One of the most extraordinary changes by which, during growth, the form and position of several important organs are affected, occurs in Flat-fishes (_Pleuronectidæ_); their young are symmetrically formed, with a symmetrical mouth, and with one eye on each side, and, therefore, keep their body in a vertical position when swimming. As they grow they live more on the bottom, and their body, during rest, assumes a horizontal position; in consequence, the eye of the lower side moves towards the upper, which alone is coloured; and in many genera the mouth is twisted in the opposite direction, so that the bones, muscles, and teeth are much more developed on the blind side than on the coloured. In a great number of other _Teleostei_ certain bones of the head show a very different form in the young state. Ossification proceeds in those bones in the direction of lines or radii which project in the form of spines or processes; as the interspaces between these processes are filled with bone, the processes disappear entirely, or at least project much less in the older than in the younger individuals (Fig. 84). The young of some fishes may be armed with a long powerful præopercular or scapular spine, or show a serrature of which nothing remains in the adult fish except some ridges or radiating lines. These processes seem to serve as weapons of defence during a period in the life of the fish in which it needs them most. In not a few instances a portion of this armature is so much developed that the disappearance of its most projecting parts with the growth of the fish is not only due to its being surrounded by other bone, but, partially at least, caused by absorption. The _Carangidæ_, _Cyttidæ_, _Squamipinnes_, _Xiphiidæ_, offer instances of such remarkable changes. A fish, described as _Tholichthys osseus_ (Fig. 85), is probably the young of a Cyttoid, the suprascapula, humerus, and præoperculum forming enormously enlarged plates. In the fish Fig. 86 those bones appear still enlarged, and the frontals develop a remarkably long and curved horn above the orbit. In the _Tholichthys_-stage of _Pomacanthus_ (specimens 10 millimetres long, Fig. 87), the frontal bone is prolonged into a straight lancet-shaped process, nearly half as long as the body; the suprascapular and præopercular processes cover and hide the dorsal and ventral fins. The plates attached to the shoulder-girdle remain persistent until the young fish has assumed the form of the adult; thus they are still visible in young _Chætodon citrinellus_, 30 millimetres long, in which the specific characters are already fully developed.--The Sword-fishes with ventral fins (_Histiophorus_) belong to the Teleosteans of the largest size; in young individuals, 9 millimetres long (Fig. 89), both jaws are produced, and armed with pointed teeth; the supraorbital margin is ciliated; the parietal and præoperculum are prolonged into long spines; the dorsal and anal fins are a low fringe, and