FIG. 120.—Weberian apparatus and air-bladder of Carp. (From Günther, after Weber.) ] To the _Ostariophysi_ belong the vast majority of the fresh-water fishes of the world. Their primitive structure is shown in many ways; among others by the large number of vertebræ instead of the usual twenty-four among the more highly specialized families of fishes. We may group the _Ostariophysi_ under four orders: _Heterognathi_, _Eventognathi_ (_Plectospondyli_), _Nematognathi_, and _Gymnonoti_. =The Heterognathi.=—Of these the order of _Heterognathi_ seems to be the most primitive, but in some ways the most highly developed, showing fewer traits of degeneration than any of the others. The presence of the adipose fin in this group and in the catfishes seems to indicate some sort of real affinity with the salmon-like forms, although there has been great change in other regards. The order _Heterognathi_, or _Characini_ (ἕτερος, different; γνάθος, jaw), contains those _Ostariophysi_ which retain the mesocoracoid and are not eel-like, and which have the lower pharyngeals developed as in ordinary fishes. In most cases an adipose fin is present and there are strong teeth in the jaws. There are no pseudobranchiæ, and, as in the _Cyprinidæ_, usually but three branchiostegals. The _Characidæ_ constitute the majority of the fresh-water fishes in those regions which have neither _Cyprinidæ_ nor _Salmonidæ_. Nearly four hundred species are known from the rivers of South America and Africa. A single species, _Tetragonopterus argentatus_, extends its range northward to the Rio Grande in Texas. None are found in Asia, Europe, or, with this single exception, in the United States. Most of them are small fishes with deep bodies and very sharp, serrated, incisor-like teeth. Some are as innocuous as minnows, which they very much resemble, but others are extremely voracious and destructive in the highest degree. Of the caribe, belonging to the genus _Serrasalmo_, known by its serrated belly, Dr. Günther observes: "Their voracity, fearlessness and number render them a perfect pest in many rivers of tropical America. In all the teeth are strong, short, sharp, sometimes lobed incisors, arranged in one or more series; by means of them they cut off a mouthful of flesh as with a pair of scissors; and any animal falling into the water where these fish abound is immediately attacked and cut to pieces in an incredibly short time. They assail persons entering the water, inflicting dangerous wounds before the victims are able to make their escape. In some localities it is scarcely possible to catch fishes with the hook and line, as the fish hooked is immediately attacked by the 'caribe' (as these fish are called), and torn to pieces before it can be withdrawn from the water. The caribes themselves are rarely hooked, as they snap the hook or cut the line. The smell of blood is said to attract at once thousands of these fishes to the spot."