[Illustration: FIG. 102.--Larva of Brook Lamprey, _Lampetra wilderi_, before transformation, being as large as the adult, toothless, and more distinctly segmented.] [Illustration: FIG. 103.--Common Eel. _Anguilla chrisypa_ Rafinesque. Family _Anguillidæ_.] Among the different groups of fishes the larval changes are brought about in widely different ways. These larval peculiarities appear at first of far-reaching significance, but may ultimately be attributed, the writer believes, to changed environmental conditions, wherein one process may be lengthened, another shortened. So, too, the changes from one stage to another may occur with surprising abruptness. As a rule, it may be said the larval stage is of longest duration in the Cyclostomes, and thence diminished in length in sharks, lung-fishes, Ganoids, and Teleosts; in the last-named group a very much curtailed (i.e., precocious) larval life may often occur. [Illustration: FIG. 104.--Larva of Common Eel, _Anguilla chrisypa_ (Rafinesque), called _Leptocephalus grassii_. (After Eigenmann.)] The metamorphoses of the newly hatched Teleost must finally be reviewed; they are certainly the most varied and striking of all larval fishes, and, singularly enough, appear to be crowded into the briefest space of time; the young fish, hatched often as early as on the fourth day, is then of the most immature character; it is transparent, delicate, easily injured, inactive; within a month, however, it may have assumed almost every detail of its mature form. A form hatching three millimeters in length may acquire the adult form before it becomes much longer than a centimeter. [Illustration: FIG. 105.--Larva of Sturgeon, _Acipenser sturio_ (Linnæus). (After Kupffer, per Dean.)] [Illustration: FIG. 106.--Larva (called _Tholichthys_) of _Chætodon sedentarius_ (Poey). Cuba. (After Lütken,)] [Illustration: FIG. 107.--Butterfly-fish, _Chætodon capistratus_ Linnæus. Jamaica.]