Consider fungi. They are their own kingdom, more closely related to animal than to plant. They grow and search for food with threadlike hyphae. They feed by excreting digestive juices onto their dinner, living or dead, to help break down nutrients before ingesting. There are more species of fungi (as many as six million) than there are known animal species (about two million, mostly insects) or plant species (about 400,000 terrestrial species). Fungi (except yeast) reproduce by putting out spores. Chemists estimate that in any given year, 50 million tons of fungal spores are blowing about in the air.
Most fungi are harmless; some are essential for our survival.
