The members of this superclass are commonly called fishes and are the
simplest and most numerous of all vertebrates. They are aquatic,
free-moving, cold-blooded (their blood maintains the temperature of the
surrounding water), and breathe primarily by means of gills. However,
some forms (the lungfishes) breathe by means of a lung developed from
the air-bladder.
The most recent fish classification recognizes four major classes, the
Agnatha (primitive jawless fishes), the Placodermi (armored fishes with
primitive jaws), the Chondrichthyes (sharks and related forms with
cartilaginous internal skeletons), and the Osteichthyes (true bony
fishes).