536 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [12] 



follow each other in rapid alternation they impel the body forward, just 

 as a boat is propelled by an oar turned right and left at its stern. 



The fins aid but little in pro- 

 pelling the body of the fish. 

 Their chief object is to keep the 

 back upward, and to give to the 

 body the direction which it is to 

 follow. The herring has two 

 pectoral fins (Fig. II, Br), two 

 ventral fins (Ban), one dorsal fin 

 (R), one anal fin (A), and one 

 caudal fin (Sch), whose lower 

 lobe is somewhat longer than 

 the upper one. The skin of the 

 fins is extended between the 

 thin bones, termed the rays of 

 the fins, at whose roots there 

 are small muscles, by means of 

 which the fins can be moved. 

 Every fin has its certain num- 

 ber of rays. The scales of the 

 herring adhere to the skin so 

 loosely that in hauling in the net 

 they come off very easily. The 

 opercles or 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 

 opercle or gill-cover consists of 

 four bony plates. In Fig. II 

 these are marked 1, 2, 3, 4. The 

 folds of the herring differ in 

 shape from those of the sprat 

 (Figs. II and III). Below there 

 is joined to the third and fourth 

 plates a skin or membrane kept 

 extended by means of small 

 bones (the gill or branch iostegal 

 rays, Kh, in Fig. II), which con- 

 tributes towards, making the 

 covering of the gills fit closely. 

 The herring has four pairs of gill-arches (Fig. IY, B.) To these are 



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