Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/681

663 VAEIATION.] ICHTHYOLOGY 663 with pointed teeth ; the supraorbital margin is ciliated ; the parietal and praeoperculum are prolonged into long spines ; the dorsal and anal fins are a low fringe ; and the ventrals make their appearance as a pair of short buds. When 14 millimetres long the young fish has still the same armature on the head, but the dorsal fin /&quot; ( i 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 FIG. 60. Yonng Cfirrfnclon citrinellus (GO mill. long). FIG. 61. Toung Sword-fish (ffistiophonif), 9 mill. long. Atlantic. (Magn.) mature individuals. Young sword-fishes without ventral fins (Xiphias) undergo similar changes ; and, besides, their skin is covered with small rough excrescences longitudin- Fm 62. Tonng Sword-fish (Ifisttophortts), GO mill. long. Mid-Atlantic. ally arranged, which continue to be visible after the young fish has in other respects assumed the form of the mature (fig. 63). FIG. Q3.Xiphias glarlim, young, about 8 inches long. The Plectognaths show changes no less extraordinary : a remarkable form caught in the South Atlantic, and named Ostradon loops, is considered by Liitken to be the young of a sun-fish (Ortha- goriscus). In still very young but more advanced sun-fishes (18 to 32 millimetres) the vertical diameter of the body is not much less than the longitudinal, and may even exceed it ; and small conical spines are scattered OVer its Various parts. Fl(J - 64 Ostracion boops (much magnified). The caudal fin is developed long after the other vertical fins. Similar changes take place in a number of other fishes, and in many cases the young are so different that they have been described as belonging to distinct genera: thus Priacantichthys has proved to be the young of Serranits, FIG. 63. Tonne of Orlhagnriscut, 18 and 32 mill, long (natural size). RhynchicJdhys that of ffolocentrum, Cephalacanthus of Dactylopterus, Dicrotus of Thyrsites, Nauclents of Nau- crates, 1 ortltmeus of Chorinemus, Lampugm of Coryphcena, Acronurus of Acantkurus, Keris of Naseus, Porolronchus of Fierarfer, Couclda of Motdla, Stomiasuncidus of Stomias, &c. The fins are most frequently subject to changes during growth ; but, whilst in some fishes parts of them are pro longed into fila ments with age, in others the filaments exist during the early life-periods only: whilst in some a part of the dorsal or of the ventral fins is normally de veloped 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 have asperities on the skin, in others the young are smooth and the old have a tubercu lar 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 bear a relation to the sexual development (Callionymus, many Laby- r mtldn Cyprinodonts). These secondary sexual differ ences do not show themselves in the male individual till it commences to enter upon its 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 secondary sexual characters of the male consist principally in the prolonga tion of some of the fin-rays, or of entire fins, and in Sal- monidce 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 is permanent in comparatively few (as in some Callionymus, Labrus mistns); 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. With regard to size, it appears that in all Teleosteous fishes the female is larger than the male ; in many Cyprino donts the male may be only one-sixth of the bulk of the female or even less. In Pala-ichthyes we possess few ob servations on the relative size of the sexes, but such as have been made tend to show that, if a difference exists at all, the male is generally the larger (Lepidostens). In the rays (Raia] the sexes, after they have attained maturity, differ in the development of dermal spines and the form of the teeth, the female being frequently much rougher than the male. There is much variation in this respect in the different species ; but the males are constantly distinguished by an oblong patch of erectile clawlike spines on each pectoral fin, and by having the teeth (all, or only a portion) pointed, and not obtuse, like those of the females. In sharks no secondary sexual differences have been observed ; the male Chimoeridoe possess a singular comb-like cartila ginous appendage on the top of the head, which can be erected or depressed into a groove, both the appendage and the anterior part of the groove being armed with booklets. The use of this singular organ is not known.