Embryology and Growth of Fishes 145 



differences between distinct species. In fact with this and with 

 other forms which change with age, almost the only test of 

 species is found in the count of the fin-rays. So far as known 

 the numbers of these structures do not change. In the moon- 

 fishes (Carangidce) the changes with age are often very con- 

 siderable. We copy Liitken's figure of the changes in the genus 

 Selene (fig. 113). Similar changes take place in Alectis, Vomer, 

 and other genera. 



The Development of Flounders. In the great group of 

 flounders and soles (Heterosomata) the body is greatly com- 

 pressed and the species swim on one side or lie flat on the bot- 

 tom, with one side uppermost. This upper side is colored like 

 the bottom, sand-color, gray, or brown, while the lower side is 

 mostly white. Both eyes are brought around to the upper 

 side by a twisting of the cranium and a modification or division 

 of the frontal bones. When the young flounder is hatched it is 

 translucent and symmetrical, swimming vertically in the water, 

 with one eye on either side of the head. After a little the young 

 fish rests the ventral edge on the bottom. It then leans to one 

 side, and as its position gradually becomes horizontal the eye 

 on the lower side moves across with its frontal and other 

 bones to the other side. In most species it passes directly under 

 the first interneurals of the dorsal fin. These changes are best 

 observed in the genus Platophrys. 



Hybridism. Hybridism is very rare among fishes in a state 

 of nature. Two or three peculiar forms among the snappers 

 (Lutianus) in Cuba seem fairly attributable to hybridism, the 

 single specimen of each showing a remarkable mixture of char- 

 acters belonging to two other common species. Hybrids may 

 be readily made in artificial impregnation among those fishes 

 with which this process is practicable. Hybrids of the different 

 salmon or trout usually share nearly equally the traits of the 

 parent species. 



The Age of Fishes. The age of fishes is seldom measured 

 by a definite period of years. Most of them grow as long as 

 they live, and apparently live until they fall victims to some 

 stronger species. It is reputed that carp and pike have lived for 

 a century, but the evidence needs verification. Some fishes, as 

 the salmon of the Pacific (Oncorhynchus] , have a definite period 



