Many improvements in this edition are the direct result of suggestions of many zoology instructors who read and commented on sections of the twelfth edition. The principal revisions for each chapter are detailed below. In general, we revised all chapters to streamline the writing style, make the text current while eliminating excessive detail, and place more emphasis on experimentation and comparative studies in zoology. While the order of chapters is unchanged from the twelfth edition, we reorganized the content of several chapters. Throughout we have updated references, and revised or replaced numerous illustrations. 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/hickmanipz13.

Biological Principles and the Science of Zoology (Chapter 1)
We updated the treatment of the animal rights debate with recent references describing the controversial use of laboratory animals in medical research. We noted in the evolution section that the religious challenge formerly called "creation science" has reappeared recently under the name "intelligent design theory."

The Origin and Chemistry of Life (Chapter 2)
We added the finding of amino acids on meteorites as evidence of spontaneous chemical assembly of protein precursors, and made reference to experimental synthesis of extinct genes and proteins as a test of evolutionary hypotheses.

Cells as Units of Life (Chapter 3)
We extensively reworked this chapter to update material throughout, adding new art coordinated with the text to improve clarity. We gave special attention to sections on components of eukaryotic cells and their functions, cell surface specializations, membrane function (with a new section on diffusion through channels), and mitosis and chromosome structure.

Cellular Metabolism (Chapter 4)
We reworked sections to improve clarity and flow, and added a chapter note on carbon dioxide production, pointing out that whereas a delicate balance exists between consumption and production of oxygen and carbon dioxide by living organisms, human industrialization leading to global warming threatens this balance. This chapter received new art throughout.

Genetics: A Review (Chapter 5)
This chapter was extensively reworked to streamline wording throughout.

Organic Evolution (Chapter 6)
We inserted a new example (bill-size polymorphism in an African finch) used to challenge Darwin's theory of gradualism, and clarified definitions of a populational bottleneck and a founder effect.

The Reproductive Process (Chapter 7)
Among the many changes made in this chapter, we rewrote the chapter note on contraception and placental hormones to reflect recent work.

Principles of Development (Chapter 8)
We rewrote the section on yolk distribution in developing embryos and revised the text to provide a broad overview of development following cleavage. New sections describe changes in an embryo during formation of three germ layers and two body cavities. Differences in the sequence of development between protostome and deuterostome taxa are clarified, and material has been rearranged into a separate section that explains variations on the typical patterns for some members of these two groups. Two new figures have been added, one to clarify the general development sequence and the other to illustrate a life cycle where an embryo has little yolk, and must develop into a feeding larva before metamorphosis to the adult body form. We added a short section to explain that biologists study development for several reasons, in particular to search for common features of development that would indicate shared ancestry among taxa, and to understand the mechanisms whereby a single-celled zygote becomes a complex organism.

Architectural Pattern of an Animal (Chapter 9)
Material in this chapter has been reordered to illustrate the body plans of animals. We added new sections to explain how various developmental sequences produce different adult structures. A new figure illustrates the origin of body cavities, including a gut, pseudocoelom, and coelom. A second new figure shows how the developmental sequence differs for diploblastic animals, and for four types of triploblastic animals. Differences in developmental sequence between deuterostomes and both lophotrochozoan and ecdysozoan protostomes are explained.

Classification and Phylogeny of Animals (Chapter 10)
In this chapter we added the observation that many taxonomic subspecies have been found upon further study to deserve species status. We also elaborated the sections on dynamism of species concepts, and the meaning of PhyloCode.

Protozoan Groups (Chapter 11)
This chapter has been completely reorganized and expanded to include new taxonomic relationships elucidated through molecular studies. We added a cladogram showing the diversification of eukaryotes from a prokaryote ancestor. This cladogram shows 13 major eukaryotic clades containing protozoan taxa. The metazoans are included in the clade Opisthokonta. We made a serious attempt to include taxon names from traditional classification schemes, as well as those used in recently constructed cladograms. Some morphological and molecular characters that appear to define particular clades are explained. Groups such as amebae, where the classification is in flux, were discussed in more general terms. We added a new description of the dinoflagellate Pfiesteria piscicida.

Mesozoa and Parazoa (Chapter 12)
Changes to this chapter include a more integrated section on canal systems and water flow in sponges, as well as a slight expansion of the section on the origin of metazoans. We added a new description of the habitat and life style of a family of cave-dwelling sponges without choanocytes; these sponges are carnivores and supplement their food supply through a symbiosis with bacteria that use methane. Recent phylogenetic analyses of sponge classification using molecular data suggest that the siliceous sponges form a clade distinct from the calcareous sponges. Several drawings were updated and clarified.

