Protozoa, are mainly microscopic organisms, ranging in size from 10 to 52 micrometers. Some, however, are significantly larger. Among the largest are the deep-sea dwelling xenophyophores, single-celled foraminifera whose shells can reach 20cm in diameter. Free-living forms are restricted to moist environments, such as soils, mosses and aquatic habitats, although many form resting cysts which enable them to survive drying. Many protozoan species are symbionts, some are parasites, and some are predators of bacteria, algae and other protists.