262 LECTURE X. 



are joined together nearly to tlieir apices, the musculo-membranous 

 medium of union extends from pair to pair throughout the entire 

 gill, forming a true ' septum branchiale,' and presenting a beautiful 

 transition to the more complete septum which divides the respiratory- 

 vascular surfaces in the Plagiostomes. 



In some osseous fishes certain of the branchial arches support only 

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

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

 of the Labroids, the genera Cottus, ScorpcBiia, Sebastes, Apistes, Zeus, 

 Antennarius, Polypterus, Gobiesox, Lejjadogaster, and the Cyclopterus 

 liparis have three biserial gills and one uniserial gill ; the genera Lo- 

 pJiius,' Batrachus, Diodoti, Tetrodon, Monopterus, Cotylis, have three 

 biserial gills ; Maltlicea and Lepidosiren have two biserial gills and 

 one uniserial gill; the Cuchia (Atnphipnotis) has only two gills. 

 The above enumeration refers only to the branchial organs of one 

 side — they are symmetrical in all fishes, and the uniserial opercular 

 gill is not counted, as not being attached to a proper branchial arch. 



The processes which radiate from the convexity of the branchial 

 arches are bony in some fishes (e. g. Salmo, Alosa), gristly in most 

 (Perca, Cottus, Trigla, &c.). They break up, in the Sturgeon, into 

 delicate branched fringes, along their outer or free margin, vSmall 

 'interbranchial' muscles extend, through the uniting septum, between 

 the bases of the processes, for effecting slight reciprocal movements.* 

 The mucous membrane supported by the processes is puckered up 

 into minute transverse folds, crossing their flat sides and producing 

 an enormous extent of surface for the branchial capillaries. f 



The concave borders of the branchial arches are usually beset by 

 defensive processes, fringes, or tubercles, and these sometimes suj)- 

 porting small teeth which aid in deglutition ; but the chief oflice of 

 these appendages, which project inwards towards the mouth, is to 

 prevent the passage of any solid, nutrimental, or other particles taken 

 into the mouth from entering the interspaces of the gills, and irrita- 

 ting their delicate texture. In the edentulous Sturgeon and Paddle- 

 fish each arch supports a close-set series of such retroverted slender 

 tapering filaments {Jig. 61. t), which are longer than the opposite 

 branchial processes (ib. ii) : they are developed even from the fifth 

 or pharyngeal arch, which has no gill. Similar fi-inges of extreme 

 delicacy defend the branchial slit in the Mullet (prep. 1034.). Fre- 

 quently such a fringe is developed only from the first branchial arch 

 (Mackerel and Cod, Jig. 69.), the rest supporting dentated tubercles, 



* cxii. cxiir. 



I See Prep. 10:-.8. (Conger), and its description (xx. 1834, p. S3.). Dr. Hyrtl 

 counted 1000 transverse folds on a single process or tooth of the pectinate gill of 

 the Salmon, and from HOG to 1600 in a similar process in the Sterlet. 