Radiate Animals (Chapter 13)
Among the few changes made in the chapter, the material on the basic structure of polyps and medusae has been moved forward to the form and function section. This will make it easier for students to understand the general plans of a cnidarian body and of a cnidarian life cycle before delving into the complexities of each class. There are minor reorganizations of a few sections of the chapter. We added a diagram of the life cycle of Tubularia to the photograph of this animal. Several new photographs of animals replace some older photographs.

Platyhelminthes & Nemertea (Chapter 14)
We updated the species numbers for each phylum and review the recent molecular evidence for relationships among the acoelomate bilateral animals. Controversy continues to surround the validity of the phylum Platyhelminthes and monophyly of the traditional classes (especially the Turbellaria, which is paraphyletic). We introduce molecular and morphological evidence that questions the relationship of the Acoela to the remainder of the bilaterian phyla. Several figures were updated, depictions of the parasitic life cycles are improved, and the relationships of platyhelminth taxa updated in the summary cladogram (Figure 14-28).

Pseudocoelomate animals (Chapter 15)
Recent molecular evidence supports a reversion to the traditional classes of nematodes, but the monophyly of this phylum is questionable. We updated the species numbers for each taxon and discuss the molecular evidence for relationships among the ecdysozoan and lophotrochozoan phyla. We revised the discussion of nutrient uptake by adult nematomorphs. Revised art includes figures depicting the rotifer life cycle and phylogenetic relationships among taxa, and a new close-up image of hookworm jaw plates added to the image of the feeding hookworm.

Molluscs (Chapter 16)
We updated the molecular evidence supporting relationships among the molluscan classes, and the relationship of molluscs to other protostome phyla. New information about the fossil record was added. Several photographs were replaced for improved clarity or color contrast and the summary phylogenetic tree redrawn.

Segmented Worms (Chapter 17)
We made numerous modifications in this chapter to improve readability, update material and changes in interpretation, and removed potentially confusing technical information. . We changed all use of somites and metameres to segments to have students better understand the significance of segmentation and segments. We incorporated new information about the independent evolution of segmentation, and added recent interpretation of annelid diversity.

Arthropods (Chapter 18)
We substantially reworked this chapter to add new information on cuticular sclerotization, morphology, and structural function We added a brief discussion of the Superphylum Panarthropoda. Throughout the chapter we discussed new information on evolutionary interpretation within the arthropods, characteristics of groups of arthropods, and morphological characteristics. New information concerning comparative molecular analyses concerning annelid and arthropodan evolution has been added as well. We provide numbers of species (indicative of species diversity) for all major groups to enable students to make a comparison of diversity. Other new topics include a prelude to the evolutionary problem of “uniramia,” and a more complete discussion of the possible reasons for arthropod success (especially concerning small body size). Interesting anecdotal information has been included to engage student interest.

Aquatic Mandibulates (Chapter 19)
We augmented the discussion of molecular evidence placing insects as a sister group within the Crustacea, and some evidence that is contrary to the so-called "uniramia" hypothesis. We also discuss tagmatization and schizocoely at various points within the chapter.

Terrestrial Mandibulates (Chapter 20
As with the other chapters, this chapter has been streamlined, corrected for content, and expanded in specific areas including the following: updating of insect pests; corrections concerning "marine" insects; a fuller discussion of evidence from development suggesting that biramous appendages may have been present in ancestral insects; the independent evolution of Malpighian tubules in insects and spiders; and the independent evolution of tracheal systems in the various arthropod groups that have them. Species diversity indices have been included throughout. Updates in classification and diversity numbers for the insects have also been included. A great deal of new information concerning specific higher taxa and species dominates the chapter revision. We improved the description of metamorphosis and clarified use of the terms molting and ecdysis. We added more information concerning medically and economically important insect pests. We removed the discredited “skimming” hypothesis and revised the discussion of the evolution of insect wings.

Smaller Protostome Phyla (Chapter 21)
The relationships of the phyla in this chapter to one another and to the remainder of the Bilateria are among the most contentious of any taxa. We introduced molecular and morphological evidence suggesting that the pentastomids are, in fact, derived crustaceans. Discussion of the Pogonophora & Vestimentifera as derived polychaetes within the family Siboglinidae is also presented. We introduced additional data that question the phylum status of the Echiura, along with updated species numbers for each taxon in the chapter. The discussion of bryozoan biology is expanded. Photographs and pictures are updated throughout the chapter.

