Amphibians:

http://www.preservearticles.com/2011082011052/characteristics-of-class-amphibian.html

Reptiles:

http://animals.about.com/od/reptiles/fl/reptile-characteristics.htm

Fish:

http://www.dtmag.com/Stories/Ocean%20Science/05-04-feature.htm

Birds:

http://animals.pawnation.com/5-major-characteristics-birds-5792.html

Mammals:

http://animals.about.com/od/mammals/a/mammals-characteristics.html

Sponges:

http://www.earthlife.net/inverts/porifera.html

http://voices.yahoo.com/4-main-characteristics-fish-11208
388.html

http://animals.pawnation.com/five-characteristics-amphibia
ns-5872.html

http://www.buzzle.com/articles/reptiles-characteristics-of-r
eptiles.html

http://birding.about.com/od/Bird-Trivia/a/What-Is-A-Bird.h
tm

http://faculty.buffalostate.edu/beaverjf/mammals/tsld001.h
tm

http://tolweb.org/treehouses/?treehouse_id=3431