In this type of nutrition, the organisms (called parasites) depend on the body of other living organisms (called their host) for getting their food. Many viruses, bacteria, fungi and animals have this mode of nutrition. The organisms which depend on other living organisms (host) for food are called as parasites.
Parasites of two types:
Ecto parasites are those parasites which obtain their food from their host by remaining outside the body of their host. For example, mosquitoes, ticks, lice and bed bug.
Endo parasites are those parasites which obtain their food from their host by remaining inside the body of their host. For example, Ascaris, plasmodium vivax and tape worm.
Symbiosis is a close ecological relationship or association between the individuals of two (or more than two) different species. In symbiosis, at least one member of the pair benefits from the relationship. The other member may be injured (parasitism, relatively unaffected (commensalism), may also benefit (mutualism). In other words, at least one member of the partner gets symbiotic nutrition.
Bacterial endosymbiosis has a recurring importance in the evolution of insects. Approximately 10-20% species of insects depend on bacterial associates for their nutrition and reproductive viability. In nutritional symbiosis, both mutuals contribute to each other organic nutrients, inorganic minerals, or digestive enzymes. Some important examples of nutritional mutualism are: nitrogen fixation, Mycorrhiza (fungus and root association), Syntrophy (mutual production of biochemical substances and nutrients), and Lichen.