What are the criteria for an organism to be classified as an Arthropod?

[wasteofo2]

My bio teacher refuses to do much outside of the lesson plan, and when I asked him what the criteria something must meet to be in arthropoda, he said that it just meant that the creature had jointed limbs. I said that humans, and many, if not all, reptiles, mamals, amphibians and birds have jointed limbs, but aren't in athropoda, to which he didn't really have a response other than that the aforementioned creatures were in chordata, and I was right, then he moved on.

So, as far as I can gather just by looking at random members of this phylum, it seems they all have exoskeletons (and are implicitly invertebrates)and have jointed limbs. Is that bassically what it means, or are there more criteria to meet?

 

[Monique]

Well, the number of legs they have maybe? There are 6 and 8 and many legged critters..

 

[iansmith]

This give a pretty good description of arthropoda.

The major criteria are

1- Exoskeleton
2- bilaterally symmetry
3- segmented
4- Jointed appendages on on all segments. Mouth part are actually modified appendages.

 

[wasteofo2]

There are many crabs which aren't bilatterally symmetrical, the only I can think of right now is the fiddler crab, does that make them not part of arthropoda?
http://www.csc.noaa.gov/acebasin/specgal/image/photos/invertpic.jpg
fiddler crab^

And there are also many insects, most notably flies, mosquitos and butterflys, which use probiscuses for nutrition, hence no jointed limbs/mouthparts and no jointed limbs on their heads. And do the external coating of things like catterpillars, butterflies, moths, flies and other such organisms count as exoskeletons? Catterpillars are pretty darn squishy...

 

[iansmith]

Originally posted by wasteofo2
There are many crabs which aren't bilatterally symmetrical, the only I can think of right now is the fiddler crab, does that make them not part of arthropoda?

It is bilateral symmetry. It is only the an oversize claw. There is still two claws and the internal organs are symmetrical.

Originally posted by wasteofo2
And there are also many insects, most notably flies, mosquitos and butterflys, which use probiscuses for nutrition, hence no jointed limbs/mouthparts and no jointed limbs on their heads.

Technically it is a limb but it is a highly speciallized limb.

Check the link
http://dev.biologists.org/cgi/content/abstract/128/14/2803

Originally posted by wasteofo2
And do the external coating of things like catterpillars, butterflies, moths, flies and other such organisms count as exoskeletons? Catterpillars are pretty darn squishy...

As far as I remember the coating is a modification of the outer layer of the exoskeletons? The catterpillar are larvae so I don't remember if the actually have an exoskeleton. The adult stage will have the exoskeleton.

Remember that if it more or less fit the description it is probably an arthrops. Higher arthropods migth not have what it takes at the first glance because they have become more specialized than the lower classes.