RESPIRATION IN FISHES. 303 



as in the mollusca, the fibres being set close to 

 each other, like the barbs of a feather, or the 

 teeth of a fine comb, and being attached, on each 

 side of the throat, in double rows, to the convex 

 margins of four cartilaginous or osseous arches, 

 which are themselves connected with the jaws 

 by the bone called the os hyoides. The mode of 

 their articulation is such as to allow each arch 

 to have a small motion forwards, by which they 

 are separated from one another ; and by moving 

 backwards they are again brought together, 

 or collapsed. Each filament contains a slender 

 plate of cartilage, giving it mechanical sup- 

 port, and enabling it to preserve its shape 

 while moved by the streams of water, which 

 are perpetually rushing past. When their sur- 

 faces are still more minutely examined, they 

 are found to be covered with innumerable mi- 

 nute processes, crowded together like the pile 

 of velvet ; and on these are distributed myriads 

 of blood-vessels, spread, like a delicate net-work, 

 over every part of the surface. The whole 

 extent of this surface exposed to the action of 

 the aerated water, by these thickly set filaments, 

 must be exceedingly great.* 



A large flap, termed the Operculum^ extends 

 over the whole organ, defending it from injury, 



* Dr. Monro computed that in the Skate, the surface of the 

 gills is, at the least, equal to the whole surface of the human 

 body. 



