RESPIRATION 



35 



upper to the lower surfaces of the body. They are normally 

 five in number (excluding a small circular opening known as 

 the spiracle, which may lie in front of the first gill-cleft), but in 

 the Frilled Shark {Chlamydoselachus) (Fig. 32c), the Comb- 

 toothed or Cow Sharks (Hexanchidae) , and one of the Saw 

 Sharks (Pliotrema), there may be as many as six or seven. In 

 the Frilled Shark each of the partitions between the successive 



^"fi 



Fig. 14. — EXTERNAL GILL-OPENINGS. 



A. Spotted Dog-fish (Scyltorhinus sp.),X^; b.i, b.2. Thornback Ray (Rata 

 clavata),X\\ c. Rabbit-fish {Chimaera monstrosa),X\ \ D. Trout {Salmo 



trutta), X i. 

 g.c, external gill-clefts of Selachians ; g.o., external gill-opening of Chimaeras 

 and Bony Fishes ; w., nostril ; sp., spiracle. 



clefts is produced backwards as a curious fold of skin covering 

 the cleft immediately behind (Fig. 32c). 



The partitions, or interbranchial septa, between the separate 

 gill-pouches are fairly thick, and are reinforced by sheets of 

 tough fibre-like substance. Further support is provided by a 

 series of bars of cartilage known as the gill-arches, which lie 

 at the inner edges of the septa and between the pharyngeal 

 openings (Fig. 15 a, g.a.). Each arch has the form of a half- 

 hoop, and is broken up into several segments movably con- 



