Many of the changes in this edition were guided by the suggestions of more than 60 zoology instructors who read and commented on sections of the 10^th edition. In addition, the vertebrate chapters of Part Three, and several chapters on functional systems (Part Four) were revised by invited Contributors, all experienced zoologists who were solicited for their interest and expertise in the subject matter of specific chapters. In general, all chapters were revised to make the text current while eliminating excessive detail, and to place more emphasis on experimentation and comparative studies in zoology.

• Separate treatments of the origin of life and chemistry of life are condensed into a single chapter (Chapter 2), thus streamlining the presentation by discussing basic chemistry in the context of origin of life.
• The order of chapters in Part Two is altered to offer a better study sequence for students, providing a grounding in genetics and evolutionary theory before undertaking the chapters on reproduction and development. There are numerous places in the development chapter in which an understanding of genetics is crucial.
• A completely new chapter on immunology (Chapter 37) was developed, covering both vertebrate and invertebrate immunology and embracing many new discoveries in this fast-moving field.

• Throughout the text we updated references, revised or replaced many illustrations, and rewrote many of the Review Questions to provoke thought and reduce emphasis on rote memorization.
• Suggested Internet topics are added at the end of each chapter.
• The end paper on Origin of Life and Geologic Time Table has been replaced with a revised version in full color.

The principal revisions are explained below.

• Chapters 2 (Chemistry) and 3 (Origin of Life) now form an integrated review of the kinds of organic molecules found in living systems and their origins in the earth's primitive reducing atmosphere. A review of basic chemistry (atoms, elements, and molecules; bonding theory; acids, bases, salts, and buffers) is available for reference.
• For Chapter 3, on cells as units of life, we revised the discussion of cell structure and cell junctions, and reorganized the sequence of certain topics. Several illustrations in this and the following chapter on cellular metabolism were redrawn for this edition.

• Chapter 5, Principles of Genetics, features a revised section on molecular genetics, adding a new coverage of genomics, and a new subsection on molecular systematics. The increasing ease with which genes can be sequenced and compared to sequences of the same gene in other taxa has led to a great many revisions of phylogenies based on sequence analysis. Such findings have made necessary many changes in the diversity chapters in Part Three of this book.
• Chapter 7, The Reproductive Process, was revised to clarify relationships among bisexual reproduction, hermaphroditism, and parthenogenesis. A new section on sex determination summarizes the most recent understanding of the male determining gene and masculinizing hormones, and discovery of the Sex Reversing X region on the X chromosome and its role in promoting ovary formation. The final section on endocrine events that orchestrate reproduction was rewritten and updated.
• Chapter 8, Principles of Development, was extensively revised in both text and line art. The order in which material on cleavage is presented was reorganized to clarify relationships among principal topics of yolk amount and distribution, cleavage type, cleavage pattern, and subtopics of direct and indirect development, mosaic vs. regulative development, and differences between protostomes and deuterostomes. Cleavage of centrolecithal eggs was added. The section on gastrulation now compares the process in sea stars, reptiles, birds, and mammals. Among other sections revised and updated were those on cytoplasmic specification and homeotic genes.

