Acanthocephala: Spiny-Headed Worms. These internal parasitic worms have a proboscis that they can evert outside of their body. The proboscis has a "head" of spines, which is used to hold onto the host. They have no mouth or stomach, but absorb nutrients directly from the host through their skin. Acanthocephalans, similar to some other parasitic phyla, are notable for having complex life cycles that involve multiple different hosts. Some species are able to alter the behavior of their intermediate crustacean hosts.
Tardigrada: Water Bears or Moss Piglets. These micro-animals have been found on every continent, including Antarctica. They are well-known for their ability to survive extreme conditions such as dangerous radiation, extreme heat/cold, or lack of food and water. Members of Tardigrada have even been studied on space missions, and survived the vacuum of space. They can endure these conditions by entering a "tun" state, where they lose almost all of their water and slow all their life systems to a near-stop. Tardigrades live on moist mosses and lichens, but can survive when their habitat dries out, and reawaken when water returns.