bY tl. J. tiLLYARb. 85 



duced by gradual lengthening of the base, concurrently with a 

 flattening down upon it of the mid-rib and its two adjacent 

 sides. At the same time, the larva ceased to rest the gill upon 

 the ground, but held it out freely into the water, so that the 

 plane of the base gradually changed from oblique to vertical, as 

 in the median gill. The vertical portion is, however, never 

 actually yW/y attained, as may be seen from Text-figs. 19, 22. 



Under this sub-type, we have to deal with two very distinct 

 groups of gill-forms, one found in the Lestidff only, the other in 

 the Agrionidoi. These must now be dealt with separately. 

 (i.) The Lestid Form of Vertical Lamellar Gill. 



(Text-figs. 19-21: Plate i., figs.1-5: Plate iii., fig.25; Plate v., 

 figs. 36, 37). 



These are always simple, unjointed, vertical lamellse. They 

 *vary much in external appearance. But a study of them by 

 means of sections shows the following characteis to be constant 

 throughout the group . — 



1. The median gill has two mid-ribs, right and left; the lateral 

 gills have only one mid-rib, on the outer face. 



2. The median gill carries two main longitudinal tracheae: the 

 lateral gills only one apiece. 



3. The rachis is constricted off into a swollen central portion 

 carrying the main trachea?, and dorsal and ventral outer por- 

 tions, not much thicker than the blade of the gill, each carrying 

 one of the blood-canals. 



4. In connection with the above constrictions, the principal 

 internal lamime are excessively shortened, and can only just be 

 made out as short remnants connecting the closely approximated 

 hypoderm-layer of right and left sides. 



5. In the median gill, only the two dorsal longitudinal nerves 

 are present, lying upon the wall of the dorsal blood-canal. No 

 nerves can be seen on the wall of the ventral canal. This 

 reduction is unique for functional caudal gills, all other foi'ms 

 showing two pairs of longitudinal nerves. 



6. The alveolar meshwork is reduced to a minimum, its rem- 

 nants surrounding the large trachea? in the central portion of 

 the rachis. 



