Parasites are animals that live at the expense of other animals. They rob the host animal of nourishment and often cause it to become sick. However, in other types of relationships, different animals can help one another.
Some hermit crabs, hiding under their protective stinging tentacles. At the same time, the sea anemone benefits because it shares the crab's food. Similarly, a species of  shrimp digs a burrow that it shares with the small goby fish. The fish benefits from being able to hide in the burrow, while acting as a lookout to warn the shrimp of approaching predators. Most true parasites are very simple animals, because they do not need complicated organs to digest their food. Some parasites are simply a mass of reproductive organs.