GENERATION, SEX AXl) ovTOOENY 



2:^0 



like and transparent. As the M\ grows okk-r tlic l^odv Ix^comes 

 more compaet, and therefore sliort<'r and thicker. Aft<^r 

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

 begins normally, ahlioiigh it lias all the time steadily increiiscd 

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

 fishes pass through stages similar to those socn in the la<lvfisli. 

 Another type of development is illustrated in tlie sworilfish. 

 The young has a bony head, bristling witli 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 uladius : 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 swinnning or when resting in the sand on the bottcHu 

 of the sea. The eyes are both on the upper side of the body, 

 and the lower side is ])\'nu\ and colorless, ^^'llen the flounder 

 is hatched it is a transparent fish, broad and flat, swimming 

 vertically in the water, with an eye on each side. .Vs its d(^ 

 velopment goes on it rests itself ()bli(|ue]y on the bottom. \\\v 

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

 passes gradually around the forehead, its socket moving with 

 it, until both eyes and sockets are transferred l)y the twisting 



