releases its oxygen rapidly. Carbon dioxide accumu- 

 lates in excess in the tissues and diffuses from the cells to the 

 lymph, thence to the plasma where much of it combines with sodium 

 as sodium carbonate. Small amounts of CO2 combine with the 

 hemoglobin. 



The gills of most aquatic forms are richly supplied with a capil- 

 lary supply of blood and then membranous surfaces are directly 

 exposed to surrounding water from which the dissolved oxygen is 

 absorbed. In many aquatic worms the gill filaments are outgrowths 

 of the sides of the body wall. Likewise, the more or less plumelike 

 gills of crayfish are pocketlike outpushings of the body wall. In 

 a number of aquatic insects, worms, fishes, and turtles, the rectum 

 serves as an accessory respiratory organ. 



