24 



RESPIRATORY MECHANISMS 



stance of the cuticle. Assuming as above A = 100 ml/ 

 kg/hour = 1/600 ml/g/m, r = 1 cm, T = 0.005 cm, and 

 £> = 0.000011, we find C = 1/4 atm. This would just about 

 allow the necessary quantity of 2 to diffuse in from saturated 

 water, provided the tension just inside the cuticle remained at 

 0, leaving nothing for the transport by convection. 



To account for the convection transport we must assume 

 that "blood" arrives at the surface containing a small amount 

 of oxygen at a tension which may approach 0. During its 

 passage along the surface it takes up the oxygen passing in. 

 The tension rises in consequence and may — when the condi- 

 tions for diffusion are optimal — approach that on the outside. 

 The mean tension difference must always be definitely lower 

 than the outside tension. It can be found by a complicated 

 integration when the tensions of the blood arriving at and 

 leaving the surface are known. 



From the formula it is seen that, assuming a constant metab- 

 olism per unit weight, the necessary tension difference is 

 proportional to the radius of the sphere, which would mean a 

 size limit to organisms respiring through the surface. When, 

 however, metabolism is proportional to W 2I ' S there should be 

 no theoretical size limit, provided the thickness and permea- 

 bility of the cuticle remains the same. In practice, however, 

 these conditions do set limits to the possible size and determine 

 the development of special respiratory surfaces. 



Fig. 3. Intraepithelial blood-vessels in the skin of the leech. A, in cross- 

 section; B, from the surface. (Hesse.) 



Respiration through the undifferentiated surface is found 

 in many Chcetopoda (Capitellida according to Lindroth, 1939), 

 Hirudinea (Fig. 3), Gephyrea, Synapla, Pantopoda. In several 