• Chapter 9 provides a concise presentation on animal architecture as an introduction to animal diversity, which is the core of most zoology courses. Several sections of this chapter were revised: complexity and body size, muscular tissue, animal body plans, body cavities, and terminology used in specifying aspects of symmetry.
• Chapter 10, Classification and Phylogeny of Animals, explains the principles of animal taxonomy and how they are applied by the competing schools of evolutionary taxonomy and cladistics. Because classification pervades every course in zoology, students should understand that systematics provides the evolutionary basis for zoological study. Changes include revision of systematics of great apes to use a cladistic classification, and updating of the material on classification of the Bilateria to incorporate results of new molecular phylogenetic studies.
• The title of Chapter 11 was changed from "The Animal-like Protista" to "Protozoan Groups." Although both Protozoa and Protista no longer are considered valid taxa, we continue to use the terms "protozoa" and "protozoan" informally to distinguish these animal-like phyla. Among sections revised in the protozoan chapter are pseudopodial movement, mechanism of contractile vacuole action, and the final sections on phylogeny and classification.
• For Chapter 12 (Mesozoa and Parazoa) we revised the sections on origin and phylogeny of Metazoa, and deleted reference to class Sclerospongiae, which is no longer recognized as a valid taxon.
• We made several changes in Chapters 14 and 15 on acoelomate and pseudocoelomate animals, including reorganization of the material on class Turbellaria, and revision of the phylogeny sections for both chapters. There is evidence now that acoels (order Acoela) are not flatworms but form the sister group for all other Bilateria. All remaining acoelomates are now placed in the newly erected protostome superphylum Lophotrochozoa.
• Each of the pseudocoelomate phyla are assigned to either Lophotrochozoa or to the alternative superphylum Ecdysozoa. Phylogeny sections for mollusc, annelid, and arthropod chapters also were revised to embrace new information from sequence analysis, which places Mollusca and Annelida in superphylum Lophotrochozoa, and Arthropoda in superphylum Ecdysozoa. We point out, however, that analysis upon which the Lophotrochozoa/Ecdysozoa hypothesis is based fails to support monophyly of Mollusca and Annelida. Nevertheless, few if any zoologists believe molluscs and annelids are not monophyletic groups.
• In Chapter 20, on terrestrial mandibulates, we introduce the term parasitoid and emphasize the importance of parasitoids in controlling populations of other insects. Among other changes in this chapter we strengthened coverage of pheromones, including use of pheromone baits in insect traps and importance of such use in monitoring insects of economic importance.
• Lophophorate animals (Chapter 22) are now assigned to Protostomia, forming an important group in superphylum Lophotrochozoa. If lophophorates are protostomes as most recent evidence suggests, the trimerous coelomic arrangement must have evolved independently in protostomes and deuterostomes.
• Chapter 25 (chordates) received minor revision, including reworking sections on ancestry and evolution, chordate fossil discoveries, and position of amphioxus in speculations on chordate ancestry.
• Chapter 26 on fishes was extensively revised. With Osteichthyes no longer considered a valid taxon, Actinopterygii and Sarcopterygii are elevated to class; this change is accompanied by a discussion of the origin and radiation of ray-finned fishes, radiation of the neopterygians, and morphological trends that permitted great diversification of the teleosts. Introductory sections on ancestry, relationships, and biology of fishes were rewritten to clarify relationships among major fish groups. Revisions in the section on sharks include discussions of sensory systems, shark attacks, and reproduction. Several changes were made in the art program, including corrections in synapomorphies in the cladogram of fishes.
• The title of Chapter 28 was changed to Reptilian Groups to emphasize paraphyly in class Reptilia. Topics revised in this chapter include lung breathing in turtles, viviparity, and characteristics that distinguish reptiles from amphibians.
• In the bird chapter (Chapter 29) we added a note on recent fossil bird discoveries, and revised discussions of skeletal weight comparisons in birds and mammals, bird kidney function, and sun-azimuth orientation of bird migration. We reorganized the treatment of forms of bird wings for flight and added a new illustration to show hovering flight in hummingbirds.
• Chapter 30, Mammals, includes an updated discussion of the first hominids to summarize recent fossil finds, and a revised illustration of hominid skulls. Other changes: adoption of a cladistic classification for primates, and revision of discussions of horns and antlers, glands, feeding specializations, body weight and food consumption, and reproductive patterns.

• The revisions for Chapter 31, Support, Protection, and Movement include discussions of skin cancer from sunlight, mechanisms of ciliary movement, energy for muscle contraction, fast and slow fibers, and description of dermal derivative in vertebrates.
• Chapter 32, Homeostasis was updated throughout. Treatments revised include hyperosmotic regulation in invertebrates, hypoosmotic regulation in fishes, shark kidney function, mechanism of contractile vacuole function, and glomerular filtration.
• A major improvement in flow and unity of Chapter 33, Internal Fluids, was transfer of defense mechanisms and immunity to a separate chapter (Chapter 37).
• Chapter 34, Digestion and Nutrition, includes a discussion on nutritional requirements to embrace new understanding of relationships among the hunger center, brown fat, the protein thermogenin, and the recently discovered hormone leptin. We also updated statistics on world meat consumption, malnutrition, and world population. The discussion on gastrointestinal hormones, previously included in the endocrine chapter, was moved to this chapter.
• The chapter on nervous coordination (Chapter 35) was revised throughout. The most important revisions appear in sections dealing with nature of the nerve impulse, synapses, evolution of invertebrate nervous systems, reflex acts and reflex arcs, autonomic nervous systems, odor reception, and color vision.
• Chapter 36, Chemical Coordination, features an updated section on second messenger system, and new sections that describe the role of growth hormone as a diabetogenic hormone, and action of the most recently discovered hormone, leptin, in regulating eating behavior and energy balance.
• Chapter 27, Immunity, is NEW and covers the topics of susceptibility and resistance, innate defense mechanisms, acquired immune response in vertebrates, blood group antigens, and immunity in invertebrates. The section on acquired immune response in vertebrates includes descriptions of self-non self discrimination (MHC proteins), recognition molecules (antibodies and T-cell receptors), cytokines, humoral response (T_H2 arm), and cell-mediated response (T_H1 arm).
• Chapter 38 concludes this unit with a discussion of animal behavior. It features an expanded explanation of the ritualization of behavior, and new sections on diversity of mating systems, altruistic behavior and kin selection, and animal cognition. The latter describing the remarkable studies of the Gardners with the chimpanzee Washoe, and Pepperberg's work with an African grey parrot.

• Chapter 39, Biosphere and Animal Distribution, includes an updated discussion of the proposed effect of carbon dioxide on the earth's climate. It also provides an expanded explanation of the earth's heat engine with accompanying new art, and added mean annual temperature and rainfall values to all biome descriptions.
• Chapter 40, Animal Ecology, was completely rewritten to provide much greater emphasis on populational and community ecology. It features expanded explanations of niche, characteristics of population (age structure, growth rates, survivorship), population regulation, and interactions among populations in communities.