Many of the changes in this edition were guided by the suggestions of the 49 zoology instructors who read and commented on sections of the eleventh edition. For the eleventh edition, 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. This twelfth edition continues to benefit from their participation. In general, we revised all chapters to make the text current while eliminating excessive detail, and placed more emphasis on experimentation and comparative studies in zoology.

CHAPTER ORGANIZATION

• Order of chapters in Parts One and Two has been maintained as in the eleventh edition. The presentation is streamlined by discussing basic chemistry in the context of the origin of life.
• Several chapters in Part Three have been reorganized. Because evidence is strong that lophophorate phyla and Chaetognatha are protostomes, separate chapters for each of these are not justified; therefore, material on these phyla has been moved to a single chapter covering all smaller phyla of coelomate protostomes (Chapter 21). Removal of chaetognaths from the chapter covering hemichordates made that chapter quite short, and accordingly, hemichordates are now covered in the same chapter as echinoderms, their closest relatives.

NEW PEDAGOGY

• 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; hyperlinks are available on this text's Online Learning Center web site at www.mhhe.com/zoology.
• The art inside the front cover has been completely revised to depict phylogenetic relationships consistent with the Lophotrochozoa/Ecdysozoa hypothesis and other phylogenetic relationships based on sequence analysis of DNA.

The principal revisions in each chapter are explained below.

Part One: Introduction to the Living Animals

• Chapter 2, The Origin and Chemistry of Life, now introduces the important concept of symbiogenesis in the context of origin of eukaryotic cells. A review of basic chemistry (atoms, elements, and molecules; bonding theory; acids, bases, salts, and buffers) is available for reference; it will be found at our Online Learning Center web site: www.mhhe.com/zoology.

Part Two: Continuity and Evolution of Animal Life

• Chapter 5, Genetics: A Review, features new art, including a revised meiosis figure that includes homologous chromosomes in synapsis during prophase of meiosis I and clarifications of 5' and 3' ends of RNA and DNA, indicating direction of synthesis. We have added new material on publication of the draft sequence of the human genome and its implications, as well as an explanation of proteomics.
• In Chapter 6, Organic Evolution, we have added information on new fossil discoveries from China. We relate the issue of gradualism to new work in developmental genetics.
• Chapter 7, The Reproductive Process, was revised extensively in the eleventh edition. Notes on contraception were updated to reflect new trends.

