258 REPORT— 1886. 



I merely mentioned these facts, without going into further details, in 

 order to show the importance of having an exact knowledge of the histo- 

 logical structure of the gills in order to be able to compare it with that 

 of the other organs mentioned. 



In the investigation I carried on at Naples during several months the 

 gills of the following Elasmobranch fishes were examined, both fresh and 

 preserved : — 



Scyllium catulus. Torpedo ocellata. 



— canicula. — marmorata. 



Trygon vtolaceus. Squafina angelus. 



Raja asterias. Musfehis Icevis. 

 — clavata. — vulgaris. 



The general structure and anatomy of the gills are of course well known, 

 and have been the subject of several important papers. 



The gills of sharks and rays are easily distinguished from the corre- 

 sponding organs of Ganoids and Teleosteans. While the rows of bran- 

 chial leaflets, which are placed upon the branchial arches in the latter 

 two groups project freely into a common cavity covered by the operculum, 

 in the Elasmobranchii they are distributed in separate branchial sacs. 

 Every one of these sacs also has its own opening to the exterior. As in 

 Teleosteans, the branchial leaflets are provided with secondary folds at 

 their sides, in which the true branchial capillaries are to be found, and 

 thus form the principal respiratory surface. 



The whole branchial leaflet has the form of the blade of a knife. The 

 base is taken up by the artery, the free margin by the vein. The trian- 

 gular shape of the cross-section is somewhat interfered with by the above- 

 mentioned lateral folds, wliich are placed upon the sides at right angles. 

 They appear as semicircular flaps. The gills, as well as the gill-clefts, 

 contain a large number of mucous cells in their outer cellular layers. As 

 far as I have been able to make out, there are no special cells having a 

 sensory function. In adult gills there are no ciliated cells ; in the fully 

 grown embryo of Mustelus, however, the cells covering the lateral flaps 

 of the branchial leaflets were ciliated. At the margin of the gill-cleft I 

 also observed ciliated cells. 



One of my principal objects in studying the gills of Elasmobranch 

 fishes was to find out the nature of the nerve-endings. I regret that, 

 although I tried a large number of different methods, and much time was 

 spent over it, I was not successful. For general purposes, however, I can 

 recommend the following method of staining with chloride of gold, which 

 did me more good service in tracing nerves than Ranvier's or any of the 

 other methods : — 



' Place the small object in a watch-glass-full of ^ per cent, chloride of 

 gold, and add one drop of hydrochloric acid. Leave this in the dark for 

 about half an hour ; then, after washing out with water, put the object in a 

 mixture of one part formic acid to four parts water, and expose to light 

 until a violet colour appears.' 



A more detailed account of the innervation, as well as the structure of 

 the mucous and other cells composing the cellular layers of the gill-cleft 

 and the branchial leaflets, will be published during the course of next 

 winter. I can only give this very short resume at present, having to 

 complete many of my observations by a series of sections which I purpose 

 making shortly. 



