418 THE LAETAL GILLS OF THE ODONATA. 



play a very imporfcaBt part in tlie excretion of carbonic acid, for 

 it has been shown before, and it should not be forgotten, that 

 its function in the gill is largely that of nourishing the tissues. 



To recapitulate : — 



1. The rectal tracheal gills of larval Odonata are prevented 



from adhering to one another by the presence of three 

 conical pillars. 



2. The main tracheal tubes alone are lodged between the two 



plates that form the gill ; the terminal loops, i. e. the 

 functional parts of the system, run within the protoplasm 

 of the subcuticular layer, 



3. A blood-space communicating with the "body-cavity" exists 



in the rectal gills. 



4. The oxygen seems to be absorbed through the tracheal loops 



by the action of the subcuticular protoplasm only, and 

 to be discharged from these into the general tracheal 

 system. 



5. Carbonic acid, on the contrary, appears not to be carried to 



the gills by the tracheal tubes but by the blood alone, 

 certain enigmatical organs borne upon a " praerectal 

 vesicle" being perhaps directly concerned in its excretion. 



6. In any case the blood would appear to play an important 



part in the excretion of carbonic acid, and a very un- 

 important one in the absorption of oxygen. 



