The Cyprinids constitute the majority of the freshwater fishes in Europe, Asia, and North America; they are comparatively few (about 100 species) in Africa, where they coexist with the Characinids. Some, like the Carp (Cyprinus carpio) and the Tench (Tinca vulgaris), are sluggish, except during the breeding season, when they show great excitement and indulge in leaps out of the water; others, like the Bleak (Alburnus lucidus) are constantly on the move in large shoals near the surface; whilst others again, like the M‘Biriki of Lake Tanganyika (Barbus tropidolepis), behave after the manner of Salmon and Trout, travelling long distances, against rapids and over waterfalls, to reach their breeding places at the heads of rivers. During the breeding season, the males of many species assume a more brilliant livery, or develop pearl-like or spiny excrescences on various parts of the head, or also on the body and fins.[[662]] Cyprinids are oviparous, with the exception of a small Barbel from Natal, discovered and described by Prof. Max Weber as Barbus viviparus.
A most striking instance of symbiosis is offered by a little Carp-like fish of Central Europe, the "Bitterling" (Rhodeus amarus). The genital papilla of the female acquires a great development during the breeding season, becoming produced into a tube nearly as long as the fish itself; by means of this ovipositor the comparatively few and remarkably large eggs, measuring 3 millimetres in diameter—the fish being only 60 to 80 millimetres long—are introduced through the gaping valves, between the branchial laminae of pond mussels (Unio and Anodonta) where, after being inseminated, they undergo their development, the fry leaving their host about a month later, having attained a length of 10 or 11 millimetres.[[663]] The mollusc reciprocates by throwing off its embryos on the parent fish, in the skin of which they remain encysted for some time, the period of reproduction of the fish and mussel coinciding.
Some members of this family grow to a very large size,—4 to 6 feet; such is the case with the Carp, a native of Asia, introduced into England towards the beginning of the seventeenth century; the Catla (Catla buchanani) of India, Burma, and Siam; the Mahaseer (Barbus mosal) of the mountain streams of Asia, the scales of which may be as large as the palm of a hand; and Hypophthalmichthys molitrix of China and Manchuria, remarkable for the low position of the eyes, the fusion of the gill-rakers into thin plates of spongious appearance, which must act as a most efficient sifting apparatus, and the presence of an involuted problematic superbranchial organ to each branchial arch.[[664]]
Among well-known aberrations produced by artificial selection may be mentioned the "Leather Carp," a race in which the scales are either lost or much reduced in number, and enlarged along the lateral line and the back, and the Gold-Fish, a variety of Cyprinus carassius, remarkable for its golden or bright red colour, or its perfect albinism, as well as its monstrous form the Telescope Fish, with enormously projecting eyes, and enlarged, horizontally spread caudal fin.[[665]] This family has also yielded numerous more or less well-established examples of hybridism, congeneric and digeneric, originally described as distinct species, the produce of which is believed to be in some cases fertile for at least one generation.
The crystalline silvery colouring matter of various Cyprinids is said to have been employed from time immemorial for ornamental purposes by the Chinese. The well-known and important industry of "Essence Orientale" and artificial pearls, carried on in France and Germany with the scales of the Bleak, was not introduced before the middle of the seventeenth century.
Fig. 354.—Pond Loach (Misgurnus fossilis), with lower view of mouth. ⅓ natural size.
The Loaches, Cobitidinae, which form a very natural sub-family, are small fishes, few species growing to a foot in length, mostly living in small streams and ponds. Many delight in the mud at the bottom, in which they move like Eels. In some cases, the branchial respiration appears to be insufficient, and the intestinal tract acts as an accessory breathing organ. The air-bladder, which is partially encased in a bony capsule, may be so reduced as to lose its hydrostatic functions and becomes transformed into a sensory organ, its outer exposed surface being connected with the skin by a meatus between the bands of muscle, and conveying the thermo-barometrical impressions to the auditory nerves; hence the name of "Wetterfisch," by which Loaches are known in some parts of Germany.
The Homalopterinae are more or less perfectly adapted to life in rapid streams, the most remarkable in this respect being Gastromyzon of North Borneo, in which the pectoral and ventral fins are much expanded to form, with the belly, a sucker by which the fish adhere to the stones of mountain torrents, showing a remarkable analogy to Exostoma among the Silurids.[[666]]
