Ostariophysi.—A large group of orders, certainly of common descent, may be brought together under the general name of Ostariophysi (ὀσταρίον, a small bone; θυσός, inflated). These are in many ways allied to the Isospondyli, but they have undergone great changes of structure, some of the species being highly specialized, others variously degenerate. A chief character is shared by all the species. The anterior vertebræ are enlarged, interlocked, considerably modified, and through them a series of small bones connect the air-bladder with the ear. The air-bladder thus becomes apparently an organ of hearing through a form of connection which is lost in all the higher fishes.
In all the members of this group excepting perhaps the degraded eel-like forms called Gymnonoti, the mesocoracoid arch persists, a trait found in all the living types of Ganoids, as well as in the Teleost order of Isospondyli. Other traits of the Ostariophysan fishes are shared by the Isospondyli (herring, salmon) and other soft-rayed fishes. The air-bladder is large, but not cellular. It leads through life by an open duct to the œsophagus. The ventral fins are abdominal in position. The pectorals are inserted low. A mesocoracoid arch is developed on the inner side of the shoulder-girdle. (See Fig. 119.) There are no spines on the fins, except in many cases a single one, a modified soft ray at front of dorsal or pectoral. The scales, if present, are cycloid or replaced by bony plates.
Many of the species have an armature much like that of the sturgeon, but here the resemblance ends, the bony plates in the two cases being without doubt independently evolved. According to Cope, the affinities of the catfishes to the sturgeon are "seen in the absence of symplectic, the rudimentary maxillary bone, and, as observed by Parker, in the interclavicles. There is also a superficial resemblance in the dermal bones." But it is not likely that any real affinity exists.
Fig. 119.—Inner view of shoulder-girdle of the Buffalo-fish. Ictiobus bubalus Rafinesque, showing the mesocoracoid (59). (After Starks.)
The sturgeons lack the characteristic auditory ossicles, or "Weberian apparatus," which the catfishes possess in common with the carp family, the Characins, and the Gymnonoti. These orders must at least have a common origin, although this origin is obscure, and fossil remains give little help to the solution of the problem. Probably the ancestors of the Ostariophysi are to be found among the allies of the Osteoglossidæ. Gill has called attention to the resemblance of Erythrinus to Amia. In any event, all the Ostariophysi must be considered together, as it is not conceivable that so complex a structure as the Weberian apparatus should have been more than once independently evolved. The branchiostegals, numerous among the Isospondyli, are mostly few among the Ostariophysi.
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.