We revised all chapters to streamline the writing and to incorporate new discoveries and literature citations. Our largest formal revision is to include a cladogram of animal phyla on the inside front cover of the book, and to reorder chapter contents in Part Three (Diver-sity of Animal Life) to match the arrangement of phyla on the cladogram. Each chapter in Part Three begins with a small image of the zoological cladogram highlighting the phylum or phyla covered in the chapter, followed by an expanded cladogram of the contents of each major phylum. We place stronger emphasis on phylogenetic perspectives throughout the book. Material formerly presented separately as biological contributions and characteristics of phyla is consolidated in a boxed list of phylum characteristics for each chapter in Part Three. New photographs are added to illustrate animal diversity in many phyla.