kingdom, which of course includes yourself. But animal di- versity extends far beyond humans and the dogs, cats, birds, and other animals we humans regularly encounter. For ex- 32 ample, the diverse organisms in Figure 32.1 are all animals, including those that appear to resemble lacy branches, thick stems, and curly leaves. To date, biologists have identified 1.3 million extant (living) species of animals. Estimates of the actual number of animal species run far higher. This An Overview vast diversity encompasses a spectacular range of morpho- logical variation, from corals to cockroaches to crocodiles. of Animal Diversity In this chapter, we embark on a tour of the animal king- dom that will continue in the next two chapters. We will con- sider the characteristics that all animals share, as well as those that distinguish various taxonomic groups. This information is central to understanding animal phylogeny, a topic that is a lively arena of biological research and debate, as you will read. CONCEPT 32.1 Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers Listing features shared by all animals is challenging, as there are exceptions to nearly every criterion for distinguishing an- imals from other life-forms. When taken together, however, several characteristics of animals sufficiently describe the group for our discussion. Nutritional Mode ᭡ Figure 32.1 Which of these organisms Animals differ from both plants and fungi in their mode of are animals? nutrition. Plants are autotrophic eukaryotes capable of gener- ating organic molecules through photosynthesis. Fungi are EVOLUTION heterotrophs that grow on or near their food and that feed by KEY CONCEPTS absorption (often after they have released enzymes that di- gest the food outside their bodies). Unlike plants, animals 32.1 Animals are multicellular, heterotrophic eukaryotes with tissues that develop from cannot construct all of their own organic molecules and so, embryonic layers in most cases, they ingest them—either by eating other living 32.2 The history of animals spans more than half a organisms or by eating nonliving organic material. But un- billion years like fungi, most animals do not feed by absorption; instead, 32.3 Animals can be characterized by “body plans” animals ingest their food and then use enzymes to digest it within their bodies. 32.4 New views of animal phylogeny are emerging from molecular data Cell Structure and Specialization OVERVIEW Animals are eukaryotes, and like plants and most fungi, ani- Welcome to Your Kingdom mals are multicellular. In contrast to plants and fungi, how- ever, animals lack the structural support of cell walls. Instead, Reading the last few chapters, you may have felt like a a variety of proteins external to the cell membrane provide tourist among some unfamiliar organisms, such as slime structural support to animal cells and connect them to one molds, whisk ferns, and sac fungi. You probably are more at another (see Figure 6.30). The most abundant of these pro- home with the topic introduced in this chapter—the animal teins is collagen, which is found only in animals. 654 UNIT FIVE The Evolutionary History of Biological Diversity Many animals have two types of specialized cells not found in other multicellular organisms: muscle cells and 1 The zygote of an animal nerve cells. In most animals, these cells are organized into undergoes a series of mitotic cell divisions tissues, groups of cells that have a common structure, func- called cleavage. tion, or both. Muscle tissue and nervous tissue are responsi- ble for moving the body and conducting nerve impulses, Zygote respectively. The ability to move and conduct nerve impulses Cleavage underlies many of the adaptations that differentiate animals from plants and fungi. For this reason, muscle and nerve cells are central to the animal lifestyle. 2 An eight-cell embryo is formed by three Reproduction and Development rounds of cell division. Most animals reproduce sexually, and the diploid stage usually Eight-cell stage dominates the life cycle. In the haploid stage, sperm and egg Cleavage cells are produced directly by meiotic division, unlike what oc- Blastocoel curs in plants and fungi (see Figure 13.6). In most animal 3 In most animals, cleavage produces species, a small, flagellated sperm fertilizes a larger, nonmotile a multicellular egg, forming a diploid zygote. The zygote then undergoes stage called a blastula. The blas- cleavage, a succession of mitotic cell divisions without cell tula is typically a growth between the divisions. During the development of hollow ball of cells most animals, cleavage leads to the formation of a multicellu- that surround a cavity called lar stage called a blastula, which in many animals takes the Cross section the blastocoel. Blastula form of a hollow ball (Figure 32.2). Following the blastula of blastula stage is the process of gastrulation, during which the layers of embryonic tissues that will develop into