species to die out if the other species begins to take advantage of the other. In the case that the mutualistic behavior persists for enough generations, the dynamic can evolve into parasitism, which is a more stable dynamic due to the increased benefit to the parasite that propagates the behavior. In this case the parasite takes advantage of the previously mutualistic host and parasite dynamic, gaining greater benefits for itself. Parasitism Parasitism is a form of symbiosis in which one species benefits from the interactions between species while the other organism is actively harmed. This is the most common form of ectosymbiotic interactions. One of the many examples of ectosymbiotic parasites includes head lice in humans, which feed off of the blood of humans by attaching to the scalp of a humans head. Additionally, mature Branchiobdellida bacteria act as a nutrient thief in the gut of crayfish species to exist. In these cases, the head lice and the Branchiobdellida are both parasites interacting with host species. See also Epixenosomes References Symbiosis