r2 CAUDAL GILLS OF ZYGOPTERID LARV.?:, 



7'he Capillary System. — The capillaries, which are the end- 

 terms in the series of branchings from the main tracheae, are 

 always quite unpigmented, and very difficult to follow. They 

 are best examined in a freshly-cut-oif gill, soon after an ecdysis. 

 They do not form clear simple loops, like the capillaries of the 

 rectal gills of Anisoptera, though a close approximation to such 

 a condition may be examined at the ext^'eme tipoi a lamellar gill, 

 as in Text-fig. 1 0. Here one can follow out the complete course 

 of each separate capillary, and it will be seen that most of them 

 make a fairly simple complete loop from one branch-trachea to 

 another. In other parts of the gill, the capillaries cannot be 

 easily followed throughout their entire courses. Though an 

 occasional one can be followed through in a course of varying 

 directness from one branch to another, yet the general rule 

 would appear to be that they form an irregular anastomosing 

 network within the gill, lying for the most part close to the 

 hypodermis upon its inner side. I have not been able to demon- 

 strate the existence of any capillaries ending blindly in any part 

 of the gill, and do not think that any such are likely to occur. 

 The condition shown at the tip of the gill (Text-fig. 10), probably 

 represents, in a somewhat simplified form, the general method 

 of linking-up of the smaller branch -tracheae by means of the 

 capillaries. 



Part ii. 

 Studies of the Separate Gill-Types. 

 A. The Saccoid Type. 

 (Text-figs.11-16; Plate i., fig.6; Plate iii., figs.21, 22; Plate iv., 



figs.28-33.) 



In this type, each gill is in the form of a swollen sac or bladder, 

 more or less sausage-shaped, and usually tapering posteriorly to 

 a fine point. Text-fig. 11 shows an ideal section of a saccoid 

 gill-system taken across the basal third of the gills. Both 

 cuticle {cu) and hypodermis {hy) are very thin compared with 

 the size of the gill. The cuticle, however, may become fairly 

 tough, especially during the last larval instar. The hypoderm 

 cells are deeply pigmented, except at their boundaries, which 

 are usually clear. Their nuclei are fairly regularly placed, and 