Echinoderms and Hemichordates (Chapter 22)
We describe the curious mutable collagenous tissue (“catch collagen”) and the importance of this tissue type to echinoderm biology. Most zoologists now agree that the Concentricycloidea is not a class, but there is little agreement on the relationship of this clade to other echinoderms. Recent molecular evidence suggests that the Enteropneusta is not a monophyletic taxon, and pterobranchs appear to be derived from within the enteropneust lineage. Molecular data also suggest that hemichordates are the sister group to echinoderms rather than chordates. The color and contrast of photographs and diagrams in the chapter are improved and the phylogenetic relationships among the taxa are updated.

Fishes (Chapter 24)
Principal changes for this chapter include rewriting the section on sarcopterygian evolution to link it with the origin of tetrapods, and inserting a new section on fish hearing and Weberian ossicles. The art was extensively revised in this chapter.

Early Tetrapods and Modern Amphibians (Chapter 25)
For this chapter we revised material on heart chambers and integumentary structures (especially granular glands), and incorporated a new convention favoring Urodela over Caudata as the main name for the salamander order.

Amniote Origins and Reptilian Groups (Chapter 26)
The relationship of turtles to other amniotes remains controversial. Although some have placed the turtles within the Diapsida, we keep them in the Anapsida for this edition while awaiting broader acceptance of recent molecular analyses.

Birds (Chapter 27)
Although debate about the origin of flight has not abated, we discuss increasing support for the “trees-down” hypothesis. Higher order bird classification also remains controversial. Because the Sibley/Ahlquist biochemical classification has not received broad acceptance, we maintain traditional classification. However, all ratites are now placed within the single order Struthioniformes. The number of recognized bird species now exceeds 9900.

Mammals (Chapter 28)
Because the formation of a squamosal-dentary jaw joint is the definitive mammal characters for fossils, we added a new figure showing the evolution of mammal skulls, especially the jaw joint. We updated the in-text box on whale evolution. Updating mammal taxonomy included splitting the marsupials into seven orders.

Support, Protection, and Movement (Chapter 29)
Among several updatings and art revisions in this chapter, we rewrote the chapter box on osteoporosis to include recent work on hormonal and non-hormonal therapy. We made numerous changes in the animal movement section to improve accuracy and clarity.

Homeostasis (Chapter 30)
We revised several figures in this chapter and added information on estivation in the section on temperature regulation.

Internal Fluids and Respiration (Chapter 31)
This chapter was extensively rewritten to improve accuracy and clarity and integrate the art program with text. Much of the art was revised for this edition. A new chapter box describes coronary artery disease.

Digestion and Nutrition (Chapter 32)
The sections on regulation of food intake and digestion were extensively revised to update information on diet-induced thermogenesis, and the role of newly discovered peptides ghrelin and PYY in short-term regulation of food intake.

Nervous Coordination (Chapter 33)
We made extensive rewrites in sections dealing with the neuron, action potential generation, the synapse, and the chemistry of vision. A new chapter box describes the ability of some fish to receive bioelectric signals, and of other fish to generate weak or strong electric fields by electric organs. Much of the art program in this chapter was revised.

Chemical Coordination (Chapter 34)
We updated this chapter throughout, and added a section to explain the role of juvenile hormone in insect diapause.

Immunity (Chapter 35)
This chapter was extensively revised to update material and improve clarity of subject matter. We also expanded a section dealing with HIV/AIDS, and prepared new artwork that is better integrated with text.

Animal Behavior (Chapter 36)
For this edition we clarified terminology of social behavior, including agonistic behavior, competitive behavior, socially coordinated behavior, and cooperative behavior. We introduced examples and citations of recent work by T. Clutton-Brock.

The Biosphere and Animal Distribution (Chapter 37)
We extensively revised the treatment of marine environments, expanding the descriptions of benthic communities, rocky intertidal and subtidal zones, coral reefs, soft sediment zones, hydrothermal vents, and the pelagic realm. These new sections are enhanced with representative photographs and references. We also expanded the section on zoogeography to emphasize the role of phylogenetic systematics in historical biogeography.

Animal Ecology (Chapter 38)
We introduce the concept of metapopulation dynamics, the interaction of multiple genetically interacting demes. Such interaction may be crucial to avoid extinction of a species when a local deme is depleted or eliminated by environmental change. We added with this edition a section on the important topics of extinction and biodiversity, drawing on major paleontological studies of macroevolutionary changes in earth's biodiversity.