424 RECTAL GILLS IN LARV^ OF ANISOPTERID DRAGONFLIES, 



longitudinal folds, forming continuous gills from end to end of 

 the branchial basket; each of these main-folds is supported at 

 intervals, to right and left alternately, by smaller cross-folds. In 

 the Duplex System, the main-folds are functionless or suppressed, 

 and the cross-folds become separate gills, arranged in six double- 

 rows. Further, the gills of the Simplex System may be classed 

 as either of the Undulate or Papillate Type, while those of the 

 Duplex System may be either of the Implicate, Foliate or 

 Lamellate Type. Whatever be the type of gill, it remains 

 essentially a delicate eversion of the rectal wall, projecting far 

 into the rectal cavity, and bathed on all sides by the water 

 drawn into that cavity. I have contented mj^self, therefore, 

 with figuring only the gill of the larva of Hemicordulia tau Selys 

 (Plate xlvii.). This belongs to the Lamellate Duplex Type, and 

 is, at the same time, both one of the most highly organised, and 

 one of the most easily understood of all rectal gill-types. 



On opening the branchial basket {i.e., the anterior portion of 

 the rectum) of this larva, we find a very beautiful and regular 

 arrangement of gills within. There are six, longitudinal, double 

 rows of lamellate gills (Fig.l) of a beautiful pale mauve tint. 

 Each single row contains twenty or more of these separate 

 lamellae, in the form of semiovals inserted on a broad base (Fig. 

 26), placed obliquely, and overlapping one another from before 

 backwards. .Consecutive lamellae are protected from rubbing 

 against one another by the action of three small tubercles (Fig. 

 2, ta, tp), which also allow a sufficient space between them for a 

 complete circulation of water around the gill. 



On the outer anterior face of the base of each gill, there is a 

 large rounded swelling, the basal pad(6/»), very conspicuous, and 

 of a semitransparent orange-colour. This pad is a turgid struc- 

 ture, apparently full of watery fluid. The boundaries of the 

 cells forming it are obliterated, but their nuclei are large, and 

 form centres from which very distinct fibrillar structures pass 

 out to the borders of the pad. Beneath the pad, the space 

 between the bases of the two walls of the gill-lamella is filled 

 with a mass of hypobranchial tissue (hb), surrounding the region 

 of the large efferent tracheae of the gill. The nature of this 



