THE GILL-CHAMBER OF DRAGONFLY NYMPHS 321 



haemocoele, is attached. At the caudal end it tapers into a 

 distinct anal canal, about 1 mm. long. On the dorsal side of 

 the rectum are three longitudinal thickened areas, each with 

 about seventeen to nineteen pectinations on each side, and on the 

 ventral side there are three more. One of these areas occupies 

 the middle of the dorsal side. Each area is the base of a set of 

 respiratory structures within the rectum, and is white and 

 opaque. The pectinations point about 45° laterocephalad on 

 the middle dorsal row. Each pectination I call a 'gill base,' 

 and each bipectinate longitudinal area, formed by the fusion of 

 adjacent gill bases, I call a 'double row,' A dense brush of 

 tracheal branches which spring from the two dorsal trunks 

 passes into these gill bases on the dorsal side, but only to the 

 bases. The dorsal trunk is rather suddenl}^ increased in diame- 

 ter just anterior to the point where the branches to the rectum 

 leave it (fig. 1), but there is no similar expansion of the ven- 

 tral trunk. No branches to any other organs or parts leave the 

 trunks in the region in which the branches to the rectum are 

 found. 



On each dorsal trunk there are eight large branches to the 

 rectum, in addition to the postdorsal trachea. Four of these 

 branches are on the dorsal and four on the ventral side of the 

 trunk. In figure 1 and other figures the branches ventral to 

 the dorsal trunk have been omitted for the most part, as they 

 simply duplicate those on the dorsal side. Where they differ, I 

 note the fact. 



Each of the four large primary branches at once splits up: 

 the first one into four or five branches, the second into four, the 

 next two into two or three each. Each of these smaller branches 

 either divides into two or proceeds at once to a gill base, be- 

 fore entering which it again divides dichotomously. One of 

 these branches passes to the anterior or proximal, the other to 

 the posterior or distal end of the gill base of pectination of the 

 double row. From the postdorsal trachea five branches to gill 

 bases are given off on each side. The branches from the dor- 

 sal side of the dorsal trunk and the postdorsal trachea pass to 

 the gill bases of the same side of the middorsal double row, and 



