GILLS OF FISHES. 



479 



317 



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

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

 Plectogonaths they are half that length ; in the Carji-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 sul)division 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 hj 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 cai-bonated 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 Marsipobi-ancldi 

 and Plagiostomi, for example, by folds of mem- 

 brane on plane surfaces : in the Lophohranddi 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, fiij. 

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

 fio;. 311, h. 



Each pair of processes has its flat 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 {Xiphlas), the processes of the 

 same pair stand quite free from each other ; whence 

 Aristotle described this fish as having double the 



' XXIII. i. p. 379. 



Abrauclijalloaf, ^vith the reppiratory caiiillarics on side, 

 Ood. CCLX"\'I1j[. 



318 



Diac^ram of tkc cii- 

 culatloii of tlie blood 

 through the bran- 

 chial leaflets, I'ish. 



Xilll. 



