22 RESPIRATORY MECHANISMS 



almost sufficient in water saturated with air, but when it is 

 remembered that the metabolism is generally proportional 

 not to the weight, but approximately to W" 2l:i or to the surface, 

 we find that even for this sphere the oxygen supply would be 

 insufficient and, generally, that the necessary tension differ- 

 ence must be proportional to the radius of the sphere. The 

 general conclusion is that when metabolism is fairly high, 

 diffusion alone can provide sufficient oxygen only to organ- 

 isms of 1 mm diameter or less, while larger forms depending 

 upon diffusion must have a low metabolism. In many cases 

 the assumption of homogeneity does not hold, and the diffu- 

 sion coefficient for the surface layer may be definitely lower 

 than we have assumed. Calculations have been made further 

 on the assumption that diffusion takes place only inside the 

 organism while the oxygen outside is replaced by convection. 

 For many small organisms (eggs) which are immobile and 

 have a specific gravity close to that of the surrounding water 

 this assumption is not justified. 



It is to be remembered on the other hand that the surface 

 of a sphere is the smallest possible, corresponding to a given 

 volume, so that conditions improve with any deviation from 

 the spherical form and are a great deal better in threadlike 

 animals. For complicated and branched forms calculations 

 cannot be made, but it can be stated generally that from the 

 point of view of oxygen supply each branch is independent of 

 the rest. 



This type of respiration is realized in a large number of 

 small animals, viz., Protozoa, Planaria, Rotatoria, copepods, 

 many mites, eggs and young embryos in all groups and young 

 larvae of many groups, e.g., within the Crustacea. 



Some fairly large forms {Spongia, Coelenterata, Cirripedia) 

 also seem to depend exclusively upon diffusion. 



The structure of the Spongia provides a very large surface 

 practically proportional to the mass of tissue along which a 

 continuous flow of water is maintained by cilia. In this case 

 the size will have no influence upon the efficiency of diffusion. 



