Muscle

Both the muscles in our bodies and mussels from the sea take their name from the common house mouse. The early Romans rather whimsically thought that the body muscles, appearing and disappearing as men competed in athletic games then resting, resembled tiny mice appearing and disappearing at play. Similarly, little dark mice were thought also to resemble the dark colored marine bivalves the Romans liked to serve at banquets. Thus, both the muscle and the mussel were named musculus, or (little mouse). The marine mussel name is spelled differently today only because this makes it easier to distinguish it from the human muscle.

Source: QBP Encylopedia of Word and Phrase Origins, By Robert Hendrickson :