RESPIRATION IN AIR 55 



roundings, but others belonging to the same group have 

 developed functional lungs and have a greater power to 

 withstand desiccation {Porcellio). 



Lungs as physiologically defined are respiratory surfaces 

 folded into the body. The gas exchange takes place through 

 these surfaces between the "alveolar" air and the blood, and 

 the gases are transported between the tissues and the lungs by 

 the circulation of the blood. The distinction between lungs 

 and tracheae carrying air directly to the tissue cells is not quite 

 sharp. In certain small insects and in several arachnoids the 

 tracheal system is poorly developed and does not reach all 

 parts of the body, so that some circulation is essential for 

 respiration. Among the Chilopoda and Arachnoidea we have 

 forms in which the tracheal system is definitely developed in 

 the form of lungs. 



Lungs are of two physiologically different types, viz., diffu- 

 sion lungs and ventilation lungs. In the former, diffusion of 

 oxygen and C0 2 between the outside air and the respiring 

 surface is normally sufficient to supply the oxygen and remove 

 the CO2, while in the ventilation lungs a mechanical transport 

 of air by movements which alter the volume of the lungs is 

 essential. 



Diffusion lungs. Diffusion is effective only over short dis- 

 tances and can serve for the gas transport to the respiring 

 surfaces only in rather small animals. The largest are prob- 

 ably the African pulmonate snails Achatina and Buiimus, having 

 a volume up to 500 ml. 



Diffusion lungs are found in all the pulmonate gastropods, 

 in the Scorpionidce, Pedipalpi and Araneidce among the Arachnids, 

 in a chilopod, Scutigera, and in the "tracheate" isopods, espe- 

 cially the Porcellionidcz (Verhoeff, 1921). 



It is a curious fact that in several quite unrelated forms 

 diffusion lungs have taken the form of a system of numerous 

 short and narrow tubes, generally called tracheae, opening 

 into a common vestibulum. This arrangement is found in 



