THE GILL-CHAMBER OF DRAGONFLY NYMPHS 327 



or four full rows, avS in certain forms to be described. The re- 

 maining rows on the ventral side are of course supplied from 

 the ventral trunks. 



5. Each gill or buttress fold is supplied with at least two 

 tracheal branches. In most forms these two branches come from 

 different sources of the different series of major branches leaving 

 the trunk or resulting from the first divisions of these major 

 branches. 



6. An epithelial cushion, of tough and thick epithelium, is 

 present on the cephalic side at the base of every gill. In many 

 Aeschnidae a similar cushion is present on the caudal side of the 

 gill or buttress fold also. 



7. In the base of all gills or buttress folds fat is present, lying 

 in the cavity of the fold, which communicates with the haemo- 

 coele. 



8. The rectum always has six bands of longitudinal muscles, 

 arranged three on the right and three on the left side. The cir- 

 cular muscles ahvays form a layer over the whole rectum. 



9. In every case the distal part of the folds, gills, or villi are 

 filled with loops of tracheoles, which usually connect fine twigs 

 from each of the branches entering the gill, but some of which 

 also connect twigs from the same branch. This part of each gill 

 is functional in respiration. There are no loops in the region of 

 fat and epithelial cushions. 



10. An anal valve, consisting of three semilunar plates, of 

 which one is situated midclorsally, is always present. 



11. The rectum occupies the seventh, eighth, and ninth 

 somites of the abdomen, but may extend forward into the sixth. 

 It never extends into the tenth somite, but whore this is long 

 there is a distinct anal canal. Where this somite is short, the 

 rectum tapers directly and abruptly to the anal valve. 



