Finding frills in a boring animal

A mystery close-up for you.  Do you have a guess about what this might be?

 

Okay, here's another clue.  The next picture was taken under a microscope, but with less magnification.

 

Still need some help?  I'll zoom out a little more:

 

It might be difficult to make out, but you're looking down on the siphons of a rock-boring clam!  This is a Flat-tipped Piddock (Penitella penita).  The inhalant siphon is on the left and was the one featured in the first two photographs.  The frilly projections extending across the inhalant siphon act as a filter to keep out large unwanted debris.  The smaller exhalant siphon is on the right and has a smooth opening.  [The frill is gone.]  The entire width of the two siphons in the view above is about 15 mm across.

Flat-tipped Piddocks are probably more common than we think.  But because they're hidden in burrows in the rock, the clams themselves are generally not that visible.  The siphons are beautiful, but you'd have to be looking in a pool or under water to find and appreciate them.

This clam happened to be on the edge of a rock that collapsed, so it became exposed and it seemed like a good opportunity for a photograph of this seldom-seen animal.  

The clam has a somewhat unusual shape (see below), with a very bulbous rounded end that sits at the bottom of the burrow and an elongated leathery portion that reaches to the surface of the burrow.  Look for the many rough lines of "teeth" on the outer surface of the shell.  The clam uses these teeth to rasp away at the rock.  It rocks back and forth, moves up and down, and rotates around and around, to create a long, smooth, circular burrow.  (You may occasionally discover these burrows in rocks washed up on local beaches!)