58 THE STORY OF FISH LIFE. 



which take their origin in the middle line of the 

 back and abdomen, and the tail or caudal fin; 

 and (2) paired fins : these are four in number, 

 and correspond to our arms and legs — these fins 

 we know as the pectoral and pelvic fiiS. We 

 will accordingly discuss the median fins first, and 

 begin with those of the back, the dorsal fins, as 

 they are usually called. 



In our typical fish, the perch or salmon, the 

 dorsal fins are two in number, called the first and 

 second dorsals respectively. If we raise, say the 

 first dorsal of a perch, we shall have an admirable 

 illustration of the constitution of a typical fin — a 

 thin sheet of membrane, supported by numerous 

 stiffened rods. On raising the second dorsal we 

 shall at once be struck with the fact that the rods 

 supporting these fins differ conspicuously. In the 

 first dorsal the rods were exceedingly hard and 

 sharp - pointed, in the second they are com- 

 paratively soft, and if we examine them further, 

 we shall find that towards the tip each rod breaks 

 up into a number of little branches, closely 

 pressed one against another. In some fishes, as 

 in the cod-fish, for instance, both fins have these 

 "soft rays," as they are called. Sometimes, as 

 in the salmon, the second dorsal fin is very small 

 and composed entirely of fatty tissue, without 

 any supporting structures. This is known as an 

 " adipose " or fatty fin. In the mackerel, and the 

 tunny, and other allied forms, the second dorsal 

 is followed by a large number of smaller fins. In 

 the bichir (Polypterus) of the Kile, the dorsal fins 

 were represented by a large number of separate 

 fins, each consisting of a spine supporting a sail- 



