The sixteen survey chapters of animal diversity that form the central theme of this book are prefaced by four chapters with discussions of the principles of evolution, ecology, classification, and animal architecture. Throughout this revision we updated references and worked to simplify and streamline the writing style.

Chapter 1 begins with a brief explanation of the scientific method—what science is (and what it is not)—and then moves to a discussion of evolutionary principles. Following an historical account of Charles Darwin’s life and discoveries, the five major components of Darwin’s evolutionary theory are presented, together with important challenges and revisions to his theory and an assessment of its current scientific status. This approach reflects our understanding that Darwinism is not a single, simple statement that is easily confirmed or refuted. It also prepares the student to dismiss the arguments of creationists who misconstrue scientific challenges to Darwinism as contradictions to the validity of organic evolution. The chapter ends with discussion of micro- and macroevolution. The essay on the animal-rights controversy is also updated.

Chapter 2 on ecology is new to this third edition, derived from the 11th edition of its larger sibling, Integrated Principles of Zoology, in which it was completely revised and updated. It explains the principles of ecology, with emphasis on populations, community ecology, and variations in life history strategies of natural populations. The treatment includes discussions of niche, population growth and its regulation, limits to growth, competition, energy flow, and nutrient cycles.

Chapter 3 on animal architecture is a short but important chapter that defines the organization and development of body plans distinguishing major groups of animals. This chapter includes a picture essay of tissue types and a section explaining important developmental features associated with the evolutionary diversification of the bilateral metazoa.

Chapter 4 treats classification and phylogeny of animals. We present a brief history of how animal diversity has been organized for systematic study, emphasizing current use of Darwin’s theory of common descent as the major principle underlying animal taxonomy. Continuing controversies between the schools of evolutionary taxonomy and phylogenetic systematics (cladistics) are presented, including a discussion of how these alternative taxonomic philosophies affect our study of evolution. Chapter 4 also emphasizes that current issues in ecology, evolution, and conservation biology all depend upon our taxonomic system. For this edition we added a new boxed essay illustrating use of molecular phylogenetic procedures, and updated higher-level phylogeny and taxonomy of the bilateria.

The sixteen survey chapters are a comprehensive, modern, and thoroughly researched coverage of the animal phyla. We emphasize the unifying architectural and functional theme of each group. Structure and function of representative forms are described, together with their ecological, behavioral, and evolutionary relationships. Each chapter begins with succinct statements of "Position in the Animal Kingdom" and "Biological Contributions." Students have found these opening statements, a distinctive feature of this text, to be important in assisting their approach to each chapter.

The classifications in each chapter are positioned following other coverage of a particular group, in most cases immediately preceding the summary at the end of the chapter. Discussions of phylogenetic relationships are written from a cladistic viewpoint, and cladograms have been presented where possible. These show the inferred branching events in each group’s history and the origin of some of the principal shared derived characters. Traditional phylogenetic trees have been drawn to agree with cladistic analyses as closely as possible. Because cladistics is not embraced by all teachers, we have presented cladograms as supplemental to the conventional Linnaean classifications.

Some of the principal changes in these survey chapters follow. Chapter 5 on protozoan groups was completely revised for this edition. Molecular sequencing of bases in genes has revealed that the former phylum Protozoa embraces numerous phyla of varying evolutionary relationships. These groups of animal-like unicellular eukaryotes are grouped in this chapter as a convenience without implying that they form a monophyletic group. In chapter 8 (acoelomate animals) we rewrote the section on turbellarians to emphasize their paraphyly, yet retaining the class Turbellaria because the taxonomic complexity of a strictly cladistic classification is beyond the scope of an introductory textbook.

While still covering all pseudocoelomates in a single chapter (chapter 9), we now group the various phyla in superphyla Lophotrochozoa (Rotifera, Acanthocephala, Gastrotricha, and Entoprocta) and Ecdysozoa (Nematoda, Nematomorpha, Kinorhyncha, and Priapulida). In the phylogeny section of chapter 12 (arthropods), we revisited the Lophotrochozoa-Ecdysozoa question, as well as the possibility of polyphyly of Arthropoda. We discuss the new hypothesis that myriapods are a sister group to all other arthropods and that insects and crustaceans form a monophyletic group. Chapter 13 (lesser protostomes) was reorganized to group lophotrochozoans (Sipuncula, Echiura, Pogonophora, Brachiopoda, Ectoprocta and Phoronida) and ecdysozoans (Pentastomida, Onychophora, Tardigrada, and Chaetognatha) together. Chaetognatha were transferred to this chapter on the strength of evidence that they are protostomes, despite their morphological similarities with deuterostomes. Molecular evidence strongly supports placement of lophophorate phyla in Protosomia, and the division of their coelom similar to deuterostomes must be convergent.

In chapter 15 (vertebrate beginnings) we revised and shortened discussions of pharyngeal filter-feeding, ancestry and evolution of chordates, and Garstang’s hypothesis of larval evolution. The many changes in chapter 16 (fishes) include revision of origin and evolution of fishes, and fish classification. In current classifications the traditional term Osteichthyes as applied to all bony fishes does not describe a monophyletic grouping; rather the two major lineages of bony fishes are now divided into two classes containing ray-finned fishes (Actinopterygii) and lobe-finned fishes (Sarcopterygii). In chapter 19 (birds) we revised the section on origin and relationships to explain the recently adopted division of living birds into paleognathous and neognathous groups, replacing the older terminology of ratite and carinate to describe flightless and flying birds, respectively. Among the many changes in chapter 20 (mammals) were revisions of the sections on horns and antlers, glands, food and feeding, primate classification, and human evolution.