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From Wikipedia, the free encyclopedia

Zoology (/zuˈɒləi, z-/) or animal biology is the branch of biology that studies the animal kingdom, including the structure, embryology, evolution, classification, habits, and distribution of all animals, both living and extinct, and how they interact with their ecosystems. The term is derived from Ancient Greek ζῷον, zōion, i.e. "animal" and λόγος, logos, i.e. "knowledge, study".[1]

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You and I both know people or dogs that we don't consider particularly sophisticated. We sometimes refer to them as "simple" or "Real Housewives." But when it comes to truly simple animals, we shouldn't underestimate them. Because the animal phyla that we describe as being the least complex actually offer us a vivid way of understanding how animals are structured, and also how they evolved. Simple doesn't always mean dumb. Unlike those dullards that we've all met in our lives, animals aren't considered simple because they apparently take things for "granite" or they think that reality TV is...reality. Their simplicity has to do with their tissue complexity. As you know, almost all animals' cells are organized into tissues that perform specialized functions. The more different kinds of specialized cells an animal has, the more complex it is, and this complexity is determined in the embryonic phase. As embryos, most animals either form two layers of early tissue, called germ layers, or they form three. By exploring the very simplest phyla, from animals with no layers at all, aka sponges, to the most basic of three-layer animals, like mollusks you can see how a not-totally-amazing-sounding change in tissue results in truly fundamental and amazing changes. So the places in the animal family tree where these transitions take place, from no layers to two layers, and from two layers to three, are some of the most important benchmarks in animal evolution. Let's start with the very simplest of animals, in the phylum Porifera: the sponges! They diverged from protists probably 600 million years ago and not a whole lot has changed for them since then. If you've been paying attention, you've noticed by now that almost nothing that applies to other animals applies to sponges. That's because they're so freaking simple. They can't move; they just hang out and filter water for food like bacteria, while some host photosynthesizing microbes and mooch off them. More important, sponge embryos don't have any layers, they just have cells. This means that sponges don't have specialized tissues or organs. And their cells can take different forms. Some have flagella to force water into the sponge; some are more amoeba-like and wander around distributing nutrients. But these cells can transform into whatever type of cell the sponge needs. For this reason, some scientists argue that sponges aren't even animals at all, they're actually colonies of cells that depend on each other to function. But for our purposes, mainly because they're multicellular, eukaryotic organisms that can't make their own food, they still count. And they've managed to diversify into nearly 10,000 different species, so good for them. Things get more interesting with Cnidaria, which include jellies, sea anemones, corals, and hydras. They got a couple of sweet evolutionary breaks that made them animals that you do NOT want to mess with. The first and most important break is that they develop two germ layers. You'll remember these layers are called the endoderm, or the "inside" -derm, and the ectoderm, the "outside" -derm, and they form a tube that allows an animal to ingest, digest and get rid of stuff. This makes Cnidaria among the oldest living descendants of the world's first diploblast, which is the common ancestor of all "true animals." But still, jellies and anemones and other cnidarians have only one hole that serves as both mouth and anus, and they don't have any organs. So, still pretty simple. Their second evolutionary break is in their ectoderm, which contains stinging cells called cnidocysts. Think Portuguese Man o' War, I once stepped on a dead one, it was dead, LONG dead, and I wanted someone to cut my foot off it hurt so much. So now we've got two-layer animals swimming around, able to move and eat and poop and defend themselves. The animal kingdom is just one evolutionary breakthrough away from a huge, like, explosion! And we can see the evidence of this breakthrough in Platyhelminthes, the phylum of soft, unsegmented worms that includes flatworms, planaria, tapeworms, and flukes. Not super-handsome, but these guys are a big deal, because they're the oldest existing phylum that is triploblastic, or has three germ layers. So in addition to an endoderm and ectoderm, their embryos form a mesoderm. I know it sounds like just another piece of toast and turkey on a club sandwich, but this development changes everything. Platyhelminthes are themselves pretty simple, but a couple of phyla up the ranks, this new layer allows animals to form true organ systems: the ectoderm forming the brain, nervous system and skin; the mesoderm forming muscles, bones, cartilage, the heart, blood and other very useful stuff; and the endoderm forming the digestive and respiratory systems. And this kind of complexity is only possible because of one of the mesoderm's key features, the coelom, a fluid-filled cavity that stores and protects the major organs. It allows the internal organs to move independent of the body wall, and the fluid can provide some shock resistance. Coeloms are where all the action happens when it comes to organ systems, but not all triploblasts have them. From here on, we can assess the complexity of an animal by whether it has a coelom or not, and if so, how complete it is. For instance, because they're the simplest of the triploblasts, Platyhelminthes have their mouths and buttholes on opposite ends of their bodies, which is awesome for them! But they're acoelomates, they don't have a coelom, which tells us they're still on the shallow end of the pool, complexity-wise. To give you an idea of how simple, you can cut a Platyhelminthes in half, and both of the pieces will happily continue on with their wormy business. That, my friends, is simplicity. Now, you probably haven't forgotten