Hi

I checked all websites that have the gropus of arthropods and I cannot still clasify crabs !

What type of crab are you trying to classify?

Since this is not my area of expertise, I searched Google under the key words "crabs arthropod" to get these possible sources:

http://www.enchantedlearning.com/subjects/invertebrates/arthropod/Arthropodprintouts.shtml
http://www.pbs.org/kcet/shapeoflife/animals/arthropods.html
(Broken Link Removed)
http://en.wikipedia.org/wiki/Arthropod

I hope this helps. Thanks for asking.