Merriam-Webster Collegiate Dictionary - mussel

 noun  Etymology: Middle English muscle, from Old English muscelle, from Vulgar Latin *muscula, from Latin musculus muscle, ~  Date: before 12th century  1. a marine bivalve mollusk (especially genus Mytilus) usually having a dark elongated shell  2. a freshwater bivalve mollusk (as of Unio, Anodonta, or related genera) that is especially abundant in rivers of the central United States and has a shell with a lustrous nacreous lining