Rotifers
The 'rotifers' are a phylum of tiny animals which are common in freshwater environments, such as ponds and puddles. There are a few saltwater species. Some rotifers are free swimming, others move by inching along, and some are fixed. They live inside tubes or gelatinous holdfasts that are attached to a substrate A few species live in colonies. Microscopic Anatomy of Invertebrates' vol 4. Wiley-Liss, New York.