Osteichthyes

this class division of vertebrata includes almost all of the worlds living fish (excepting jawless fish of the class agnatha and cartilaginous fish of the class Chondrichthyes). the defining characteristics of these fish are a skeleton at least partly made up of bone, an air-filled bladder which provides buoyancy, gill covers, bony scales and external fertilization of eggs.

A note to ophie's previous writeup. Current theory is that unlike sharks, which first appeared in the oceans, these fishes (commonly called bony fishes) first developed in fresh water and then moved to salt water. The swim bladder, common to almost all bony fishes, originated as a lung to aid in respiration in swamps and other oxygen-poor environments. That capability has been lost in most fishes and now the swim bladder is mainly used to control buoyancy.

Class Osteichthyes is a problem to taxonomists because it's too diverse to conveniently define. Due to its age and diversity (over 21,000 species have been recognized), there are exceptions to every "defining" characteristic. (Members of Osteichthyes do have gill covers, also called opercula, but I don't consider this a defining characteristic as they also appear in all non-bony fishes.)

Most of the web sites that I checked list the characteristics of Osteichthyes that ophie listed. If your zoology instructor does this and you want to cause trouble, here are some exceptions:

• bony skeleton - Sturgeons (order Acipenseriformes) have cartilagenous skeletons. (Adults, however, do have bony plates that cover the cartilagenous skull.)
• bony scales - All eels (order Anguilliformes) and many catfishes (order Siluriformes) are scaleless.
• swim bladder - Some species of tuna have no swim bladder.
• external fertilization - Many (e.g., family Poeciliidae) are viviparous; at least one species of molly reproduces through a form of parthenogenesis; and some deep water fish are hermaphroditic.