that I mentioned an explosion a minute ago. Well, I'm not going to taunt you with talk of explosions without giving you one. [BIOLO-GRAPHY] The Cambrian Explosion! Not long after germ layers became a thing, say 535 million years ago, life on earth was undergoing some pretty terrific and rapid innovations. Over about 10 or 12 million years, about half of the animal phyla that exist today started to appear. It remains the most biologically productive period in history. Think of the most exciting, vibrant, creative, dangerous experience and then invite all of Kingdom Animalia to the party. Like Burning Man, ComicCon, and Coachella all at once. This is when animals started to look and behave as we know them today. Before the Cambrian, most of the big animals were slow and soft-bodied and ate algae or scavenged. But this explosion of diversity brought all kinds of new adaptations, including predatory ones, like claws, and defensive ones like spikes and armored plates. Shells and mineral skeletons made their first appearances. In fact, the adaptations were so many and so abrupt that in the 1800s the abundance of fossils from this period was used to argue against evolution. Scientists offer a lot of different theories about what caused this explosion. It was probably a combination of a few of these things. For one, oxygen levels became very high in Cambrian seas, which allowed for larger bodies and higher metabolisms. It's also thought that ocean chemistry changed, with more minerals becoming available for the production of shells and skeletons. And of course, with more diversity comes more competition and predation, which drove selective pressures on animals to become either better at hunting or better at defending themselves. It's pretty near the top of my list of places I want to go once I put the finishing touches on my time machine. But for now, we still have many modern animal phyla to remind us of this time of crazy awesomeness. So flukes are cool and all, but things start to get more complex with another phylum of mostly nasty parasites, Nematoda, unsegmented roundworms. These guys are pseudocoelomates, meaning they have an incomplete body cavity. Unlike a true coelomate, whose body cavity is contained within the mesoderm, pseudocoelomates sort of improvise one between the mesoderm and the endoderm. The vast majority of nematodes live in soil, where they eat bacteria or fungus or parasitize plant roots. But humans host at least 50 nematode species, including hookworms, which burrow into our intestines and treat us like some kind of food court. But most nematodes are very very small: a single teaspoon of forest soil can have several hundred in it! Rotifera, meanwhile, are tiny filter-feeding animals that live mostly in fresh or salt water, though some of them can live in damp soil. They're also pseudocoelomates like nematodes, and although they are way smaller than most flatworms, a big honkin' rotifer is like 2 millimeters long, they're anatomically more complex, as they have a stomach, jaws and a little tiny anus. My favorite fun fact about Rotifera is that many of its species are known to exist entirely of females, and they reproduce through unfertilized eggs. Fossils of rotifers have been found as old as 35 million years, and in many cases, there's not a dude to be found. You go girls! Okay, so now for the big dogs: the phylum Mollusca. Molluscs might be kind of simple, but they're amazing and some of them are incredibly smart. They take four different basic forms: chitons, snails, bivalves, and octopi and squid. Now, I realize it can be hard to see how an oyster and an octopus might be related, but molluscs have some important similarities: They all have a visceral mass, which is a true coelom a body cavity completely within the mesoderm that contains most of the internal organs. They also have a big, muscular foot which takes different forms in each class of mollusc. They have a mantle, which in some molluscs makes a shell and in others just covers the visceral mass, And finally, all molluscs except bivalves have a radula, or a rasping organ on their mouths that they use to scrape up food. So, chitons, are these headless marine animals, covered with a plated shell on one side, and they use their foot to move around on rocks, scraping off algae with their radula. You know about bivalves. They have shells that are divided into two hinged halves, like clams and scallops. They're filter feeders, so they trap particles of food in the mucus that covers their gills. Snails and slugs are gastropods. One thing that sets them apart is a process called torsion, in which the visceral mass twists to the side during embryonic development, so that by the end of it, its anus is basically right above it's head. Most gastropods also have a single, spiraled shell and most use their radula to graze on algae and plants. And last, but certainly not least, we have the cephalopods, which are the kings of the Molluscs, as far as I'm concerned. Cephalopods include octopi and squid, and they are obviously a lot different from other molluscs. For starters, they have tentacles that they use to grab their prey, which they then bite with their beaks and immobilize with poisonous saliva. And the foot of a cephalopod has been modified into a really powerful muscle that shoots out water to help it move and steer through the water. But probably the coolest thing about cephalopods is how smart they are. While a typical mollusk might have 20,000 neurons, an octopus has half a billion neurons. If you just do a YouTube search for octopus, you'll find all kinds of videos of them opening jars and stealing peoples' video cameras. They're like freaking ocean ninjas. Cephalopods got skillz. So remember, simple doesn't equal dumb, there's a lot to learn from our less-developed cousins. Next time we'll talk about even more complex animals and what we have to learn from them. Until then: Thank you for watching Crash Course Biology, If you want to review anything that we discussed in this video we've put a table of contents over on one of my sides, I can never remember which one it is. I think it's THAT side, yeah THAT side? I'm getting a nod. If you have questions on simple animals, or other topics, you can get in touch with us on Facebook or Twitter, or of course, down in the comments below. Goodbye.



