GILLS OE FISHES. 



479 



317 



A branchial leaf, with the respiratory capillaries on side, 

 Cod. CCLXVIII. 



are long, e.g. in the Mackerel ; short in the Eel : in the Lopho- 

 branchs they are one-third the length of the arches : in the 

 Plectogonaths they are half that length ; in the Carp-tribe they 

 are nearly as long, in the Salmon-tribe quite as long, as the 

 branchial arches themselves. 



The main purpose of the gills of fishes being to expose the 

 venous blood in a state of minute subdivision to streams of water, 

 the branchial arteries rapidly divide and subdivide until they 

 resolve themselves into mi- 

 croscopic capillaries. These 

 constitute a network in one 

 plane or layer, fig. 317, 

 supported by an elastic 

 plate, and covered by a 

 tessellated and non-ciliated 

 epithelium. This covering 

 and the tunics of the capil- 

 laries are so thin as to 

 allow the chemical inter- 

 change and decomposition 

 to take place between the carbonated blood and the oxygenated 

 water. The requisite extent of the respiratory field of capillaries is 

 gained by various modes of multiplying the surface 

 within a limited space. In the Marsipolranchii 

 and Plagiostomi, for example, by folds of mem- 

 brane on plane surfaces : in the Lopholranchii by 

 clavate processes grouped into tufts : in the Pro- 

 topteri, by double or single fringes of filaments : 

 in the rest of the class by the production of the 

 capillary-supporting plates from each side of long, 

 compressed, slender, pointed processes, extending, 

 like the teeth of a comb, but in a double row, fig. 

 3 1 8, d, d, from the convex side of each branchial arch, 

 fig. 311, b. 



Each pair of processes has its fiat sides turned 

 toward contiguous pairs, and the two processes of 

 each pair stand edgeways toward each other,, and 

 are commonly united for a greater or less extent 

 from their base : hence Cuvier describes each pair 

 as a single bifurcated plate, ( feuillet.' ] 



In the Swordfish (Xiphias), the processes of the 

 same pair stand quite free from each other ; whence 



318 



Aristotle described this fish as having double the 



1 xxin. i. p. 379. 



Diagram of the cir- 

 culation of the blood 

 through the bran- 

 chial leaflets. Fish. 



XXIII. 



