The Five Kingdom System of Classification, proposed by R.H. Whittaker in 1969, categorizes organisms into five main kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. However, several entities are excluded from this system because they do not meet the primary criteria (such as cellular organization and metabolism) used for these classifications:

Viruses: These are considered acellular (non-cellular) and are not truly "living" because they lack their own metabolic machinery and can only replicate inside a host cell.

Viroids: Similar to viruses, viroids are infectious agents composed only of naked RNA. Because they lack a cellular structure, they are not included in Whittaker's system.

Lichens: These are symbiotic associations between a fungus and an alga (or cyanobacterium). Since they are a composite of two different organisms from different kingdoms, they cannot be classified as a single entity within the five-kingdom framework.

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