ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 301 



gill in company with the afferent branchial artery. Similarly, both the efferent branchial 

 arteries of a gill are associated with lymph channels placing the lymphatic spaces on that 

 side of the gill in communication with the dorsal sinus system (cp. Part IV. pp. 220-1 ), 

 but the lymphatics are not so well developed on this side of the gill. 



The large mucous cells, which are so characteristic of the Myxinoid gill gut do not 

 occur within the gills themselves. In one case, however, 1 found numerous mucous 

 cells of the glassy type in the thicker epithelium on the efferent gill-duct side of 

 the gill. 



The whole of the complex cavity of the gill is lined by epithelium, which is better 

 developed near the two great surfaces of the gill, where the gill lamellae are simple in 

 structure, and which, further, is stronger on the afferent arterial side than the efferent. 

 There is roughly an extra rank of nuclei on the afferent side, and in some specimens 

 the disproportion is greater than this. The free surface in places is bounded by 

 flattened squamous cells, and under these there may be up to four irregular ranks of 

 nuclei, some of which here and there are observed to be undergoing mitosis. The cell 

 boundaries can be easily distinguished under the higher powers of the microscope, the 

 epithelium then having in vertical section the appearance of a mosaic. 



In the complex or more respiratory portions of the gill lamella the blood is only 

 separated from the water by what appears to be, in longitudinal sections of the vessel, 

 a single row of shallow cells about 2 /a thick, and having flattened nuclei up to 10m 

 long, separated by intervals of 20 m- In transverse sections of the vessels these 

 nuclei are seen to follow the curves of the vessel, and are hence markedly crescentic. 

 They measure about 10 m this way also, and are therefore circular concavo-convex 

 discs. Under the highest powers of the microscope, however, this boundary separating 

 water from blood appears as a very thin, almost structureless, membrane having definite 

 outer and inner borders, the nuclei above described being situated on the vascular side, 

 and internal to the inner border. The above statements apply only to that part of 

 the vessel which projects or bulges into the cavity of the gill, i.e. that portion of it in 

 direct contact with water. Elsewhere the nuclei are of a different character, and are 

 embedded in the wall of the vessel. 



Although appearances very often suggest it, I have not satisfied myself of the 

 existence of intra-cellular capillaries in the Myxinoid gill. 



The efferent gill duct is lined internally by a layer of mucosa thrown into about 

 twelve conspicuous folds. Underneath this is a somewhat extensive vascular zone of 

 fibrous connective tissue forming the submucosa. This in its turn is surrounded by 

 the irregular but well-developed muscular coat of striated fibres, of which the external 

 fibres are mostly circular and the internal ones longitudinal. External to the muscular 

 coat we have no definite boundary, but some very loose fatty connective tissue. The 

 lining epithelium is very similar to that of the gill itself : there are about three ranks 

 of nuclei ; cell boundaries are easily distinguishable in methyl-blue-eosin preparations — 

 the superficial row of cells being flattened, the cytoplasm vacuolated, with a definite 



TRANS. ROY. SOC. EDIN., VOL. XLIX. PART II. (NO. 3). 39 



