•icSO ANATOMY OF VERTEBRATES. 



usual number of gills. 1 But to compensate for this independ- 

 ence, and to prevent the inconvenience of mutual pressure, 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 the Orthagoriscus the processes of each series are 

 not opposite, but alternate. In a few species the processes of 

 each pair are joined together to near their apices, as in the 

 Sturgeon, in winch the musculo-membranous medium of union 

 extends from pair to pair throughout the entire gill, forming a 

 true ' septum branchiale,' and presenting a transition to the more 

 complete septum which divides the respiratory vascular surfaces 

 in the Plagiostomes. 



In fig. 318, the course of the blood through a pair of branchial 

 processes is diagrammatically shown : a is a section of the 

 branchial artery ; d is the branch sent along the outer margin of 

 the process ; e is the vessel receiving the blood from the capil- 

 laries after the respiratory change has been effected, and returning 

 it, along the inner border of the process, to the branchial vein, the 

 sectional area of which is shown at c. In fig. 319 are shown the 

 vascular plates or lamella?, b, of the branchial processes, a', in the 

 Cod (Morrhua vulgaris), in which they are confined to the inner 

 half or two-thirds of the process. Fig. 317, representing a trans- 

 verse section of the process, shows the degree and form in which 

 the plates extend from it on each side : the arrows indicate the 

 course of the blood from the outer to the inner border of the 

 plate-bearing process. Fig. 320 represents the frame-work 

 supporting the vascular structure of the gill : a is a section of the 

 branchial arch ; b is the base of the branchial process attached to 

 but distinct from the arch : c its outer obtuse border ; cl its inner 

 border, from which are continued the elastic cords, f, extending 

 along the outer margin of the lamella?, fig. 317, i, and maintaining 

 them outstretched. 2 The number of plates on one process has been 

 estimated at 55 in the Gudgeon, 96 in the Tench, 106 in the 

 Barbel, 135 in the Carp, 700 in the Eel, 1000 in the Cod, 

 1400 in the Salmon, 1600 in the Sturgeon. 



In some Osseous Fishes certain of the branchial arches support 

 only one series of processes ; such are called e uniserial,' or ' half' 

 gills ; but, as a general rule, they support ' biserial,' or ' whole ' 

 gills. Most of the Labroids, the genera Cottus, Scorpcena, 

 Sebastes, Apistes, Zeus, Antennarius, Pohjpterus, Gobiesox, 



1 XXIII. t. viii. p. 192. 



- For the histology of these structures, sec Dr. Williams's minute description in 

 cclxviii. pp. 288-290. 