Ancient history to Darwin

Conrad Gesner (1516–1565). His Historiae animalium is considered the beginning of modern zoology.
Conrad Gesner (1516–1565). His Historiae animalium is considered the beginning of modern zoology.

The history of zoology traces the study of the animal kingdom from ancient to modern times. Although the concept of zoology as a single coherent field arose much later, the zoological sciences emerged from natural history reaching back to the biological works of Aristotle and Galen in the ancient Greco-Roman world. This ancient work was further developed in the Middle Ages by Muslim physicians and scholars such as Albertus Magnus.[2][3][4] During the Renaissance and early modern period, zoological thought was revolutionized in Europe by a renewed interest in empiricism and the discovery of many novel organisms. Prominent in this movement were Vesalius and William Harvey, who used experimentation and careful observation in physiology, and naturalists such as Carl Linnaeus, Jean-Baptiste Lamarck, and Buffon who began to classify the diversity of life and the fossil record, as well as the development and behavior of organisms. Microscopy revealed the previously unknown world of microorganisms, laying the groundwork for cell theory.[5] The growing importance of natural theology, partly a response to the rise of mechanical philosophy, encouraged the growth of natural history (although it entrenched the argument from design).

Over the 18th, 19th, and 20th centuries, zoology became an increasingly professional scientific discipline. Explorer-naturalists such as Alexander von Humboldt investigated the interaction between organisms and their environment, and the ways this relationship depends on geography, laying the foundations for biogeography, ecology and ethology. Naturalists began to reject essentialism and consider the importance of extinction and the mutability of species. Cell theory provided a new perspective on the fundamental basis of life.[6][7]


These developments, as well as the results from embryology and paleontology, were synthesized in Charles Darwin's theory of evolution by natural selection. In 1859, Darwin placed the theory of organic evolution on a new footing, by his discovery of a process by which organic evolution can occur, and provided observational evidence that it had done so.[8]

Darwin gave a new direction to morphology and physiology, by uniting them in a common biological theory: the theory of organic evolution. The result was a reconstruction of the classification of animals upon a genealogical basis, fresh investigation of the development of animals, and early attempts to determine their genetic relationships. The end of the 19th century saw the fall of spontaneous generation and the rise of the germ theory of disease, though the mechanism of inheritance remained a mystery. In the early 20th century, the rediscovery of Mendel's work led to the rapid development of genetics, and by the 1930s the combination of population genetics and natural selection in the modern synthesis created evolutionary biology.[9]



Cell biology studies the structural and physiological properties of cells, including their behavior, interactions, and environment. This is done on both the microscopic and molecular levels, for single-celled organisms such as bacteria as well as the specialized cells in multicellular organisms such as humans. Understanding the structure and function of cells is fundamental to all of the biological sciences. The similarities and differences between cell types are particularly relevant to molecular biology.

Anatomy considers the forms of macroscopic structures such as organs and organ systems.[10] It focuses on how organs and organ systems work together in the bodies of humans and animals, in addition to how they work independently. Anatomy and cell biology are two studies that are closely related, and can be categorized under "structural" studies.


Animal anatomical engraving from Handbuch der Anatomie der Tiere für Künstler.
Animal anatomical engraving from Handbuch der Anatomie der Tiere für Künstler.

