UnIdentified Marine Invertebrate

I found this interesting blob on the beach in St. Augustine, Florida. It was attached to the outside surface of a broken sea shell, so my guess is that it did not at one time live inside the shell. The ridges on the shell make it look sort of like a scallop, although it doesn't have the "wings" on the hinged side of the shell that you normally associate with a scallop.

The thought did cross my mind that the blob was one time the adductor muscle of a scallop (the part we eat). If you sliced it up, each section would be a disk that looked like a scallop on your dinner plate. But if someone or something opened up the scallop shell and pulled out the adductor muscle, how would it then re-attach itself to the outside of its old shell?

I'm thinking that this blob is some type of creature independent from the shell, which attached itself to the shell when they were both living in deeper waters. When the shell washed up on shore, its hitchhiker came with it. The hole in the shell at the bottom of the photo is also odd - I'm wondering if that is how the inhabitant of this shell met his end.

Here is a better view of the "foot" of this creature, with its "toes" securely attached to the shell. Or maybe it is the "mouth", and it was busy eating microorganisms off the shell?


Here is the other side.


And here is the end.


This whole animal was about 3 inches long and 2 inches in diameter, although it was longer and skinnier when I first approached it. As it "sensed" I was taking its picture, it got shorter and fatter, as if it were tensing up from stress. About the only other thing I can tell you is that I found this in salt water (i.e. on the ocean side, not the bay side).

If anyone has any ideas as to what this might be, please leave them in the comments!

UPDATE 8/31/10: Thank you to Anonymous, who identified this creature for me in the Comments below. I have to agree, it does appear to be an anemone with its tentacles withdrawn. I had never seen anything like this before, but after seeing many anemones in Washington state recently, I saw quite a few with their tentacles retracted. The ones I saw had more of a "puckered up" look to them, but I definitely see the resemblance. The mystery is solved!