THE STRUCTURE OF FISHES 



57 



Diagram 2. Left, gill-membranes united to isthmus. Right, gill-membranes 



free from isthmus. 



as pharyngeal teeth, are found in the throat region on the inner margin 

 of the gill-bars. 



Barbels are present on the heads of some fishes. These are soft, thread- 

 like structures above or below the mouth. Barbels form the so-called 

 whiskers of the bullhead. 



The nostrils are usually double apertures on each side of the snout 

 which open into a pair of blind pits. They do not open into the throat 

 as they do in land animals. The nostrils serve only as organs of smell 

 and have the same nerve connections as those in the higher land 

 animals. 



Four pairs of gills are present. Each gill is composed of a gill-bar, or 

 gill-arch, on the outer edge of which are two rows of delicate red fila- 

 ments. These are the gill- filaments, the respiratory structures of the fish. 

 They contain numerous small blood vessels, which lies close to the sur- 

 face, thus enabling the blood to obtain oxygen from the water which 

 flows through the openings, the gill-clefts or gill-slits, between the gills. 

 The gill-cleft behind the fourth gill is closed in some species. The gill- 

 clefts are guarded by numerous projections, known as the gill-rakers, 

 attached to the inner surface of the gill-bar. In most fishes the gill-rakers 

 are a series of coarse projections along the inside surface of each gill- 

 arch, but in some fishes, the tullibee for example, the rakers consist of 

 long filaments almost as fine as the gill-filaments. The purpose of the 

 gill-rakers is to strain the water and prevent any solid objects or food 

 from passing out through the gill-clefts. 



Rudimentary gills are frequently present. These are relics of a time 



