THE 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 



a 



FIG. 50. Three stages in the development of the sword-fish (Xiphias gladius). 

 a, very young ; b, older ; c, adult. Partly after LUTKBN. 



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 



