RESPIRATION IN AIR 61 



showing that at the low concentration the possible uptake of 

 O2 was the limiting factor. 



No satisfactory explanation has been found for the "dysp- 

 noeic" movements of Helix in low oxygen. 



The fresh-water Pulmonata were studied recently by Precht 

 (1939) . These animals move about on plants and stones under 

 water and come up to the surface at intervals to breathe. 

 Very often they contract the lung somewhat just before open- 

 ing the pneumostome in the surface layer, but otherwise they 

 do not make respiratory movements. Precht shows that two 

 different stimuli serve to drive the snails to the surface to 

 breathe. One is oxygen lack and the other is the amount of 

 air present in the lung which serves also hydrostatic purposes. 

 If a snail is exposed to an atmosphere of pure nitrogen it will 

 remain as usual for a couple of minutes at the surface and then 

 go down, but it will return very soon. An animal exposed to 

 pure oxygen will remain the same time at the surface, but will 

 return before the supply is exhausted when the volume of air 

 enclosed in the lung becomes sufficiently reduced. A return 

 towards the surface can also be induced, when the snails are 

 breathing atmospheric air, by increasing the pressure to 2 

 atm and thereby reducing the pulmonary volume, and when 

 the pressure was again reduced before the animals reached the 

 surface they turned back towards deeper layers. C0 2 in low 

 concentration has no influence, in high concentrations the 

 narcotic effect is apparent. The relative insensitivity to CO2 

 seems to be typical for diving animals, as we shall see later. 



In ventilation lungs the exchange of air with the atmosphere 

 takes place by volume changes of the lung. In inspiration 

 the lung volume is increased, a certain amount of outside air 

 flows in and is more or less completely mixed by convection 

 and diffusion with the gases already there. In expiration a 

 corresponding amount of the mixture is driven out. 



Ventilation lungs have been developed only in the verte- 

 brate phylum and are one of the essential conditions for the 

 attainment of a large or even a medium size, combined with 

 a high rate of metabolism, in terrestrial animals. By increas- 



