Weird thingy

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Okay First off I will appoligies for not having pic. I have this thing on one of my rocks. It has build a bone like structure around it's self. White in coulor. It appears to have antenas that it expands and retracts. What it seems to do is let out a string of mucus or something and then pull it back in. Does anyone want to take a guess. Also where is a good place to look for a pic to compare. Thanks