RESPIRATION IN AIR 53 



As in the water-breathing animals, we find a series of 

 mechanisms of increasing complexity and effectivity running 

 more or less parallel with the general level of organization. 



Respiration by diffusion through the general undiffer- 

 entiated surface and unassisted by circulation is the rule for 

 eggs and embryos during the first stages. Many eggs (e.g., 

 those of birds) are quite large, but the metabolism calculated 

 for such eggs as a whole is extremely low. Most eggs are 

 placed in surroundings where there is normally a high degree 

 of moisture, but there are numerous exceptions among insects, 

 and it is often difficult to see how such eggs avoid desiccation. 

 It is possible for a membrane to be almost (or perhaps com- 

 pletely) impermeable to water and at the same time to allow 

 the diffusion of some oxygen and CO2, as found by Gray (1 932) 

 and by Krogh and Ussing (1937) for trout eggs, but the point 

 should be studied on suitable eggs in air. 



A few terrestrial arthropods of about 1 mm length or less 

 (Tardigrada, Pauropoda) and also the parasitic Linguatulidce 

 which reach several cm in length manage, even as adults, 

 without respiratory or circulatory organs. 



In embryos at later stages the respiration through the 

 general surface of the egg is aided by the circulation of the 

 blood in the embryo and, as in aquatic eggs, there is often a 

 network of vessels on the surface of the yolk sac. 



Adult animals having circulation of blood just below the 

 surface can attain a weight of several grams and a length of 

 30-40 cm (Earthworms, Lumbricus) without developing special 

 respiratory organs when they live in a moist atmosphere, but 

 most such animals are very small, like Koenenia (Pedipalpi), 

 many Acarina, and some Apterygota. 



As in the aquatic animals, the body surface will generally 

 take part in the respiratory exchange, even when specific 

 organs exist, and in some animals this accessory function is, 

 under certain conditions, of vital importance. 



Cutaneous uptake of oxygen must take place when the O2 

 tension of the blood and tissues just below the surface is lower 

 than the atmospheric and the skin is permeable to oxygen. 



