FISHES 



21 



by the hranchiostegal rays, or as they are usually called, the branchio- 

 stegals. The gills lie beneath the gill-covers, borne on four gill-arches 

 (Fig. 9) ; posterior to these arches is a fifth arch which rarely bears gills 

 but is usually massively built and bears teeth; it is called the pharyngeal 

 arch. On the inner surface of the opercle near its anterior margin are, 



Fig. 7. — Head of a fish: i, nostrils; 2, premaxillary; 3, mandible; 4, lateral portion of 

 premaxillary; 5, maxillary; 6, supplementary maxillary; 7, cheek; 8, interopercle; 9, pre- 

 opercle; 10, branchiostegals; 11, subopercle; 12, opercle. 



in most fishes, small gills called the pseudobranchice. In most fishes 

 from the pharyngeal margins of each gill-arch a double series of car- 

 tilaginous or bony rods project towards the gill-cleft; these are the gill- 







® 



Fig. 8. — Position of the mouth: a, mouth terminal in position; b, mouth superior; c, mouth 

 inferior (from Siissw. Fauna Deiit.). 



rakers and they serve as a sieve to prevent particles of food from passing 

 through the gill-clefts with the respiratory water. 



Teeth. — A few species of fish are without teeth. They are, however, 

 usually present, and may be found, in the various species, on the pre- 

 maxillaries, dentaries (lower jaw), vomer, palatines, pterygoids, sphe- 



