Mollusca is indeed a very large phylum, with bivalves, gastropods, cephalopods, monoplacophorans, scaphopods, aplacophorans and chitons. However, this is not nearly enough to cap the largest phylum, which is .......
Phylum Arthropoda, with about 1 million insect species, and plenty of myriapods, arachnids and crustaceans, not to mention horseshoe crabs and pycnogonids.