GENERATION, SEX AND ONTOGENY 



239 



like and transparent. As the fish grows older the body oecomes 

 more compact, and therefore shorter and slimmer. After 

 shrinking to the texture of an ordinary fish, its growth in size 

 begins normally, although it has all the time steadily increased 

 in actual weight. Many herring, eels, and other soft-bodied 

 fishes pass through stages similar to those seen in the ladyfish. 

 Another type of development is illustrated in the swordfish. 

 The young has a bony head, bristling with spines. As it 

 grows older the spines disappear, the skin grows smoother, and, 



a 



FIG. 142. Three stages in the development of the swordfish, Xiphias gladius : a, Very 

 young; b, older; c, adult. (After Liitken.) 



finally, the bones of the upper jaw grow together, forming a 

 prolonged sword, the teeth are lost and the fins become greatly 

 modified. Fig. 142 shows three of these stages of growth. The 

 flounder or flatfish (Fig. 143) 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 de- 

 velopment goes on it rests itself obliquely on the bottom, the 

 eye of the lower side turns upward, and as growth proceeds it 

 passes gradually around the forehead, its socket moving with 

 it, until both eyes and sockets are transferred by the twisting 



