10'2 CAUDAL GILLS OP ZYGOPTERlD LARV^, 



median gill possessing a pair of dorsal longitudinal nerves and a 

 pair of ventral ones, while the lateral gills have only one of each, 

 placed on the outer wall of the corresponding blood-canal. 



The rachis of the median gill is bounded internally on either 

 side by a series of secondarily developed interned lamina' (Plate 

 vi., fig.43,i/) passing fi'om the dorsal to the ventral side of the 

 gill. vSimilar but less strongly developed laminae occur at inter- 

 vals at different levels along the blade of the gill {il'). These 

 laminae are not, of course, the original laminae of the gillbefore 

 it became horizontally flattened. The remnants of the original 

 series of principal laminje, running horizontally across the gill, 

 are to be seen only in a set of slender laminse crossing the rachis 

 below the ventral blood-canal (Plate vi., fig. 4 2, v/). These laminae 

 mark off ventrally what was the original rachis of the gill from 

 the remnant of its ventral blade, which has now become the 

 portion just above the mid-rib. The present blade of the gill, to 

 right and left of the new rachis, is purely an extension of the 

 old rachis itself, due to intense flattening and widening in the 

 horizontal plane. As will be seen from Text-fig. 2?^, the original 

 right and left mid-ribs of the gill, before it became horizontal, 

 are still preserved as tlie swollen, somewhat knobbed, edges of 

 the new blade. 



The cuticle is rather thin, aN eraging about 6/x in thickness, of 

 which the tough outer sti'atum occupies from 2 to ofx. The 

 hypodermis is slightly thinner than the cuticle, a\eraging about 

 5/x in width, but somewhat irregular. The nuclei mostly lie 

 with their long axes parallel with the cuticle. They are of 

 tmall size, only 5-6/x in length, except near the base of the gill, 

 where many of the nuclei are considerably larger. The hypo- 

 dermis is almost everyAvhere pigmented, but in certain transverse 

 zones the pigment-granules are exceedingly abundant, almost 

 hiding the nuclei from view. 



Considering how narrow the space in the interior of these gills 

 is, it is rather surprising to find the ah eolar meshwork present, 

 not only in the rachis, but also throughout the blade. The 

 meshwork is most extensively developed in the space between 



