42 RESPIRATORY MECHANISMS 



as Bevelander thinks, be an adaptation to a low oxygen habi- 

 tat, but the conclusion is by no means binding. At low 2 

 concentrations a very high utilization is not called for so much 

 as a large ventilation for which the honeycombed gills do not 

 appear specially well fitted. It is conceivable that they may 

 serve for respiration of air, although the capillary resistance 

 of the very narrow passages must be rather high. 



Branchial surfaces. Gill surfaces were measured in a few 

 cases on Crustacea (Putter, 1909) and fishes (Riess, 1881; 

 Putter). Putter thought he could show that within the same 

 species of fish the gill surface is proportional to the outer 

 surface or, what amounts to the same thing, the square root 

 of the gill surface divided by the cube root of the weight should 

 be constant. This relation is rather probable a priori, but 

 Putter's measurements are not sufficient to prove that it is 

 true. It seems certain, however, that gill surfaces per unit 

 weight are larger in small specimens. When different ani- 

 mals are compared, some relation should be found to the 

 maximal metabolism, but the material available is insufficient. 



In the lower gill-breathing forms the circulation is not 

 definitely adapted to the branchial respiration, but in most of 

 the Gastropoda, the Cephalopoda, the higher Crustacea, and the 

 fishes the adaptation is perfect, the venous blood being col- 

 lected from all other tissues and passed through the gills 

 before being returned to the general circulation. In the in- 

 vertebrates the gills are located on the venous side of the heart, 

 which is therefore the first organ to receive the oxygenated 

 blood. In the fishes the heart is supplied with venous blood, 

 and discharges directly to the gills. From the point of view 

 of respiratory efficiency there is nothing to choose between the 

 two systems. 



Regulation of branchial respiration. The mechanisms for regu- 

 lating respiration in aquatic animals have been the subject 

 of a great deal of discussion. In many lower forms the regula- 

 tion is very poor, or even absent when the movements serve 



