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Fishes
There are about 25,000 described species of fishes living today, more than the number of species in all other vertebrate groups combined. Ichthyologists (biologists who study fish)have various definitions for what a fish is, but the most basic would be as "aquatic vertebrates that have gills throughout life and limbs, if any, in the shape of fine" (Nelson, 1994). Some ichthyologists exclude the agnatha (jawless vertebrates such as lampreys and hagfish ) from the fishes, but we include them here as one of five classes of extant (living) fishes.
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Epinephalus tauvina, class Osteichthyes See also:
Fishes, taxonomic overview
Fish body structures
Measuring fish
Fish tails and fins
Fish head structures
Fish body and mouth types
Fish teeth and gill structure
Carcharodon carcharias, great white shark, class Chondrichthyes
Fishes as a group are extremely diverse in form, physiology, behaviour , trophic position, habitats, and general life history. Fishes of the Osteichthyes (bony fishes) include an enormous variety of shapes and sizes, however, a generalized form that comes to mind for most people might be one similar to Epinephalus tauvina (a member of the sea bass family, the Serranidae ). The class Osteichthyes contains most of the species of extant (living) fishes.

The sharks , rays , chimaeras , and their relatives are members of the class Chondrichthyes , the cartilaginous fishes, and consists of about 800 species worldwide.

There are several other groups of living fishes, including the sarcopterygii (lobe finned fishes, such as lungfish and Coelacanths ), the myxini (hagfish ), and Cephalaspidomorphi (lampreys ).

References:

Moyle & Cech, 1982
Nelson, 1994
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