What are these?

[kirkandkhristi]

I picked up a couple of chunks of liverock to replace the bio balls on my bio cube. A couple of the rocks have these little critters on them. What are they?

http://web.mac.com/kswinneyfour/iWeb/The%20Swinney%27s%20website/Photos%202_files/DSCF0013_2.jpg

[josephatmbimortgage]

Sponge.

-Joseph