shell thing embedding in florida rock

[tiredfish]

its me again i have about 4 of these on 2 different rocks. they have 2 little openings inside but one is bigger than the other. also the bigger one has filter things around the rim. what are they? they open and close every now and then they dont have much room to move cause they is all embedding in the rock.

 

[reefrunner69]

Some species of non photosynthetic clam. The larger opening is the inhalent siphon, the smller is the exhalent siphon.