38 



A HISTORY OF FISHES 



such as the Salmon [Salmo salar) must be famiUar to everybody, 

 and may be readily seen by lifting up the bony plate lying on 

 either side of the head behind the eye. How do these gills 

 differ from those already described? In the first place, although 





A^sa/ CccSe. 



Sra-in. 



Gill -openi/iq 

 Fig. l6. — GILLS OF MARSIPOBRANCHS. 



Dissection of anterior part of Hag-fish (Myxine glutinosa), X i . 



the same internal pharyngeal openings (ph.) are present, these 

 do not open separately to the exterior, but into a common 

 chamber, the branchial chamber, with a single external 

 aperture behind (Fig. 15B). The outer wall of the chamber is 

 provided by the movable flap already mentioned, which is 



7. — CROSS-SECTIONS OF GILL-ARCHES IN DIFFERENT FISHES. 



A. Typical Selachian ; b. Chimaera ; c. Sturgeon (Acipenser) ; d, E. Bony 



Fishes. Gill-arch (dots) ; interbranchial septum (white) ; gill-filaments (cross 



lines). (After Boas.) 



known technically as the operculum or gill-cover (Fig. 140), 

 and is supported by a series of broad, flat scale-like bones, with 

 or without the addition of a number of slender bony rods 

 below (Fig. 65) . The hinder and lower edges of the operculum 

 are nearly always free, so that the external opening is com- 

 paratively spacious, but sometimes these margins become more 



