the ventrals make their appearance as a pair of short buds. When 14 millimetres long (Fig. 90) the young fish has still the same armature of the head, but the dorsal fin has become much higher, and the ventral filaments have grown to a great length. At a third stage, when the fish has attained to a length of 60 millimetres, the upper jaw is considerably prolonged beyond the lower, losing its teeth; the spines of the head are shortened, and the fins assume nearly the shape which they retain in mature individuals. Young Sword-fishes without ventral fins (_Xiphias_) undergo similar changes; and, besides, their skin is covered with small rough excrescences longitudinally arranged, which continue to be visible after the young fish has assumed the form of the mature in other respects (Fig. 92). [Illustration: Fig. 89.--Young Sword-fish (Histiophorus), 9 mill. long. Atlantic. (Magn.)] [Illustration: Fig. 90.--Young Sword-fish (Histiophorus), 14 mill. long. South Atlantic. (Magn.)] [Illustration: Fig. 91.--Young Sword-fish (Histiophorus), 60 mill. long. Mid-Atlantic.] [Illustration: Fig. 92.--Xiphias gladius, young, about 8 inches long.] The Plectognaths show no less extraordinary changes: an extraordinary form taken in the South Atlantic, and named _Ostracion boops_, is considered by Lütken to be the young of a Sun-fish (_Orthagoriscus_). In very young more advanced Sun-fishes (18 to 32 millimetres) the vertical diameter of the body exceeds, or is not much less than, the longitudinal; and small conical spines are scattered over its various parts. The caudal fin is developed long after the other vertical fins. [Illustration: Fig. 93.--“Ostracion boops” (much magnified).] [Illustration: Fig. 94.--Young of Orthagoriscus, 18 and 32 mill. long. (Natural size.)] Similar changes take place in a number of other fishes, and in many cases the young are so different that they were described as distinct genera: thus _Priacantichthys_ has proved to be the young of _Serranus_, _Rhynchichthys_ that of _Holocentrum_, _Cephalacanthus_ of _Dactylopterus_, _Dicrotus_ of _Thyrsites_, _Nauclerus_ of _Naucrates_, _Porthmeus_ of _Chorinemus_, _Lampugus_ of _Coryphæna_, _Acronurus_ of _Acanthurus_, _Keris_ of _Naseus_, _Porobronchus_ of _Fierasfer_, _Couchia_ of _Motella_, _Stomiasunculus_ of _Stomias_, etc.