320 STEPHEN G, RICH 



Throughout the Uterature of the subject there is a tendency 

 grossly to overestimate the number of gills in each rectum. 

 The statement of Oustalet that one species of Anax has 50,000 

 villi is copied into the standard text-books: Scott ('05) seems to 

 think that Plathemis has as large a number of gills. The table, 

 included in my summary, will show the facts as I have found 

 them. 



THE GILL-CHAMBER OF CORDULEGASTER 



The gill-chamber of Cordulegaster diastatops serves as a 

 rather primitive form, by reference to which the features of 

 other genera may be described with ease and clearness. The 

 present description is from a dissection of a full-grown nymph, 

 freshly killed. 



A very large tracheal trunk lies on each side of the rectum 

 and extends to a point near its caudal end. Here this trachea 

 passes ventrad and unites with a similar but smaller ventral 

 trachea. I call these two, dorsal and ventral trunks, following 

 the accepted usage. Near where the dorsal bends around into 

 the ventral trunk, a thinner trunk is given off, which runs along 

 the lateral margin of the abdomen. I call this the 'lateral 

 trunk.' 



As shown in figures 1 and 2, large tracheae continue the dor- 

 sal and the ventral trunks caudad. These are respectively the 

 'post dorsal' and 'postventral tracheae.' The postdorsal on 

 each side sends one of its divisions to the extreme caudal part 

 of the rectum, the anal canal; the postventral does not supply 

 parts so far caudad. The lateral trunk does not share in any 

 way in the tracheation of the rectum. 



These tracheal trunks, at least at their thickest parts, are en- 

 veloped by a whitish patch of adherent fatty tissue. In some 

 genera (Ophiogomphus in particular) this is fused into the 

 general fat-body occupying the haemocoele, and I am inclined to 

 think that it is always a part of this, though detached from the 

 rest. 



The rectum itself is a much enlarged portion of the gut, 

 about 2x6 mm. in size. It is rounded at the cephalic end, to 

 which the intestine proper, coming from the ventral part of the 



