SOME OTHER TYPES OF INDIVIDUAL ORGANIZATION 187 



must be eliminated are proportional to the amount of protoplasm and 

 the rate of metabolism. When the animal is very small, the ratio of its 

 body surface to the volume of its protoplasm is comparatively great, and 

 if the surface is permeable to oxygen and carbon dioxide, it may be 

 sufficient to meet all respiratory needs. Several factors, however, set a 

 limit to the utilization of the body surface as a respiratory membrane. 

 As the size of the body increases, two distinct types of respiratory com- 

 plications result: (1) the ratio of body surface to volume decreases and 

 becomes inadequate for a sufficient respiratory exchange; (2) a high 

 proportion of body tissues are now internal, with no direct access to the 

 external medium. Any increase in the metabolic rate will, of course, 

 increase the minimum ratio of surface to volume necessary for an ade- 

 quate respiratory exchange ; and the various adaptations of body surfaces 

 for protection against loss of water or mechanical injury decrease or 

 wholly destroy their properties as respiratory membranes. All but the 

 simplest and smallest metazoans have consequently had to develop 

 special respiratory processes and structures. 



Types of respiratory processes. Four more or less distinct types of 

 respiratory processes can be distinguished. Simple respiration involves 

 a direct exchange through the cell membrane between the protoplasm 

 of the cell and the gases of the external medium. When a circulating 

 medium (blood) is utilized to provide for the respiratory needs of internal 

 tissues, respiration involves two distinct processes: external respiration, in 

 which the blood makes a gaseous exchange with the external medium; 

 and internal respiration, in which a gaseous interchange takes place 

 between the aerated blood and the protoplasm of the cells. Finally, in 

 many organisms the necessity of providing special respiratory surfaces 

 (which in most instances must be protected against loss of water, mechan- 

 ical injury, or both) is associated with breathing, a process that results 

 in the continual aeration of the respiratory surface. 



Simple direct respiration. The simplest of all respiratory processes 

 is found in the Protozoa, the lowest Metazoa, and in certain minute 

 representatives of the intermediate and higher Metazoa. Here, since the 

 ratio of surface to volume is large, since all protoplasm is within a very 

 short distance of the body surface, and since the latter is freely permeable 

 to oxygen and carbon dioxide, an adequate gaseous exchange is provided 

 by a direct diffusion between the protoplasm and the external medium. 



The direct respiration of insects. A far more complicated type of 

 direct respiration has been developed by the insects and certain of their 

 relatives. Here the tough exoskeleton and the comparatively huge pro- 

 portion of internal tissues preclude gaseous interchange through the body 

 surface, and there has been developed a complex network of ramify- 

 ing air tubes (tracheae), that lead from external openings to all of the 



