CHAPTER III 

 RESPIRATION 



RESPIRATION in its widest sense is the sum total of the processes 

 by which the ultimate elements of the body gain the oxygen 

 they require, and get rid of the carbon dioxide they produce. 



Comparative. In a unicellular organism no special mechanism of 

 respiration is needed ; the oxygen diffuses in, and the carbon dioxide 

 diffuses out, through the general surface. The simple wants of such 

 multicellular animals as the coelenterates, the group to which the 

 sea-anemone belongs, are also supplied by diffusion through the 

 ectoderm from and into the surrounding water, and through the 

 endoderm from and into the contents of the body-cavity and its 

 ramifications. 



But in animals of more complex structure special arrangements 

 become necessary, and respiration is divided into two stages : 

 (i) External respiration, an interchange between the air or water 

 and a circulating medium or blood as it passes through richly 

 vascular skin, gills, tracheae, or lungs ; and (2) internal respiration, 

 an interchange between the blood, or lymph, and the cells. 



In the lower kinds of worms respiration goes on solely through the 

 skin, under which plexuses of bloodvessels often exist, but in some 

 higher worms there are special vascular appendages that play the 

 part of gills. The Crustacea also possess gills, while in the other 

 arthropoda respiration is carried on either by the general surface of 

 the body (in some low forms), or more commonly by means of 

 tracheae, or branched tubes surrounded by blood spaces and com- 

 municating externally with the air and internally by their finest 

 twigs with the individual cells. Most of the mollusca breathe by 

 gills, but a few only by the skin. 



Among vertebrates the fishes and larval amphibians breathe by 

 gills, but most adult amphibians have lungs. The skin, too, in such 

 animals as the frog has a very important respiratory function, more 

 of the gaseous exchange taking place through it in some conditions 

 than through the lungs. 



One small group of fishes, the dipnoi, has the peculiarity of pos- 

 sessing both gills and a kind of lungs, the swim-bladder being sur- 

 rounded with a plexus of bloodvessels and taking on a respiratory 

 function. 



In all ths higher vertebrates the respiration is carried on by lungs ; 

 the trifling amount of gaseous interchange which can possibly take 

 place through the skin is not worth taking into account. The lungs 

 are to be regarded as developed from outgrowths of the alimentary 

 canal, beginning near the mouth. 



206 



