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 moonfishes (_Carangidæ_) the changes with age are often very considerable. We copy Lütken's figure of the changes in the genus _Selene_ (fig. 113). Similar changes take place in _Alectis_, _Vomer_, and other genera. [Illustration: _Fig. 108._--_Mola mola_ (Linnæus). Very early larval stage of the Headfish, called _Centaurus boöps_. (After Richardson.)] [Illustration: FIG. 109.--_Mola mola_ (Linnæus). Early larval stage, called _Molacanthus nummularis_. (After Ryder.)] [Illustration: FIG. 110.--_Mola mola_ (Linnæus). Advanced larval stage. (After Ryder.)] =The Development of Flounders.=--In the great group of flounders and soles (_Heterosomata_) the body is greatly compressed and the species swim on one side or lie flat on the bottom, 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 characters 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 of growth (usually four years) before spawning. After this act all the individuals die so far as known. In Japan and China the Ice-fish (_Salanx_), a very long, slender, transparent fish allied to the trout, may possibly be annual in habit, all the individuals perhaps dying in the fall to be reproduced from eggs in the spring. But this alleged habit needs verification.