This third edition of the first part of "Special Zoology" will continue to be the authoritative textbook on systematic zoology for students of the life sciences. While the systematization of eukaryotic unicellular organisms has required extensive restructuring, the major classification of Metazoa has reached a high degree of reliability. The most significant innovations include the separation of Protostomia into Spiralia (Lophotrochozoa) and Ecdysozoa, the removal of Acoelomorpha from Platyhelminthes and their unification with Xenoturbella as Xenacoelomorpha, as well as the dissolution of Articulata into Annelida and Arthropoda, with a redefinition of Annelida that includes the unsegmented Sipuncula. All other chapters have also been revised by their previous authors or by new co-authors. The illustration material has been improved.