fills up all the interspaces between them, and over this are situated 

 the plicae or transverse folds of the respiratory mucous membrane, 

 which, in some of the large fishes are upwards of a thousand in number, 

 the outer margin of each fold, as before stated, being supported by a 

 process of the outer filament of the lamina. In Plate I. fig. 1, is 

 shown one of the laminae from the eel, and in fig. 1 a, are seen both 

 the outer filament, and the plicae of the mucous membrane, each 

 supported by a process from the large filament. The filaments 

 have by some authors been described as bony, but even in the salmon 

 and eel they contain no ossific matter, being composed of a cellular 

 structure, somewhat of the nature of cartilage or newly- developed 

 horn ; the outer margin is made up of small circular cells arranged 

 in rows at right angles to the length of the filaments, whilst on the 

 inner surface the cells are of an elongated oval figure, and arranged 

 in lines parallel to the length of the filament. 



The arteries termed branchial, which bring the impure blood to 

 the gills, run along the convex border of the branchial arch, and 

 opposite each pair of laminae, divide into two or more branches, 

 which run along the outer thick margin of the lamina to its extre- 

 mity, and in their course give off two transverse branches, one for 

 every fold of mucous membrane of the two flat or opposed surfaces 

 of the laminae ; in these folds the artery breaks up into a net-work of 

 most wonderfully minute capillaries ; the branchial vein begins in 

 these capillaries, and the entire series unite into transverse branches, 

 from which large venous trunks are formed : these run along the 

 inner or attached edges of the laminae, as shown in Plate I. figs. 3 

 and 3 a, there being branchial arteries on one margin and branchial 

 veins on the other, and between the two the net-work of capillaries 

 shown in figs. 1 a, 2 a, &c. 



The osseous fishes in which the gills have been more particularly 

 examined, are the salmon, wolf-fish (Anarrhicus) and eel, of the latter 

 the river eel requires special mention. Three of the lancet-shaped 

 lamellae of this fish are represented in different positions in Plate I. 

 figs 1, 2, 3, as seen under a magnifying power of twenty diameters ; 

 portions of the same magnified a hundred diameters, are represented 

 at figs. 1 a, 2 a, 3 a ; the first three figures serve to show the lateral 

 lamellae and plications of the mucous membrane, whilst the last three 

 exhibit very plainly the beautiful net-work of the pulmonary capil- 

 laries, distributed upon it ; these are of nearly uniform size ( T7 yi^) 

 in diameter, and the interspaces of the net-work much smaller 

 than the diameter of the capillaries themselves, but still so cha- 



