612 



CAUt)AL GILLS OF ZYGOPTERID LARV^, 



the lateral 



the opposite is the case. Thus the blood will How 



faster dorsally tlian ventral ]y in the median gill, but faster ven- 

 tral]}^ than dorsally in the lateral gills. Therefore, in each case, the 

 afferent blood-stream is the faster of 

 the two, flowing along the narrower 

 channel. Thus arises the first sign 

 of a differentiation between the pri- 

 mary or afferent blood-canal, and the 

 secondary or efferent blood -canal. 

 D 



rext-fiir.38. 



nv 



Text-%39.t 



In Text-tig. 39, I have shown a transverse section across the 

 gills of a small larva of Neosticta ainesceiiti (Saccoid Type), which 



* Two T. 8. through median and left lateral caudal gills of A ustroagrion 

 cyane tSelys, fifth instar, a at about one-third from base, h at l^eyond half- 

 way along the gill. Lettering as in Text-fig. 37; nv, main nerve; the arrow 

 VD shows the direction of the sagittal plane, the median doi-sal gill being 

 uppermost; ( x 370). 



t T.S. through median and right lateral caudal gills of Neosticta canes- 

 cem Tillyard, fourth or fifth instar. Lettering as in Text-figs. 37 and 38; 

 h, hair; ( x 370). 



