André Martel © Canadian Museum of Nature

In this group of eastern elliptios (Elliptio complanata), the incurrent siphon (the siphon the mussel uses to draw water) is most visible on the centre mussel. The siphon has hair-like papillae to detect particles in the water that are undesirable or too large. The mussel shuts quickly to eject such particles. A second siphon, at the top of the shell, expels the filtered water.

This image is from a page of the Nature of the Rideau River Web site, created by the Canadian Museum of Nature. Visit the page.