The fins are most frequently subject to changes; but, whilst in some fishes parts of them are prolonged into filaments with age, in others the filaments exist during the early life-periods only; whilst in some a part of the dorsal or the ventral fins is normally developed in the young only, in others those very parts are peculiar to the mature age. The integuments are similarly altered: in some species the young only has asperities on the skin, in others the young are smooth and the old have a tubercular skin; in some the young only have a hard bony head; in others (some Siluroids) the osseous carapace of the head and neck, as it appears in the adult, is more or less covered with soft skin whilst the fish is young. In not a few fishes the external changes are in relation to the sexual development (_Callionymus_, many _Labyrinthici_, Cyprinodonts). These _secondary sexual differences_ show themselves in the male individual, only when it commences to enter upon his sexual functions, and it may require two or more seasons before its external characteristics are fully developed. Immature males do not differ externally from the old female. The male secondary sexual characters consist principally in the prolongation of some of the fin-rays, or of entire fins; and in _Salmonidæ_ in the greater development of the jaw-bones. The coloration of the male is in many fishes much brighter and more variegated than that of the female, but in comparatively few permanent (as in some _Callionymus_, _Labrus mixtus_); generally it is acquired immediately before and during the season of propagation only, and lost afterwards. Another periodical change in the integuments, also due to sexual influence and peculiar to the male, is the excrescence of wart-like tubercles on the skin of many _Cyprinoids_; they are developed chiefly on the head, but sometimes extend over the whole body and all the fins.