What kingdom has only one cell organisms?

The kingdom Monera has only one cell organisms. Monera is the kingdom that includes all prokaryotes, which are organisms that lack a nucleus and other membrane-bound organelles. Prokaryotes are the simplest and most abundant organisms on Earth, and they include bacteria and cyanobacteria.

Has it been estimated that fewer than 24 percent of the animal species living today are likely to be preserved as fossils?
What vertebrate animal cannot reproduce itself?