If your specimen is indeed a Mollusc, then I would agree with Geoff that Teredinidae is your best bet. However, keep in mind that many non-molluscan marine worms (Phylum Annelida) also construct calcareous tubes, usually attached to various solid substrates. In the Philippines are some really huge Teredinidae, of the genus Kuphus, which can exceed 1 meter in length and 50 mm in diameter. But I'm not aware of anything similar in the Caribbean area. Paul M.