Help me, Dr. LJ, you're my only hope!

OKAY. So-- there is a kind of marine animal which builds nests/tubes out of detritus it finds-- sand, coral, shells, small stones. And the nests are pretty cool-looking. And in my novel's first draft, I couldn't remember what they were called, so I just put "[that one kind of marine animal]." In the second draft, I spent a bunch of time looking around on the internet, but couldn't figure out what it was, and eventually went on. This time, I searched lots, and could come up with nothing beyond "maybe some sort of worm or something."

HEEEELLLLPPP!!!! That thing! I know it exists. What is it? If you don't know, how would you find out? Arrrgghhh!

(thank you)

--R

ETA: Thank you, all! Caddisfly larvae does sound like what I was thinking of-- dash it all that they're freshwater, not marine. Now I need to think about whether that will still work... (Many thanks, ab3nd for your worm suggestions-- they might be better.) You are all excellent!