38 NATURAL HISTORY OF AQUATIC INSECTS CH. 



chambers of its shell, some Zoophytes their floats, 

 some aquatic Crustacea and Polyzoa their eggs with 

 air or some other gas. Even the microscopic Proto- 

 zoon, Arcella, possesses the same faculty. 1 Green 

 plants, under the influence of sunlight, liberate 

 gaseous oxygen from their tissues. This is probably 

 not due to the direct action of the sun's heat, for 

 it can be observed in cold running water, just 

 escaped from the fissures of a limestone hill. 



The passage of dissolved gas through living mem- 

 branes is equally mysterious. Direct experiment upon 

 the freshly excised lung of the Frog shows that no 

 diffusion takes place until the cells are dead. 2 



It is well known to the makers of microscopic 

 preparations that tissues will not take up colouring 

 matter so long as the cells are alive, nor will the 

 natural colouring matter, e.g. of Beet-root, pass out 

 into water, until the cells are killed. 



Bohr 3 finds that the air-bladder of a Fish refills 

 after puncture and complete emptying, the new gas 

 containing as much as 80 per cent, of oxygen. But 

 if the branches of the vagus nerve which supply the 



1 In this case the gas is carbon dioxide, whereas in others 

 it is apparently oxygen or some mixture in which oxygen pre- 

 dominates. Our information as to the composition of the 

 gases concerned is unfortunately most imperfect. Biot in 1807 

 showed that the gas in the air-bladder of Fishes is rich in 

 oxygen. 



2 Tigerstedt und Santesson. Bihang till K. Svenska Vet. 

 Akad. Handl. Bd. XI. No. 2 (1886). NaCl could not be made 

 to diffuse through the wall of the lung so long as the epithelial 

 cells were alive. 



3 C. R. CXIV. p. 1560(1892). 



