YHE LIFE CYCLE 99 
it has steadily increased in actual weight. Many herring, 
eels, and other soft-bodied fishes pass through stages simi- 
lar to those seen in the lady-fish. Another type of devel- 
opment is illustrated in the sword-fish. The young has a 
bony head, bristling with spines. As it grows older the 
spines disappear, the skin grows smoother, and, finally, the 
bones of the upper jaw grow together, forming a prolonged 
sword, the teeth are lost and the fins become greatly modi- 
fied. Fig. 50 shows three of these stages of growth. The 
Fie. 50.—Three stages in the devclopment of the sword-fish (Xiphias gladius). 
a, very young ; 0, older; e, adult.—Partly after LUTKEN. 
flounder or flat-fish (Fig. 51) when full grown lies flat on 
one side when swimming or when resting in the sand on 
the bottom of the sea. The eyes are both on the upper 
side of the body, and the lower side is blind and colorless. 
When the flounder is hatched it is a transparent fish, broad 
and flat, swimming vertically in the water, with an eye on 
each side. As its development (Fig. 52) goes on it rests 
itself obliquely on the bottom, the eye of the lower side 
turns upward, and as growth proceeds it passes gradually 
