Why is a sponge not in the fungus kingdom?
Because it belongs to the animal kingdom rather than the fungi or plant kingdoms. Sponges are heterotrophic- they feed on existing organic matter, unlike autotrophic plants that make it themselves. They filter feed rather that feed saprotrophically, which distinguishes them from fungi. EDIT: Look, I took the time to answer you question, at least read my answer properly. At no point did I say they were autotrophic. What I said was (just to make it clear) they are HETEROTROPHIC, unlike plants which are AUTOTROPHIC. They filter feed SAPROTROPHICALLY which is what makes them different from fungi. Please at least read my answer thoroughly before you criticise it.