Physiology studies the mechanical, physical, and biochemical processes of living organisms by attempting to understand how all of the structures function as a whole. The theme of "structure to function" is central to biology. Physiological studies have traditionally been divided into plant physiology and animal physiology, but some principles of physiology are universal, no matter what particular organism is being studied. For example, what is learned about the physiology of yeast cells can also apply to human cells. The field of animal physiology extends the tools and methods of human physiology to non-human species. Physiology studies how for example nervous, immune, endocrine, respiratory, and circulatory systems, function and interact.


Evolutionary research is concerned with the origin and descent of species, as well as their change over time, and includes scientists from many taxonomically oriented disciplines. For example, it generally involves scientists who have special training in particular organisms such as mammalogy, ornithology, herpetology, or entomology, but use those organisms as systems to answer general questions about evolution.

Evolutionary biology is partly based on paleontology, which uses the fossil record to answer questions about the mode and tempo of evolution,[11] and partly on the developments in areas such as population genetics[12] and evolutionary theory. Following the development of DNA fingerprinting techniques in the late 20th century, the application of these techniques in zoology has increased the understanding of animal populations.[13] In the 1980s, developmental biology re-entered evolutionary biology from its initial exclusion from the modern synthesis through the study of evolutionary developmental biology.[14] Related fields often considered part of evolutionary biology are phylogenetics, systematics, and taxonomy.


Scientific classification in zoology, is a method by which zoologists group and categorize organisms by biological type, such as genus or species. Biological classification is a form of scientific taxonomy. Modern biological classification has its root in the work of Carl Linnaeus, who grouped species according to shared physical characteristics. These groupings have since been revised to improve consistency with the Darwinian principle of common descent. Molecular phylogenetics, which uses DNA sequences as data, has driven many recent revisions and is likely to continue to do so. Biological classification belongs to the science of zoological systematics.

Linnaeus's table of the animal kingdom from the first edition of Systema Naturae (1735).
Linnaeus's table of the animal kingdom from the first edition of Systema Naturae (1735).

Many scientists now consider the five-kingdom system outdated. Modern alternative classification systems generally start with the three-domain system: Archaea (originally Archaebacteria); Bacteria (originally Eubacteria); Eukaryota (including protists, fungi, plants, and animals)[15] These domains reflect whether the cells have nuclei or not, as well as differences in the chemical composition of the cell exteriors.[15]

Further, each kingdom is broken down recursively until each species is separately classified. The order is: Domain; kingdom; phylum; class; order; family; genus; species. The scientific name of an organism is generated from its genus and species. For example, humans are listed as Homo sapiens. Homo is the genus, and sapiens the specific epithet, both of them combined make up the species name. When writing the scientific name of an organism, it is proper to capitalize the first letter in the genus and put all of the specific epithet in lowercase. Additionally, the entire term may be italicized or underlined.[16]

The dominant classification system is called the Linnaean taxonomy. It includes ranks and binomial nomenclature. The classification, taxonomy, and nomenclature of zoological organisms is administered by the International Code of Zoological Nomenclature. A merging draft, BioCode, was published in 1997 in an attempt to standardize nomenclature, but has yet to be formally adopted.[17]


Kelp gull chicks peck at red spot on mother's beak to stimulate the regurgitating reflex.
Kelp gull chicks peck at red spot on mother's beak to stimulate the regurgitating reflex.

Ethology is the scientific and objective study of animal behavior under natural conditions,[18] as opposed to behaviourism, which focuses on behavioral response studies in a laboratory setting. Ethologists have been particularly concerned with the evolution of behavior and the understanding of behavior in terms of the theory of natural selection. In one sense, the first modern ethologist was Charles Darwin, whose book, The Expression of the Emotions in Man and Animals, influenced many future ethologists.[19]


Biogeography studies the spatial distribution of organisms on the Earth,[20] focusing on topics like plate tectonics, climate change, dispersal and migration, and cladistics. The creation of this study is widely accredited to Alfred Russel Wallace, a British biologist who had some of his work jointly published with Charles Darwin.

Branches of zoology

Although the study of animal life is ancient, its scientific incarnation is relatively modern. This mirrors the transition from natural history to biology at the start of the 19th century. Since Hunter and Cuvier, comparative anatomical study has been associated with morphography, shaping the modern areas of zoological investigation: anatomy, physiology, histology, embryology, teratology and ethology.[21] Modern zoology first arose in German and British universities. In Britain, Thomas Henry Huxley was a prominent figure. His ideas were centered on the morphology of animals. Many consider him the greatest comparative anatomist of the latter half of the 19th century. Similar to Hunter, his courses were composed of lectures and laboratory practical classes in contrast to the previous format of lectures only.

