Parameter Tree:
EARTH SCIENCE > BIOLOGICAL CLASSIFICATION > ANIMALS/VERTEBRATES > FISH

Definition:
Fish are, broadly speaking, any poikilothermic (or organism whose
body temperature varies according to the temperature of its surroundings)
legless, aquatic vertebrate that possesses a series of gills on each side
of the pharynx, a two-chambered heart, no internal nostrils, and at least
a median fin as well as a tail fin.  


Reference:
Allaby, M. 1998. A Dictionary of Ecology. Oxford University Press. Oxford, New
York.