RKSriKATluX UF MBSOGOMPIILS- 



429 



are longitudinal bands of muscle, M. Figure 4 shows these 

 details. 



It we ()))en the rectum, we shall find within it a mass of 

 thin white filmy folds. There are six longitudinal folds, which lie 

 loosel}-, and are much folded upon themselves. They appear in 

 cross-section in Figure 5. FF. Arranged alternately along eacti 

 side of the longitudinal folds, at the base of these, are 20 of 

 what I may call buttress folds, adopting the terminology from 

 another genus. These leaf-like folds, BF, which are of tlie 

 shape shown in figtire 5. hang loose within the rectum. 



D 



GrB M G.B 



iM.y. 4,— Dt'tail ^f Fig. 2. 



Fis. 



-Cross-Section. 



All the folds are respiratory organs. They are all filled 

 with a network of repeatedly-branching tracheoles, which be- 

 come finer and finer as they approach the edge of the folds. 

 Near the edge they form an immense number of recurving 

 loops. These loops are not blind, but lead back again into the 

 larger tracheal branches. Sadones, 1895, has shown that for 

 other species of dragon-flies these loops are actually imbedded 

 in the epithelium of the folds or gills ; and I have reason to think 



