Changes of form normally accompanying growth (after absorption of the vitelline sac) are observed in all fishes; but in the majority they affect only the proportional size of the various parts of the body. In young fishes the eyes are constantly larger than in adult relatively to the size of the head; and again, the head is larger relatively to that of the body. Changes amounting to metamorphosis have been hitherto observed in _Petromyzon_ only. In the larval condition (_Ammocætes_) the head is very small, and the toothless buccal cavity is surrounded by a semicircular upper lip. The eyes are extremely small, hidden in a shallow groove; and the vertical fins form a continuous fringe. In the course of three or four years the teeth are developed, and the mouth changes into a perfect suctorial organ; the eyes grow; and the dorsal fin is divided into two divisions. In Malacopterygians and Anacanths the embryonal fringe from which the vertical fins are developed, is much longer persistent than in Acanthopterygians. A metamorphosis relating to the respiratory organs, as in Batrachians, is indicated in the class of Fishes by the external gills with which fœtal Plagiostomes (Fig. 58, p. 136) and the young of some Ganoids, viz. the _Protopterus_ and _Polypterus_, are provided. [Illustration: Fig. 82.--Mouth of Larva of Petromyzon branchialis.] [Illustration: Fig. 83.--Mouth of Petromyzon fluviatilis. _mx_, Maxillary tooth; _md_, Mandibulary tooth; _l_, Lingual tooth; _s_, Suctorial teeth.] [Illustration: Fig. 84.--Armature of præoperculum of young Caranx ferdau. (Magnified.) I. Of an individual, 1¼ inch long. II. Of an individual, 2 inches long.] [Illustration: Fig. 85.--Tholichthys osseus. Six times the natural size.] [Illustration: Fig. 86.--Tholichthys-stage of Heniochus (?).] [Illustration: Fig. 87.--Tholichthys-stage of Pomacanthus (magn.) Atlantic.] [Illustration: Fig. 88.--Young Chætodon citrinellus (30 mill. long).]