LAEVAL GILLS OP THE ODONATA. 415 



seems to be the result of tlie association and complete fusion of 

 t\Yo distinct elements — the subcuticular epithelium and the 

 tracheal cells. 



As before remarked, the gills contain an intralamellar cavity 

 in which the tracheal tubes are lying. The existence of this 

 cavity was not easily observable, as the two plates of the 



Kg. 3. — Part of section through a rectal gill. 

 tl., tracheal loops within the subcuticular layer, m.ir., main tracheal tube. 

 6.sp., blood-space, n., nucleus. 



lamella, in sections of hardened objects, are usually found stick- 

 ing tightly to each other. We endeavoured to determine its 

 limitations by cautiously injecting Indian ink into the "body- 

 cavity." The black particles were found between the two plates 

 up to the free edge. The existence of the cavity may, however, 

 be sometimes detected without any injection, and, even when the 

 plates are in contact, blood-cells are sometimes noticeable between 

 them. There is, therefore, not the slightest question about 

 the existence of an intralamellar cavity communicating with 

 the ccelom, and the presence of blood in the gill cannot be 

 doubted. The necessity for definitely establishing these points 

 is suificient when it is remembered that the process of respira- 

 tion would appear to be very different in a "bloodless" gill 

 from what it is in an organ supplied with an elaborate blood- 

 system. 



The tracheal loops of the gill were known to Leydig, and were 

 first described by Oustalet *. But no one, so far as we know, 

 has ever noticed that they are enclosed within the proto- 

 plasmic layer. Such gaseous interchange as takes place between 

 the contents of the tracheal loops and the exterior must first 

 involve this protoplasmic layer ; and this fact appears to us im- 

 portant in its bearings on the conclusion now gaining ground, that 



* Oustalet, " Memoire sur la respiration des larves des Libellules." Ann. 

 Sci. Nat. ser. 5, Zoologie, torn. 11 (1869). 



