IX] 



THE RESPIRATORY SYSTEM 



187 



for a respiratory function. The main-folds were quickly lost, 

 and the beautiful and very regular formation, which we now see, 

 has been formed by constant addition of extra lamellae. In the 

 Synihemini, the most archaic of Libellulidae, these still only 

 number twelve in a row. 



Fig. 78 shews diagrammatically the connection between the 

 three main types. 



4. Histology of the Rectal Gills (fig. 83). 

 All forms of gill have the same histological structure, with 



1am 



B. 



Fig. 83. Histology of the rectal gill. A. T.S. through base of gill from Aeschna 

 brevistyla Ramb. B. The same through several lamellae of Cordulephya pygmaea 

 Selys ( x250). bp basal pad; cm circular muscle; Tib hypobranchial tissue; 

 lam blades of lamellae, cut at different levels in the one section ; pg pigmented 

 epithelial syncytium of gill; tr tracheae. Original. 



only unimportant differences of detail. The gill consists of the 

 following parts: 



(1) An external cuticle. This is continuous with the general 

 intima of the hind-gut. It is excessively fine, and covers the 

 gill on both sides. Where it turns over at the free edge of the 

 gill, it is armed with small spines. It is quite colourless. 



