FISHES 



23 



gill-covers. Ventral to them lies the hranchiostegal membrane supported 

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

 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, 

 in most fishes, small gills called the pseudohranchicE. 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- 



— ecp 



, — -enp 



:;.-.uph 



a b 



Fig. 9. — The inside of the mouth (of the rock bass, AmblopUles rupeslris) showing the 

 dentition and the gill-arches: a, the floor of the mouth; b, the roof of the mouth: dn, 

 dentary; ecp, ectopterygoid; enp, entopterygoid; hy, hyoid; Iph, lower pharyngeal; pi, 

 palatine; pmx, premaxillary; tn, tongue; uph, upper pharyngeal; vo, vomer {from Fishes of 

 Illinois). 



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- 

 noids, gill-arches, tongue and pharyngeals (Fig. 9). Short, slender, 

 close-set teeth are called villiform: long, sharp teeth which project 

 beyond the level of the others are called canine; teeth with blunt tips 

 used for crushing shells are called molar; flat, mosaic-like teeth are called 

 paved or tessellated; front teeth with cutting edges are called incisor. 