Gradually zoology expanded beyond Huxley's comparative anatomy to include the following sub-disciplines:

Related fields:

See also


  1. ^ "zoology". Online Etymology Dictionary.
  2. ^ Bayrakdar, Mehmet (1986). "Al-Jahiz and the rise of biological evolution" (PDF). Ankara Üniversitesi İlahiyat Fakültesi Dergisi. 27 (1): 307–315. doi:10.1501/Ilhfak_0000000674. Retrieved 21 December 2012. Text "Ankara Üniversitesi İlahiyat Fakültesi" ignored (help)
  3. ^ Paul S. Agutter & Denys N. Wheatley (2008). Thinking about Life: The History and Philosophy of Biology and Other Sciences. Springer. p. 43. ISBN 1-4020-8865-5.
  4. ^ Saint Albertus Magnus (1999). On Animals: A Medieval Summa Zoologica. Johns Hopkins University Press. ISBN 0-8018-4823-7.
  5. ^ Lois N. Magner (2002). A History of the Life Sciences, Revised and Expanded. CRC Press. pp. 133–144. ISBN 0-8247-0824-5.
  6. ^ Jan Sapp (2003). "Chapter 7". Genesis: The Evolution of Biology. Oxford University Press. ISBN 0-19-515619-6.
  7. ^ William Coleman (1978). "Chapter 2". Biology in the Nineteenth Century. Cambridge University Press. ISBN 0-521-29293-X.
  8. ^ Jerry A. Coyne (2009). Why Evolution is True. Oxford: Oxford University Press. p. 17. ISBN 0-19-923084-6.
  9. ^ "Appendix: Frequently Asked Questions". Science and Creationism: a view from the National Academy of Sciences (php) (Second ed.). Washington, DC: The National Academy of Sciences. 1999. p. 28. ISBN -0-309-06406-6. Retrieved September 24, 2009.
  10. ^ Henry Gray (1918). Anatomy of the Human Body. Lea & Febiger.
  11. ^ Jablonski D (1999). "The future of the fossil record". Science. 284 (5423): 2114–2116. doi:10.1126/science.284.5423.2114. PMID 10381868.
  12. ^ John H. Gillespie (1998). Population Genetics: A Concise Guide. Johns Hopkins Press. ISBN 978-0-8018-8008-7.
  13. ^ Chambers, Geoffrey K.; Curtis, Caitlin; Millar, Craig D.; Huynen, Leon; Lambert, David M. (2014-01-01). "DNA fingerprinting in zoology: past, present, future". Investigative Genetics. 5 (1): 3. doi:10.1186/2041-2223-5-3. ISSN 2041-2223. PMC 3909909. PMID 24490906.
  14. ^ Vassiliki Betty Smocovitis (1996). Unifying Biology: The Evolutionary Synthesis and Evolutionary Biology. Princeton University Press. ISBN 978-0-691-03343-3.
  15. ^ a b Woese C, Kandler O, Wheelis M (1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya". Proc Natl Acad Sci USA. 87 (12): 4576–4579. Bibcode:1990PNAS...87.4576W. doi:10.1073/pnas.87.12.4576. PMC 54159. PMID 2112744.
  16. ^ Heather Silyn-Roberts (2000). Writing for Science and Engineering: Papers, Presentation. Oxford: Butterworth-Heinemann. p. 198. ISBN 0-7506-4636-5.
  17. ^ John McNeill (4 November 1996). "The BioCode: Integrated biological nomenclature for the 21st century?". Proceedings of a Mini-Symposium on Biological Nomenclature in the 21st Century.
  18. ^ "Definition of ETHOLOGY". Merriam-Webster. Retrieved 30 October 2012. 2 : the scientific and objective study of animal behaviour especially under natural conditions
  19. ^ Black, J (Jun 2002). "Darwin in the world of emotions" (Free full text). Journal of the Royal Society of Medicine. 95 (6): 311–313. doi:10.1258/jrsm.95.6.311. ISSN 0141-0768. PMC 1279921. PMID 12042386.
  20. ^ Wiley, R. H. (1981). "Social structure and individual ontogenies: problems of description, mechanism, and evolution" (PDF). Perspectives in ethology. 4: 105–133. Retrieved 21 December 2012.
  21. ^ "zoology". Encyclopedia Britannica. Retrieved 2017-09-13.

External links

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