An aggregation of Spirostomum ciliate protozoans, undulating in pond water. Ostracods, small crustaceans, travel rapidly across the white mass of worm-like ciliates. Spirostomum species are among the largest ciliates and can grow to several millimetres in length. They move by beating hair-like cilia that cover their bodies. Ostracods are small crustaceans whose bodies are enclosed within a bivalve-like shell, which is hinged at the top and opened to allow their antennae to protrude. The antennae are used for locomotion. Spirostomum and ostracods are common in freshwater and marine habitats worldwide.