P.S. With all due respect, I question the etymology I just quoted for echidna.

Another source, http://www.geocities.com/etymonline/e1etym.htm, something similar, including I.E. *angwhi- "snake, eel." However, it also has the following entry:

echinoderm - 1835, from Mod.L. Echinodermata, from Gk. ekhinos "sea urchin," originally "porcupine, hedgehog" + derma (gen. dermatos) (emphases added)

And that echidna pictures certainly resembles a porcupine, does it not? or the type of sea urchin pictured in the link below:
http://www.humboldt.edu/~natmus/NorthcoastNature/Tidepools/purple-urchin.jpg