Many parasites absorb or ingest nutrients from the intestinal contents of the host. Although this reduces food availability to the host, these parasites do not cause physical damage to host tissue as a result of their feeding.
Protozoan parasites feed by engulfing or absorbing nutrients much as their free-living counter parts do.
Giardia is an absorber.
Balantidium ingests particles.
Cestodes lack the mouth and intestine that is present in other platyhelminths. All nutrition is obtained by absorption of molecules, principally carbohydrates, across their body surface, the tegument. The tegument secretes enzymes that, among other things, break down molecules outside the body into absorbable constituents. The cestode tegument is covered with a microtriche layer (arrow, but requiring electron microscopy for clear view) that greatly increases its surface area.
Acanthocephalans also lack a mouth and intestine, and possess a tegument across which molecules are absorbed. The tegument contains a series of internal channels, or lacunae (arrows), connected by pores to the surface. One of its functions is probably to increase absorptive surface area, and act as a transport system.
Digenea possess a functional tegument like the cestodes, but also have a mouth and an incomplete digestive system. They are able to feed on particulate matter with their mouth. The tegument, which has a digestive-absorptive function, may be modified more for defense against host responses.
Nematodes possess a complete digestive system. Particles are taken in through the mouth (M), with the esophagus (E) or muscular bulb (B, in some species) acting as a pump, and passed to the intestine (I).
