Fig. 106.—Larva (called Tholichthys) of Chætodon sedentarius (Poey). Cuba. (After Lütken,)
Fig. 107.—Butterfly-fish, Chætodon capistratus Linnæus. Jamaica.
Peculiar Larval Forms.—The young fish usually differs from the adult mainly in size and proportions. The head is larger in the young, the fins are lower, the appendages less developed, and the body more slender in the young than in the adult. But to most of these distinctions there are numerous exceptions, and in some fish there is a change so marked as to be fairly called a metamorphosis. In such cases the young fish in its first condition is properly called a larva. The larva of the lamprey (Petromyzon) is nearly blind and toothless, with slender head, and was long supposed to belong to a different genus (Ammocœtes) from the adult. The larva of sharks and rays, and also of Dipnoans and Crossopterygians, are provided with bushy external gills, which disappear in the process of development. In most soft-rayed fishes the embryonic fringe which precedes the development of the vertical fins persists for a considerable time. In many young fishes, especially the Chætodontidæ and their allies (butterfly-fishes), the young fish has the head armed with broad plates formed by the backward extension of certain membrane-bones. In other forms the bones of the head are in the young provided with long spines or with serrations, which vanish totally with age. Such a change is noticeable in the swordfish. In this species the production of the bones of the snout and upper jaw into a long bony sword, or weapon of offense, takes place only with age. The young fish have jaws more normally formed, and armed with ordinary teeth. In the headfish (Mola mola) large changes take place in the course of growth, and the young have been taken for a different type of fishes. Among certain soft-rayed fishes and eels the young is often developed in a peculiar way, being very soft, translucent, or band-like, and formed of large or loosely aggregated cells. These peculiar organisms, long known as leptocephali, have been shown to be the normal young of fishes when mature very different. In the ladyfish (Albula) Dr. Gilbert has shown, by a full series of specimens, that in their further growth these pellucid fishes shrink in size, acquiring greater compactness of body, until finally reaching about half their maximum length as larvæ. After this, acquiring essentially the form of the adult fish, they begin a process of regular growth. This leptocephalous condition is thought by Günther to be due to arrest of growth in abnormal individuals, but this is not the case in Albula, and it is probably fully normal in the conger and other eels. In the surf-fishes the larvæ have their vertical fins greatly elevated, much higher than in the adult, while the body is much more closely compressed. In the deal-fish (Trachypterus) the form of the body and fins changes greatly with age, the body becoming more elongate and the fins lower. The differences between different stages of the same fish seem greater than the 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 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.
Fig. 108.—Mola mola (Linnæus). Very early larval stage of the Headfish, called Centaurus boöps. (After Richardson.)
