VASCULAR SYSTEM OF FISHES. 261 



tliey are blended into a single orifice in Symbrcmchns, analogous to 

 that in the Myxine. In some Ganoids, many Plagiostomes, and all 

 Sturgeons, a canal leads from the fore part of each side of the bran- 

 chial chamber to the top of the head ; the outlets are called ' spiracles,' 

 the canals ' spiracular.' The nasal sac communicates in the Lamprey 

 with the single homologous canal. 



The main purpose of the gills of fishes is to expose the venous 

 blood in a state of minute subdivision to the influence of streams 

 of water ; for that purpose the branchial arteries rapidly divide 

 and subdivide until they resolve themselves into microscopic capil- 

 laries, which are supported by a delicate membrane. Both this 

 membrane and the tunics of the capillaries which it covers are so 

 thin as to allow the chemical interchange and decomposition to take 

 place between the carbonated blood and the oxygenated water. 

 The requisite extent of the supporting membi'ane, or the respiratory 

 field of capillaries, is gained by various modes of multiplying the 

 surface within a limited space. In the Marsrpohrancliii and Pla- 

 giostomi, for example, by folds of the membrane on plane surfaces : 

 in the Lophobranchii by filamentous processes of the membrane 

 grouped into tufts : in the Protopteri, by double or single fringes of 

 filaments : in the rest of the class by the production of the membrane 

 upon a double row of long, compressed, slender, pointed processes, 

 extending, like the teeth of a comb, from the convex side of each 

 branchial arch. 



Each pair of processes has its fiat sides turned towards contiguous 

 pairs, and the two processes of each pair stand edgeways towards 

 each other, and are commonly united for a greater or less extent 

 from their base : hence Cuvier describes each pair as a single bifur- 

 cated plate (' feuillet').* 



In the Swordfish {Xiphias), the processes of the same pair stand 

 quite free from each other ; whence Aristotle described this fish as 

 having double the usual number of gills.f But to compensate for 

 this independence, and to prevent the inconvenience of mutual pres- 

 sure, the processes of the same series are united together by little 

 vascular lamella?, so that the surface of the gill is reticulate rather 

 than pectinate. 



In a iaw species the processes of each pair are joined together to 

 near their apices : the most common extent to which they are con- 

 nected is about two-fifths of their whole length, as in the Salmon. 

 In the Orthagoriscus the processes of each series are not opposite, but 

 alternate. In the Sturgeon, in which the processes of the same pair 



* XXIII. i. p. 379. f xxiii. t. viii. p. 192. 



