Ciliates like these Suctoria (left) and bell animalcules (right) are perhaps much more common in freshwater but marine forms do occur. You can find them attached to all kinds of substrates. I found the Suctoria (a strange ciliate that doesn't look like a ciliate and which feeds on ciliates) attached to floating colonies of Phaeocystis. Bell animalcules live on stalks, often in branching colonies.