Plasma-membrane extensions that serve as a means of enhancing nutrient absorptive area.
Haustoria tend to be associated with parasitic fungi though also with . Their function is to increase the amount of membrane across which nutrients are in effect stolen from host organisms.
Haustoria tend to penetrate to but not through the plasma membranes of cells being parasitized. As such, the haustoria are not involved in the killing of target cells but rather in an ongoing parasitism of those cells, indeed a parasitism that befits a long-term association that is based on one organism nutrients from the other.