THEIR GENERAL CHARACTER AND STRUCTURE 13 



gases generated by decomposition cause the body to rise to 

 the surface it floats belly uppermost, the back being heavier 

 than the lower parts. How comes it that without apparent 

 effort a live fish swims or rests with its back uppermost ? By 

 the same unconscious adjustment of balance whereby a man 

 stands and walks erect. True, a man's feet support him 

 against the firm ground, whereas a fish is in every part in 

 contact with a fluid medium ; nevertheless, it is chiefly by 

 the pair of ventral fins (Fig. I., 81, 82), homologues of the 

 legs and feet in man, that the perch maintains its balance in 

 the water. The pectoral fins (Fig. I., 53, 53A) doubtless assist 

 in this function, for they are constantly in motion, but their 

 chief use is in directing the course of the fish, in regulating its 

 speed, in stopping its course, as when a rower '* backs water," 

 and in supporting the head, which, in the perch and most other 

 fishes, is the heavier end. How little it takes to throw the 

 creature ofi^ its lateral balance has been proved repeatedly by 

 an experiment which it would be needlessly cruel to repeat : if 

 the pectoral and ventral fins on one side are cut off, the fish 

 falls over to the other side ; if both pectorals are removed, the 

 horizontal position is lost and the head sinks. The loss of an 

 eye on one side does not cause a man to walk bent to the other 

 side ; but the eye of a perch is far heavier than a man's in 

 proportion to respective gross weight, and a perch deprived 

 of one eye keels over permanently to the other side. The 

 action of the fins as balancers has been demonstrated by 

 removing all the fins from a live fish. Thus mutilated, the 

 animal when placed in the water floats belly upwards. 



That the ventral fins are the chief organs of lateral balance 

 is well illustrated by their total disappearance in such fish 

 as live habitually in or rest upon muddy bottoms, like the 

 Murcenida^ or Eel Family. Nevertheless the flat-fishes, which 

 live entirely on the bottom, retain their ventral fins, which are 

 useful in helping them to turn — a difficult operation in fishes 

 of such abnormal design. 



