CHAPTER III. 

 BESPIRATION. 



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 ccelenterates, 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 arthro- 

 poda 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 communicating 

 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 than through the lungs. 



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



