416 PEOr. G. GILSOJT AND J. SADOKES 01^ THE 



the absorption of oxygen is not a mere physical process but a 

 more complex one, in which the living protoplasm plays an active 

 part. It has been experimentally shown that the death of the 

 epithelial cells causes a striking change in the action of an organ 

 functioning, during life, as an osmotic divider between two dif- 

 ferent liquids or gases. As Professor Miall very rightly remarks, 

 the first setting up of the process in young larvae, when small 

 bubbles of gas appear in the liquid that fills the tracheal tubes, 

 cannot possibly be explained as a mere physical phenomenon. 

 Whatever may be the mechanism of respiration, it is a process 

 much more intricate than the play of an ordinary osmotic appa- 

 ratus ; a process that deserves the term " vital," which we are 

 wont to apply to complex activities, the actual workings of which 

 escape our observation. The living protoplasm is the agent of 

 absorption and setting free of the oxygen as well as of the 

 emission of carbonic acid. 



No wonder, therefore, that in the gills of Odonata the func- 

 tional air-tubes are completely imbedded in the protoplasm of 

 the subcuticular layer. And, as regards the absorption of 

 oxygen, there is no wonder that no special mechanism is provided 

 to renew or to remove the contents of the tracheal loops. If the 

 oxygen extracted from the surrounding water is actively dis- 

 charged into the tracheal cavity by the protoplasm, and a stream 

 of gas is continually blown out of the loops into the general 

 tracheal system, no external mechanism is wanted to clear out 

 the gaseous contents of the gill and transmit the oxygen to the 

 other parts of the body. 



As regards the emission of carbonic acid a difficulty arises ; for 

 if the function of the gill be to excrete the carbonic acid as well 

 as to absorb the oxygen, it seems likely that the former must be 

 carried to the organ by some mechanism. If the tracheal tubes 

 furnished the only apparatus through which the carbonic acid 

 could be carried, it seems that a " propelling" mechanism, though 

 unnecessary as regards oxygen, would be required. But this is 

 not the case. Carbonic acid is carried away from the organs hy 

 the blood and the blood-system enters the gills, as we have said 

 before. There it may be directly absorbed and ejected by the 

 subcuticular layer without ever entering the functional part of 

 the tracheal system. The only objection to this view is that the 

 blood is not very abundant in the gill, and that no special 

 mechanism is known to make it circulate through the organ. 



