A fish is a poikilothermic (cold-blooded)* water-dwelling
vertebrate with gills. There are over 27,000 species of
fish, making them the most diverse group of vertebrates.
Taxonomically, fish are a paraphyletic group whose exact
relationships are much debated; a common division is into
the jawless fish (class Agnatha, 75 species including
lampreys and hagfish), the cartilaginous fish (class Chondrichthyes,
800 species including sharks and rays), with the remainder
classed as bony fish (class Osteichthyes). Fish come in
different sizes, from the 14 m (45 ft) whale shark to
a 7 mm (just over 1/4 of an inch) long stout infant fish.
Many types of aquatic animals named "fish",
such as jellyfish and cuttlefish, are not true fish.
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