Mussel is the common name for many members of a species of clams or bivalve mollusca from saltwater or freshwater habitats. These mussels are closely related to clams but have an elongated shell. In fact mussels can actually move and have a muscular system. The mussel I am going to use is the blue mussel mytilus edulis or the blue mussel. The system is compromised as follows.
Like most bivalves the mussel has a foot that is largely muscular and useful to them. To the left you notice the white posterior adductor muscle in the upper image on the left shell. In the lower image, the muscle has been cut revealing the inside of the mussel. The adductor muscle is used to keep the muscle closed and prevent animals from eating out the mussel. In specific marine blue mussels the foot is smaller and has a tongue like shape. On the surface the groove in the muscle leads to the byssus. The byssus is not a muscle but rather a filament that is secreted by the mussel to attach it to rocks. Lastly, the foot can propel the mussel and also grip on to certain substances. As the mussel evolved into fresh and saltwater mussels, their foot size evolved as well and its size and ability differed.
The picture above is from flexmussels.com
http://en.wikipedia.org/wiki/Mussel#General_anatomy
The picture above is from flexmussels.com
http://en.wikipedia.org/wiki/Mussel#General_anatomy