Some of the Burgess Shale animals were originally misclassified - and we can't entirely rely upon books like Gould's
Wonderful Life that were written before that came to light.
Hallucigenia for instance has now been identified as a member of a phylum that still exists - the "velvet worms" (and yes, the spines are on the back).
http://www.ucmp.berkeley.edu/onychoph/onychophorafr.html