Fenestella was a moss animal. Moss animals are tiny organisms that use an arm-like structure bearing tentacles to filter out plankton. Each individual, only about a millimetre across, lives in a hole in a calcareous shell. Individuals live together in a mass, forming a large colony.
Moss animal colonies vary in shape, and Fenestella formed fan-like webs that stood upright on the sea bed in places where the water currents were weak.