Chordata: Class Actinopterygii

Class Actinopterygii

The bony fish (superclass Osteichthyes), a group that has retained the bony skeleton and evolved lungs and swim bladders, are subdivided into two classes. Actinopterygii, the ray-finned fishes, constitutes the predominant modern group of both fish and vertebrates. Ray-finned fishes are highly specialized for aquatic life; they have developed thin, lightweight fins supported by slender rays, and used only for balance and steering. The lung, a ventral outpocketing of the pharynx, was not necessary as these fish invaded freshwaters and oceans throughout the world; in most ray-finned fishes, it has shifted to a dorsal position and evolved into a hydrostatic organ called the swim bladder, or air float. The swim bladder, along with the strong, lightweight skeletal construction, makes ray-finned fishes much lighter-bodied than sharks. The gill passages of ray-finned fishes resemble those of sharks, but have a bony covering, called the operculum, over the external gill slits. Ray-fins have a typical vertebrate kidney which, in freshwater forms, maintains the proper salt concentration in the tissues by excreting excess water. In the marine forms the activity of the kidney is offset by the activity of salt-secreting glands; in addition, the kidney may be modified so as to produce a more concentrated urine. The heart, like that of sharks, has two chambers, and there is no separation of oxygenated and deoxygenated blood in the circulatory system.

A few primitive ray-fins (the sturgeon, the paddle fish, and the bowfin) have asymmetrical tails and thick scales regarded as primitive in construction. The higher ray-fins, or teleosts, have more or less symmetrical tail fins extending above and below the vertebral column, and typical fish scales made of very thin layers of bone. Most marine teleosts produce enormous numbers of small eggs that are externally fertilized and float in plankton; only a few of these survive. In many species there is a larval stage that is quite dissimilar to the adult. Teleosts have evolved a tremendous variety of forms and occupy very diverse ecological niches, both freshwater and marine.

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