i68 



ELEMENTS OF BIOLOGY 



that traverse the tissues of the body. In an insect on both sides in 

 each segment are small apertures called spiracles (Fig. iii). These 

 openings lead into membranous tubes, termed trachea, which sub- 

 divide and ramify throughout the insect body. Thus the air is 

 plumbed directly to the tissues. Recent investigations indicate that 



TRACHEAL TUBE 



AIR SAC 



Fig. III. — Diagram to show the position and distribution of the respiratory sys- 

 tem of an insect. (After Kiihn: Griindriss der aUgemeinen Zoologie. Georg Theime, 

 Leipzig.) 



the more anterior tracheae are incurrent, while the posterior are ex- 

 current, so that the air follows an established course through the 

 tissues. 



Among vertebrate animals three main types of respiratory systems 

 are employed. One of these, the skin, has a limited respiratory func- 

 tion in most forms, but in the frog and other Amphibia a very 

 considerable proportion of the gas interchange between the blood 

 and the environment takes place through the skin. In fishes, in 

 some adult Amphibia, for example the salamanders, and in most 



