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If you're trying to distinguish a true animal from, say, a paramecium or an amoeba, it's not very hard: animals, by definition, are multicellular creatures, though the number of cells varies greatly across species. (For example, the roundworm C. elegans, which is widely used in biology experiments, consists of exactly 1,031 cells, no more and no less, while a human being is composed of literally trillions of cells.) However, it's important to keep in mind that animals aren't the only multicellular organisms; that honor is also shared by plants, fungi, and even some species of algae.

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Possibly the most important split in the history of life on earth is the one between prokaryotic and eukaryotic cells. Prokaryotic organisms lack membrane-bounded nuclei and other organelles, and are exclusively single-celled; for example, all bacteria are prokaryotes. Eukaryotic cells, by contrast, have well-defined nuclei and internal organelles (such as mitochondria), and are capable of grouping together to form multicellular organisms. While all animals are euakaryotes, not all eukaryotes are animals: this hugely diverse family also includes plants, fungi, and the tiny marine proto-animals known as protists.

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One of the most remarkable things about animals is how specialized their cells are. As these organisms develop, what seems to be plain-vanilla "stem cells" diversify into four broad biological categories: nervous tissues, connective tissues, muscle tissues, and epithelial tissues (which line the organs and blood vessels). More advanced organisms display even more specific levels of differentiation; the various organs of your body, for example, are made up of liver cells, pancreatic cells, and dozens of other varieties. (The exceptions that prove the rule here are sponges, which are technically animals but have virtually no differentiated cells.)

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Most animals engage in sexual reproduction: two individuals have some form of sex, combine their genetic information, and produce offspring bearing the DNA of both parents. (Exception alert: some animals, including certain species of sharks, are capable of reproducing asexually.) The advantages of sexual reproduction are huge, from an evolutionary perspective: the ability to test out various genome combinations allows animals to adapt quickly to new ecosystems, and thus out-compete asexual organisms. Once again, sexual reproduction isn't restricted to animals: this system is also employed by various plants, fungi, and even some very forward-looking bacteria!

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This one is a bit complicated, so pay attention. When a male's sperm encounters a female's egg, the result is a single cell called a zygote; after the zygote undergoes a few rounds of division, it's called a morula. Only true animals experience the next stage: the formation of a blastula, a hollow sphere of multiple cells surrounding an inner fluid cavity. It's only when cells are enclosed in a blastula that they start differentiating into different tissue types, as described in slide #4. (If you're interested in further study, or if you're just a glutton for punishment, you can also explore the blastomere, blastocyst, embryoblast and trophoblast stages of embryonic development!)

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Fish swim, birds fly, wolves run, snails slide, and snakes slither--all animals are capable of movement at some stage in their life cycles, an evolutionary innovation that allows these organisms to more easily conquer new ecological niches, pursue prey, and evade predators. (Yes, some animals, like sponges and corals, are virtually immobile once they're fully grown, but their larvae are capable of movement before they become rooted to the sea floor.) This is one of the key traits that distinguishes animals from plants and fungi, if you ignore relatively rare outliers like venus flytraps and fast-growing bamboo trees.

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All living things need organic carbon to support the basic processes of life, including growth, development, and reproduction. There are two ways to obtain carbon: from the environment (in the form of carbon dioxide, a freely available gas in the atmosphere), or by feeding on other carbon-rich organisms. Living organisms that obtain carbon from the environment, like plants, are called autotrophs, while living organisms that obtain carbon by ingesting other living organisms, like animals, are called heterotrophs. However, animals aren't the world's only heterotrophs; all fungi, many bacteria, and even some plants are at least partially heterotrophic.

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Have you ever seen a magnolia bush with eyes, or a talking toadstool mushroom? Of all the organisms on earth, only mammals are sufficiently advanced to possess more-or-less acute senses of sight, sound, hearing, taste and touch (not to mention the echolation of dolphins and bats, or the ability of some fish and sharks to sense magnetic disturbances in the water using their "lateral lines."). These senses, of course, entail the existence of at least a rudimentary nervous system (as in insects and starfish), and, in the most advanced animals, fully developed brains--perhaps the one key feature that truly distinguishes animals from the rest of nature.