Turbellaria are a group of generally small and free-living members of the flatworm phylum Platyhelminthes. Their locomotion depends on cilia. Most turbellaria are carnivorous, and actively scavenge for food. Most species are marine and freshwater dwellers, but some have evolved to dwell on humid land, as well. Planarians and other flatworms lack organs specialized for gas exchange and circulation. Their flat body places cells close to the water, and the detailed branching of the gastrovascular cavity distributes food throughout the body. Nitrogenous waste, in the form of ammonia, diffuses directly from the cells into its surroundings. Flatworms also have a simple excretory apparatus that functions in maintaining an osmotic balance between the flatworm and the surrounding water.