20 THE SILVERY HOSTS OF THE NORTH SEA. 



fin, the lower lobe of which is somewhat longer 

 than the upper; the tail is considerably forked. 

 Dorsal fin, 19 ; pectoral, 17 ; ventral, 9 ; anal, 17 ; 

 caudal, 20 rays ; and 56 vertebrae. 



The scales adhere so loosely that in hauling the 

 nets, or in handling the fish, they come off with the 

 greatest ease. 



" The gill-covers at the sides of the head, back of 

 and below the eyes, are alternately opened and 

 closed by the fish, in order to draw in water for 

 breathing through the mouth over the gills, to be 

 discharged through the gill opening. Each gill-cover 

 consists of four bony plates. Below these is joined 

 to the third and fourth plates a skin or membrane, 

 kept extended by means of small bones. It has four 

 pairs of gill-arches ; to these are fastened at the back 

 two rows of gristly gill-leaflets, which are covered 

 with so thin a skin that in the live fish they look 

 dark red from the blood passing through them. In 

 front the arches of the gills have a dense row of 

 slender appendages, technically called gill-rakers, on 

 each side of them. All water which the herring 

 takes in its mouth to let it flow out again over the 

 gills in breathing, must pass through the fine grating 

 formed by these spinous points. By this process all 

 small animals which, with the water^ enter the mouth 

 and gill-cavity are retained in it, and accumulate 

 till they are swallowed. This enables the herring, 

 whenever the water is full of small life, to fill its 

 stomach in a short time with thousands of them. In 

 the abdominal cavity of fully grown herrings, the 



