40 CAUbAL GILLS OF ZYGOPTfiRit) LARV^, 



B. Whole Mounts of the Gills. 



Cleared unstained whole mounts of the separate gills can be 

 made use of in all cases, except for the very thick and opaque 

 saccoid gills of Dvphlehia. The larva should be taken preferably 

 near the middle of an instar: not just after ecdysis, when the 

 cuticle is weak and liable to shrivel, and iiot just before ecdysis, 

 when there are two cuticles, and the new inner cuticle is liable 

 to shrink away from the old hard outer cuticle. 



The gills should be cut off from the living larva, at the break- 

 ing-joint, and allowed to drop into some fixing fluid, or simply 

 into 70 per cent, alcohol; the object being simply to study the 

 external form of the gill, and the courses of the main and branch 

 tracheje. By passing rapidly up through 90 per cent, to absolute 

 alcohol, clearing in clove oil, and mounting in Canada Balsam in 

 the usual way, the tracheal system can usually be preserved 

 intact for months or even years. Saccoid and triquetral gills 

 require a longer time than lamellar gills for dehydrating and 

 clearing, and must be mounted within a raised ring. 



Stained whole mounts are best made from gills fixed for 24 

 hours in Carls' Fixative (see below, section C). The fixative 

 must be thoroughly washed out of the tissues (several changes of 

 70 per cent, alcohol during 24 hours or more), and the gill passed 

 down through 50 per cent, alcohol to 35 per cent, alcohol. It 

 should then be stained for 24 hours or longer in 35 per cent, 

 alcoholic solution of Grenacher's borax carmine, and the stain 

 differentiated out for 24 hours or more in acid alcohol (0*5 per 

 cent. HNOg in 70 per cent, alcohol). Dehydrate, clear and 

 mount as usual. The stained gill shows very clearly the hypo- 

 derm cell-layers and nuclei, as well as the courses of the dorsal 

 and ventral blood-canals. 



C. Serial Sections. 



It is not easy to obtain good serial sections of the caudal gills, 

 chiefly because of the chitinous cuticle, which not only resists 

 the knife, but also makes it very difficult to fine a suitable fixa- 

 tive, which will penetrate the gill thoroughly, and yet, at the 



