58 CAtTDAL GILLS Of ZTGOPTERID LARVvE, 



In the Horizontal Lamellar Type, the horizontal flattening 

 undergone by the gill has resulted in a more or less complete 

 suppression of the principal zones of laminae. Numerous second- 

 ary short laminae are developed at intervals across the interior 

 of the narrow blade of the gill (Plate vi., fig.43,i/, W). In the 

 Lestid form of Vertical Lamellar Gill, the two walls of the 

 lamella lie mostly so close together that the internal laminje are 

 not developed at all (Plate v., figs. 36, 37). 



I propose to call all the lamime developed along either of the 

 principal zones of the gill the principal internal laniince. All 

 the rest I shall term the secondary internal laniince. There is 

 no difference in their structure. But, along the principal zones, 

 the laminae occur more regularly and in greater number, forming 

 a kind of open fence across the gill. If a lightly-pigmented 

 Vertical Lamellar Gill {e.g., that of Ischnura heterosticta Burm.) 

 be stained in toto with borax-carmine, cleared and mounted, the 

 two sets of principal internal laminae will appear like two long 

 straight rows of slender colonnades, rising from the " floor " to 

 the " roof " of the gill. 



The two principal zones of laminje separate off the central 

 thicker portion ^of the gill, termed the rachis, with its main 

 tracheae, nerves, and blood-vessels, from the two narrower outer 

 portions, which, in the case of Lamellar Gills, together form the 

 blade. 



A section along an internal lamina is shown in Text-fig. 3. 

 The lamina appears to be an ingrowth of the whole hypoderm- 

 layer at a given point. The lamina itself exhibits a kind of 

 stratified structure, being apparently formed of a number of 

 closely appressed parallel layers, the sections of which resemble 

 strongly-developed fibrillse. Minute pigment-granules lie between 

 (or upon) each separate layer, but they are of such small size 

 that their exact positions are not easily determinable. Hypo- 

 derm-nuclei, together with a certain amount of undifi'erentiated 

 cytoplasm from the hypoderm-layer, can often be seen supporting 

 the bases of the laminae on either side. 



Ris described the internal laminae of Pseudophcea as being 

 chitinous (28). But Ris' larva, as he clearly shows, was just at 



