BY R. J. TILLYARD. 47 



hard layer seldom exceeds one-fourth, though along the niid-rili 

 of a Lamellar Gill it may occup}' as much as one-half the total 

 thickness. 



The cuticle appears to be everywhere clear and unpigmented, 

 though granules of pigment (pg) can frequently be seen adhering 

 to its inner surface at places where the hypodermis has shrunk 

 away from it (Text-fig. 1). 



At ecdysis, the whole cuticle of the gill is cast oft' with the 

 )v)dy-cuticle of the larva. A study of the morphology of the 

 cuticle during ecdysis was not one of the objects of this paper: 

 Init one cannot pass over this question without calling attention 

 to the remarkable little ball-like bodies oi- secretions (also seen 

 by Ris) which accumulatt^ between cuticle and hypodermis just 

 prior to ecdj'sis, and which certainly appear to be the first de- 

 positions of a new cuticle, underneath the old one which is soon 

 ro be cast oft'. 



The hi/podn-mis is e\'ery where a single layer of cells, except at 

 the extieme base of the gills, and, exceptionally, under the mid- 

 I ib of Lajnellar Gills, where it sometimes becomes two or three 

 layers of cells deep. The typical epithelium forming the hypo- 

 dei-mis is a layer of rather ftat, polygonal cells from 4/x to 6/x in 

 thickness (Text-ftg.3, hy). The cell-boundaries are generally 

 indicated by pale intervals separating the darkly pigmented cells, 

 it being a general rule that the pigment-granules, whether few 

 or abundant, are most numerous around the cell-nucleus, and 

 tend to be absent from the outer border of the cell. The whole 

 hypodermis may be deeply pigmented (as in Dijjhlebia), or almost 

 without pigment (as in Aryiocnemis). Between these two ex- 

 tremes lie the larger number of gill-types, in which the pigment 

 is chietiy collected into certain areas of dense pigmentation, as, 

 for instance, in the gills of Austrolestes j^syche (Plate i., fig.l), 

 Argiolestes iet e7'omel as (Plate i., figs.8-9), Pseudagrion australasiw 

 (Plate ii., fig. 1 3), and A )isfrocnemi)< sjdpndida (Plate ii., figs. 1 8-1 9). 

 This arrangement often gives a beautiful pattern to the gills, and 

 may be of value for the protection of the larva in its natural 

 haunts. 



