I wish I knew which species of clam this is. (It was found in a low intertidal cobble on 7 July 2012.) But even without a name, it's easy to appreciate the attractiveness of this bivalve's siphons. Can you tell which is the inhalant siphon (taking in water) and which is the exhalant siphon (expelling water out)?
Here's another view:
The exhalant siphon is smaller, with lobes along the fringe extending outward. It's at the top of these photos. The smaller diameter of the exhalant siphon increases the velocity of exiting water, thereby reducing the chance that the clam will draw in water that's already been filtered.
The inhalant siphon is larger, with tree-shaped structures (sometimes called papillae) extending towards the center (especially visible in first photo). The papillae help prevent larger particles from entering the siphon.