But many micro-organisms are neither plant nor animal. In many cases we grouped them by their major features. A very interesting book is 'Five Kingdoms' by Margulis and Schwartz. In this book life forms are divided into 5 kingdoms. Furthermore the book divides all these kingdoms into many phyla.
Humans belong to the phylum Chordata.
Rotifers that are also animals have their own phylum Rotifera. More about this subject can be read in David Goldstein's Micscape article Classification of Living Things . Scientists decided to use the term Protoctist instead of Protist since protist was always used for single celled creatures. Several Protoctists are multi-celled or large. I n our identification kit we often use the term 'green algae'.