Mollusk is used in North America, but mollusc is the common European spelling. The latter probably developed first, but in any case, both of these terms derive from French mollusque, which is, as many animal names are, from Carolus Linnaeus (see infographic), who first noted the word mollusca in 1783 when he was being a nerd and categorizing everything. This he borrowed directly from the Latin word molluscus, which meant "thin skinned", from the root mollis, which meant "soft". This is especially ironic, when you consider that some mollusks, like snails, have pretty hard shells. Mollus may be a corruption of an earlier word molduis, which through a brief jaunt in Proto-Italic goes back to the Proto-Indo-European term moldus, which in turn can be followed to the earlier Proto-Indo-European mel. All these following words meant "soft" as well, but they lead to words from Prussian maldai ("boys") to Ancient Greek bladus ("weak"). So, yeah, pretty interesting!
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.