was a SEA PIG!  (Sorry, giant squid. Maybe next time.)
This cool critter is an Echinoderm, which means it's related to sea stars, sea urchins, and sea cucumbers.  Despite several months studying invertebrates with my Marine Science class this year, I was stumped when trying to figure out which phylum this little critter belonged in- but the internets saved me, of course.

Here is another picture where you can see it more clearly.  It's the Scotoplanes globosa in the middle column about halfway down.

Lastly, a shout-out to the new Sant Ocean Hall at the Smithsonian National Museum of Natural History, where I took almost 200 students on a field trip today, and where the "sea pig" was sighted.  It's a great exhibit and 1.5 hours was not nearly enough time to see it all in detail- I plan to go back and I highly recommend it if you are in the area!