RESPIRATION 



39 



or less joined to the body of the fish and the outer opening is 

 correspondingly reduced to a narrow slit or even to a minute 

 upwardly directed pore. The same hoop-like gill-arches 

 support the walls of the pharynx between the internal openings 

 as in the Selachians, but are here composed of bone instead of 

 cartilage. The more or less extensive interbranchial septa of 

 the Shark have been reduced to minute proportions, and the 

 deUcate red filaments form a double row of processes attached 

 by their bases to the convex outer edge of each gill-arch (Figs. 

 17, 18). The half gill formed by the filaments on the anterior 

 wall of the first cleft in the Selachians has either disappeared 



GtU-rakcrs 



GUI -arch. 



Fig. 18. — GILL-RAKERS. 



A Gill-arch of Allis Shad {Alosa alosa) ; b. The same of Twaite Shad {Alosa 

 finta) ; c.The sanneofFerch (Per cafluviatilis) ; d. Isolated gill-rakers of Basking 

 Shark {Cetorhinus maximus). 



All about f . 



or is represented by a mere rudiment (pseudobranch) , and the 

 fifth (last) branchial arch is gill-less (Fig. 15B). The spiracle 

 is almost invariably wanting, at least in the adult fish. 



Certain of the Selachians, namely the Chimaeras and their 

 alUes (Holocephali), present a type of gill arrangement roughly 

 midway between that of a Shark on the one hand and a Bony 

 Fish on the other. The gills lie in a common branchial chamber, 

 bordered on the outside by a skinny flap, foreshadowing the 

 operculum of the Bony Fishes, and opening to the exterior by a 

 single sUt-like aperture (Fig. 140). The interbranchial septa 

 are somewhat shorter, so that the filaments project a httle 

 beyond their outer margins. Passing to some of the more 

 primitive Bony Fishes {e.g. Sturgeons), the septa become pro- 



