82 CAUDAL GILLS OF ZYGOPTERID LARV^, 



Hairs are absent from the cuticle. Stiff spines or bristles are 

 developed along each mid-rib, alsO along the dorsal and ventral 

 edges of the blade. 



The hypodermis is, on the whole, a very thin, somewhat 

 irregular layer of cells, usually only 3-4/x in thickness. Around 

 the four angles of th^ cross-section, it is thicker, especially 

 beneath the two mid-ribs. Pigmentation is only of moderate 

 density. There appear to be two transverse zones in the distal 

 half of the gill (Calopteryx) where the pigmentation is much less 

 dense than elsewhere. 



The central portion or rachis of the gill may be considered to 

 be bounded by the two zones of principal internal laminae (dl, 

 vl), which bound the two blood-canals externally. Within the 

 cavity of the rachis, all the more important structures of the gill 

 are collected. There are well-defined, oval, dorsal and ventral 

 blood-canals (dh, vh). In the walls of each canal, there runs a 

 pair of longitudinal nerves (nv), of which the dorsal pair are con- 

 siderably larger than the ventral pair. A little nearer to the 

 centre of the gill, and almost touching the blood-canals, lie the 

 two pairs of main longitudinal tracheae (tr). The rest of the 

 rachis is occupied with the alveolar mesh work, which is much 

 less abundant than in the more capacious saccoid gills. 



The interior of the blade of the gill, both dorsally and ven- 

 trally, is occupied by a small amount of alveolar meshwork, sup- 

 ported at irregular intervals by short, transverse, secondary 

 internal lamince (Plate v., fig. 3 4, dl'). Branch tracheae of con- 

 siderable size are to be seen traversing this part of the gill 

 obliquely. 



ii. The Lateral or Triquetral Gills (Plate iii., fig. 24; Plate v., 

 fig. 35). The cuticle and hypodermis resemble those of the 

 median gill, but are considerably thicker, the cuticle averaging 

 13/x or more in thickness, the hypodermis 6/x or more. The 

 cuticle is thickened along all three edges, and carries spines or 

 bristles there. The thickening is, however, especially noticeable 

 along the outer edge (Text-fig. 17, mr), which forms the true 

 mid-rib of the gill, corresponding with the mid-rib seen in the 

 lateral gills of Lestidce. 



