372 Fortieth Annual Meeting 



only the tip of the opercular element of the gill cover is 

 turned in, while in other cases it is turned in clear to the 

 preopercular bone. The latter is the usual condition and a 

 large area of the gill filaments is thus exposed. I have in 

 this jar such a specimen. Only one side is affected, and 

 you can readily see from this how much of the gill is 

 exposed. 



We might naturally expect that the exposure of the 

 gill directly to the exterior would have some effect on it. 

 We know very well th^t when epidermal tissue is exposed 

 to friction it tends to become thickened, and that is the 

 condition here. 



I have here a diagram of a normal gill filament, with the 

 smaller lamellae on either side, showing the general pro- 

 portions of the blood-vessels in comparison with the very 

 thin layer of epidermis which covers them. This layer is 

 composed of only a single series of very much flattened cells, 

 so that the blood in the capillaries comes very near to the 

 surface. 



Now, comparing with this normal filament an abnormal 

 one, we find that while the axis and the general arrangement 

 of the main blood-vessels are the same, at the tip there is 

 often a portion which is very greatly modified. Sometimes 

 as much as half of the gill filament is without the secondary 

 lamellae, and frequently it is abnormally .enlarged at the 

 end. Often a number of the lamella; are fused into one 

 mass, or partially fused at the base, or, again, they may 

 be greatly abbreviated. Wherever the filaments are ex- 

 posed to any extent we find them knobbed at the end, and 

 the external layer of cells is very greatly modified. The 

 cells, instead of being much flattened, become cuboid or 

 columnar, and, especially at the ends where they are more 

 exposed, we often find two or even three layers of these cells 

 separating the water from the blood. 



Naturally a gill filament so modified cannot be as ef- 

 fective in absorbing oxygen from the water and giving off 

 carbon dioxide to it as one that has a thin, unmodified layer. 