Part Three: Diversity of Animal Life

• Chapter 10, Classification and Phylogeny of Animals, has been reorganized. We present species concepts and diagnosis and then proceed to higher-level taxonomy, rather than vice versa, as in the eleventh edition. We revised coverage of subspecies and added the process of species recognition. A boxed essay has been added to describe how phylogenies are constructed from DNA sequences, and a new taxonomic system called PhyloCode is explained.
• Chapter 11, Protozoan Groups, was completely reorganized in an effort to make it consistent with current classification of unicellular eukaryotes. Traditional taxa such as Sarcomastigophora, Sarcodina, Mastigophora, Zoomastigophora, and Phytomastigophora are all discarded. Flagellated forms that we discuss are distributed among separate phyla: Retortamonada, Axostylata, Chlorophyta, Euglenozoa, and Dinoflagellata. Protozoologists now disperse ameboid forms into numerous lineages, and it is impractical for us to place them all in formal taxa; therefore, we simply use an informal heading "Amebas" to introduce some of the most important ones. The section on phylogeny was rewritten to reflect current evidence, including the likelihood of secondary symbiogenesis in origins of several groups.
• In discussing the phylogeny of Mesozoa in Chapter 12, we note that molecular evidence supports a relationship with flatworms. The description of Hexactinellida was completely rewritten, and a new figure depicting their structure is included.
• Chapter 13, Radiate Animals, underwent several important revisions. Most noticeable to users of earlier editions is a preference for use of Octocorallia over Alcyonaria for this anthozoan subclass. Because members of the subclass always have eight tentacles, we believe that Octocorallia is easier for students to remember. In addition, the discussion of cnidarian phylogeny was rewritten to include evidence from Hox genes and sequence data that Anthozoa is basal for the phylum. A new cladogram reflects this relationship.
• In Chapter 14, Acoelomate Animals, the description of both form and function in Nemertea and phylogeny of flatworms were rewritten.
• Chapter 15, Pseudocoelomate Animals, was completely reorganized. We hope that placing all animals with a common body plan in the same chapter will be helpful for students, rather than separating pseudocoelomate lophotrochozoans from pseudocoelomate ecdysozoans in separate chapters. Thus, phyla in each superphylum were grouped together under appropriate headings (see also Chapter 21). Furthermore, because Nematoda is the largest and most important of all pseudocoelomate phyla, their coverage was moved to the beginning of the chapter. Recognizing molecular evidence of a rotifer-acanthocephalan affinity, coverage of Acanthocephala was moved to follow immediately after Rotifera.
• Chapter 16, Molluscs, was not changed extensively. A note on the evolution of eyes in animals was added, with the observation that all animals with eyes apparently share two conserved genes: Pax 6, the "master control gene for eye morphogenesis," and the gene encoding rhodopsin, a visual pigment. The eyes of cephalopod molluscs are amazingly similar to those of vertebrates. Although this has long been considered an astonishing example of convergence, both groups must have inherited the genes from a common ancestor, and thus their eyes would be homologous.
• Chapter 18, Arthropods, is an introduction to Arthropoda and a more detailed coverage of Chelicerata. In this chapter, many sections on spiders, scorpions, and harvestment were rewritten and revised.
• Chapter 19, Aquatic Mandibulates, benefits from much new art. The phylogeny section in this chapter was rewritten and includes a discussion of how biramous limbs in Crustacea probably arose by a mutation resulting in a modulation of Distal-less gene expression.
• Chapter 20, Terrestrial Mandibulates, is a large chapter because insects are a large group. There were many small rewrites and additions, including the discussion and illustration of sucking mouthparts. The phylogeny discussion was heavily rewritten, including roles of Hox genes and Distal-less gene and the evolution of flight.
• Chapter 21, Smaller Protostome Phyla, was extensively reorganized, as in Chapter 15, grouping lophotrochozoan and ecdysozoan phyla under their appropriate headings. Lophophorate phyla and Chaetognatha were brought into this chapter because present evidence lends little justification to covering them in separate chapters. Significant changes in the phylogeny discussion were required.
• Chapter 22, Echinoderms and Hemichordates, now includes all deuterostomes other than chordates. After removal of chaetognaths from the chapter covering hemichordates, a very short chapter resulted; therefore, hemichordates were moved to join Echinodermata.
• Chapter 23, Chordates, underwent many changes throughout, incorporating new ideas about chordate origins and character evolution, primarily from new fossil finds in China, molecular phylogenetics, and studies of developmental biology. Specifically, we modified and added figures, revised characteristics of chordates and vertebrates, revised discussion of hypotheses of origins of chordates and vertebrates, and added a discussion on Hox genes. Discussions of conodonts, ostracoderms, origin of jaws, and origin of paired appendages were revised to better emphasize critical concepts and include more recent information.
• Coverage of the ecology of amphibian declines in the world is increased in Chapter 25, Early Tetrapods and Modern Amphibians.
• The title of Chapter 26, Amniote Origins and Reptilian Groups, was changed to better reflect expanded coverage of derived features of amniotes, including the amniote egg. Relationships of major groups were updated, especially showing that snakes and amphisbaenians evolved from a lizard-like ancestor. Discussions of dinosaur behavior and extinction, snake feeding methods, snakebite statistics, and ectothermy were revised. A new illustration of a snake skull was added to show the extreme mobility of their head bones.
• Chapter 27, Birds, incorporates material on new fossil dinosaur-bird finds and their significance to flight. The section on mating systems was also modified.
• In Chapter 28, Mammals, coverage on synapsid evolution has been expanded and supporting artwork is modified. We added a section on whale evolution, revised the section on the fermentation system, and updated coverage of human evolution, incorporating information from recent fossil finds and analyses. Coverage of evolution of reproductive systems was changed extensively, including evidence that the chorioallantoic placenta of placental mammals evolved in a common ancestor of marsupials and placentals, and was secondarily lost in marsupials.

Part Four: Activity of Life

• In Chapter 29, Support Protection and Movement, we clarified the role of exercise in differential stimulation of fast and slow fiber types, and we updated the microanatomy and physiology of skeletal muscle contraction.
• We elaborated on the concept of homeostatic setpoint in Chapter 30, Homeostasis, and sections on the role of urea in osmoregulation of elasmobranchs and in urine concentration by mammals were revised. Also revised were coverage of aldosterone secretion, glomerular tubular secretion, ADH mechanism of action, and countercurrent multiplication.
• Chapter 31, Internal Fluids and Respiration, includes revisions on excitation and control of heart function, structure and function of the arterial system, respiration in birds, regulation of respiration, and transport of respiratory gases in mammals.
• In Chapter 32, Digestion and Nutrition, we added a consideration of the brainstem as a hunger center and of obesity as a lifestyle problem in developed countries. New information on leptin resistance and obesity and on the role of inflammation in development of atheroscleroisis was added.
• Chapter 33, Nervous Coordination, includes new information on the vomeronasal organ as a pheromonal sense organ in terrestrial vertebrates and the possibility of pheromonal communication in humans. Coverage of taste discrimination was updated, including discovery of a new taste, umami.
• Chapter 34, Chemical Communication, includes new information on steroid hormone membrane receptors, steroid abuse by young people, insulin action in the brain, and the importance of leptin during periods of decreased food intake.
• Within the Immunity chapter (35), we significantly updated the section on innate immunity by addition of material on the exciting discoveries of antimicrobial peptides produced by many invertebrates and vertebrates, responding on first exposure to broad categories of invaders such as gram-positive bacteria, gram-negative bacteria, and fungi.
• Chapter 36, Animal Behavior, covers advances in application of phylogenetic approaches to study of animal behavior, which highlights an important recent advancement in the field. Material was added on the important concepts of reciprocal altruism and evolutionarily stable strategies.

Part Five: Animals and Their Environment

• Chapter 37, The Biosphere and Animal Distribution, features new information on Wallace's Line and its illustration of the importance of plate tectonics in biogeography. Connections between material in this chapter and that in Chapters 6 and 10 are strengthened.
• In Chapter 38, Animal Ecology, we elaborate the niche concept and have made it more consistent with current leading ecology texts. The discussion of interactions among species in a community (for example, commensalism and mimicry) has been extended.