In the third edition of this textbook all the chapters, each devoted to a phylum, have been completely revised and the proposed new phylum Loricifera has been added. The opportunity has been taken to revise classification of groups as and where appropriate, especially in the Protozoa. New information has been incorporated where available and several new illustrations included. The text on key words has been expanded and the generic lists now include more genera from parts of the world other than northern Europe.