Orb Mussel

I had no idea that there were bivalve molluscs in our garden pond until I found this amongst the waterweeds. I think it's orb mussel or freshwater cockle in the genus Sphaerium, only about 7mm. long. I suspect that it may have arrived on the feet of a heron that was a regular visitor until I protected the frog population from its predation by covering the pond with a net.
There's a young water hog louse Asellus aquaticus sitting on top of the shell, just below the piece of green alga.

Here's a view of the oldest part of the shell - the umbo - where the two halves are hinged.

 

I think I can see a pair of siphons (the pink objects on the right) poking out from under the shell, which suggests that it is Sphaerium. Water enters through one siphon, is wafted over the gills by thousands of beating cilia, and then expelled through the other. These are unusual amongst bivalve molluscs in that they have an extendable muscular foot (not visible here) that they use for clambering around amongst waterweeds, as well as for anchorage.

In this view you can see the soft mantle tissue, which lays down new shell material around the edge of the existing shell as the animal grows. There's a small amount of damage to the edge of the shell and to the umbo, but it should be able to repair this - old bivalve shells often bear scars of past misfortunes.