adult body parts are produced. The resulting developmental stage is called a 4 Most animals also undergo gastrulation, a process in which gastrula. one end of the embryo folds Although some animals, including humans, develop directly inward, expands, and eventually into adults, the life cycles of most animals include at least one fills the blastocoel, producing layers of embryonic tissues: the larval stage. A larva is a sexually immature form of an animal ectoderm (outer layer) and the that is morphologically distinct from the adult, usually eats dif- endoderm (inner layer). ferent food, and may even have a different habitat than the Gastrulation adult, as in the case of the aquatic larva of a mosquito or drag- onfly. Animal larvae eventually undergo metamorphosis, 5 The pouch formed a developmental transformation that turns the animal into a by gastrulation, called the Blastocoel archenteron, opens to the juvenile that resembles an adult but is not yet sexually mature. outside via the blastopore. Endoderm Although adult animals vary widely in morphology, the genes that control animal development are similar across a Ectoderm 6 The endoderm of the broad range of taxa. All animals have developmental genes that archenteron develops Archenteron regulate the expression of other genes, and many of these regu- into the tissue lining the animal’s Cross section Blastopore latory genes contain sets of DNA sequences called homeoboxes digestive tract. of gastrula (see Chapter 21). Most animals share a unique homeobox- ᭡ Figure 32.2 Early embryonic development in animals. containing family of genes, known as Hox genes. Hox genes play important roles in the development of animal embryos, controlling the expression of dozens or even hundreds of other earlier homeobox genes. Over time, the Hox gene family under- genes that influence animal morphology (see Chapter 25). went a series of duplications, yielding a versatile “toolkit” for Sponges, which are among the simplest extant animals, lack regulating development. In vertebrates, insects, and most other Hox genes. However, they have other homeobox genes that in- animals, Hox genes regulate the formation of the anterior- fluence their shape, such as those that regulate the formation posterior (front-to-back) axis, as well as other aspects of develop- of water channels in the body wall, a key feature of sponge ment. Similar sets of conserved genes govern the development morphology (see Figure 33.4). In the ancestors of more com- of both flies and humans, despite their obvious differences and plex animals, the Hox gene family arose via the duplication of hundreds of millions of years of divergent evolution. CHAPTER 32 An Overview of Animal Diversity 655 animal species are extinct.) Various studies suggest that this CONCEPT CHECK 32.1 great diversity originated during the last billion years. For ex- 1. Summarize the main stages of animal development. ample, some estimates based on molecular clocks suggest What family of control genes plays a major role? that the ancestors of animals diverged from the ancestors of 2. WHAT IF? What animal characteristics would be fungi about a billion years ago. Other such studies have esti- needed by an imaginary plant that could chase, cap- mated that the common ancestor of living animals lived ture, and digest its prey—yet could also extract nutri- sometime between 800 and 675 million years ago. ents from soil and conduct photosynthesis? To learn what this common ancestor may have been like, 3. MAKE CONNECTIONS Humans have about the same scientists have sought to identify protist groups that are number of protein-coding genes as do animals such as closely related to animals. As shown in Figure 32.3, a combi- tunicates (see photograph) that have nation of morphological and molecular evidence indicates very simple body forms and few that choanoflagellates are among the closest living relatives of neurons. In contrast, humans have animals. Based on such evidence, researchers hypothesize that many more microRNA molecules the common ancestor of living animals may have been a sus- (miRNAs) than these animals. Re- pension feeder similar to present-day choanoflagellates. We view Concept 18.3 (pp. 365–366); will next survey the fossil evidence for how animals evolved then suggest a possible reason for from their distant common ancestor over four geologic eras this observation. (see Table 25.1 to review the geologic time scale). For suggested answers, see Appendix A. Neoproterozoic Era (1 Billion–542 Million Years Ago) CONCEPT Despite the molecular data indicating an earlier origin of 32.2 animals, the first generally accepted macroscopic fossils of The history of animals spans animals date from 565 to 550 million years ago. These fos- more than half a billion years sils are members of an early group of soft-bodied multicellu- lar eukaryotes, known collectively as the Ediacaran The animal kingdom includes not only a great diversity of biota.