Is a sea urchin a arthropod?
No. Sea urchins are echinoderms, meaning, appropriately enough,
"spiny skin". Some of the characteristics of arthropods are jointed
appendages, an exoskeleton, and a segmented body. While the hard
test, or shell, of sea urchins could be thought of as an
exoskeleton, sea urchins have no jointed appendages and do not have
a segmented body.
Some examples of arthropods are shrimps, crabs, lobsters, and
insects.