INTRODUCTION TO THE METAZOA 73 



the complex chemical compounds into the kinetic energy of the 

 living organism. The term respiration is applied to the process 

 of admission of oxygen into living protoplasm and the subsequent 

 giving off of carbon dioxide. More strictly this process should 

 be termed aerobic respiration, for there are some organisms which 

 obtain their energy release in the total absence of oxygen and 

 these are said to undergo anaerobic respiration. Since aerobic 

 respiration is by far the more common, the term respiration when 

 not modified is usually taken to mean this type. 



Respiration is a process essential for the existence of every 

 living cell, yet in the many-celled metazoans respiration as a 

 cellular process becomes masked or lost sight of through the 

 introduction of organs which facilitate the process for the entire 

 organism. Since the respiratory process involves an exchange 

 of oxygen and carbon dioxide, any surface which serves for this 

 exchange must be moist and delicate in order to permit a diffusion 

 of the gases. In animals which are diploblastic, as the coe- 

 lenterates, conditions for respiration are not essentially different 

 from those found in the single-celled Protozoa, for practically 

 every cell has a surface exposed to the water through which the 

 gases may diffuse. Even in some of the coelomate animals, 

 such as the earthworms, the body surface provides an area 

 sufficient for the respiratory exchange but in these as well as in 

 all the higher animals the body fluid plays an important part 

 in that it absorbs or loosely combines the gases within the body. 



In many animals, the body surface is unable to supply all of 

 the cells with oxygen because of insufficiency or because of dry- 

 ness which inhibits diffusion. Special organs for respiration 

 have been developed in all such forms. Roughly, in inverte- 

 brates, these modifications may be classified as gills, tracheae, 

 book-lungs, and lung sacs. 



Of gills, there are two distinctly different kinds, blood gills 

 and tracheal gills. The former usually consist of portions of 

 the body wall drawn out into thin filaments or thin lamellae the 

 cavities of which are continuous with the body cavity and are 

 filled with body fluids or contain blood vessels through the 

 walls of which the respiratory exchange takes place. Tracheal 

 gills are evaginations or invaginations of the body wall within 

 which or around which air tubes or tracheae are distributed. 

 Gills occur on almost any part of the body, wherever their func- 

 tion may be carried out. 



