414 



PROF. G. GULSON- A.KD J. SADOWES OK THE 



soon into a bundle of very fine tubes that neither divide again 

 nor end freely, as is the case in other organs, but bend into a 

 series of very long, curved, intra-lamellar loops. These loops, 

 running parallel to the surface of the gill, reunite to form other 

 main trunks which shortly leave the lamella and open into some 

 branch of fJie same main tracheal tube tbat gives off the original 

 trunks from which tbey are derived (fig. 2). As this is so, tbe 



Kg. 1. 



Libellula depressa. 



Pig. 2. 



Fig. 1. — Schematic sections through five larval gills, p, p',p", pillars. 

 Pig. 2. — Surface view of one gill, p, p', p'', pillars, il., tracheal loops, m.tr., 

 main tracheal tubes, tr., external tracheal trunk, ep.d., epithelial disc. 



air would not appear to circulate regularly tbrougb the system 

 of tracheal loops, as might be supposed to be the case if the 

 main lamellar trunks were branches of larger tubes coming from 

 different parts of the body. The contents of the loops must 

 be renewed, if at all, by some special mechanism, but we do not 

 here propose to further investigate this point. 



The tracheal loops, which evidently constitute the functional 

 part of the system, do not hang freely in the space between the 

 two laminae : they enter the subcuticular layer and run their 

 whole length through it, usually not in contact with the outer 

 cuticle (fig. 3). 



The subcuticular layer is a syncytium in which no cell- 

 boundaries can be detected. It contains two kinds of nuclei, and 



