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 off 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 
Murenide, 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. 
