36 



A HISTORY OF FISHES 



nected with one another, the lowest of which is nearly always 

 joined by a coupling piece with its fellow of the opposite side 

 {cf. Fig. 46A). In this way, the inside of the pharynx is sup- 

 ported by a series of encircling jointed girders, the outer 

 convex faces of which are fringed by a number of slender rods 

 of cartilage, which project outwards into the septa and help to 

 strengthen them. All the parts of the gill-arches are provided 



Fig. 15. GILLS OF SHARK AND BONY FISH COMPARED. 



A. Dissection of head of Spotted Dog-fish {Scyliorhinus sp.) seen from beloWj 

 X ^ ; B. The same of Salmon (Salmo salar), x -}. 



g.a., gill-arch ; g.c, external gill-cleft ; g.f. gill-filaments ; g.o., external gill- 

 opening ; ph., pharyngeal opening ; sp., spiracle. 



with their special muscles, which by their appropriate ex- 

 pansion or contraction bring the hoops closer together or move 

 them wider apart, and thus diminish or enlarge the size of the 

 intervening openings. 



The opposing walls of each gill-pouch bear a number of 

 closely pleated folds of skin, the branchial lamellae or gill- 

 filaments ig.f'), whose free edges project into the cavity of the 

 pouch. These are richly supplied with fine blood-vessels, and 

 present the appearance of a series of thin red straps or plates, 



