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Animals (from the Latin animale for "having breath")[note 1] are a diverse group of life on Earth, consisting currently of around two million known species belonging to the biological kingdom Animalia, also known as Metazoa (literally "following animals", see below). Most of these species are insects; many of those are beetles. Animals are all motile at some point during their lives, have many cells (except in the zygotic stage), get their nutrients from other sources and have cells without walls. However, the definition of "animal" has changed over the centuries.

History and definition[edit]

In all likelihood, humans have recognised the basic concept of "animal" as long as we have had the capacity to do so, by circular definition, long before we knew anything about cells. Woolly mammoths move around, eat food, and when killed can be considered meat. This made "animals" (such as edible locusts) distinct from "bugs". This simple, intuitive method would have worked well enough for our hunter-gatherer ancestors, because it was important to recognize what you can eat and what can eat you.Wikipedia When humans begun to take a look at the seas, they saw that things like "shellfish" didn't noticeably move, but were still a form of "meat". "Animals" could be considered as having a "flesh and blood", as reflected in a biblical statement, Genesis 9:2-4:

...the dread of you [Noah and his family] shall be upon every beast of the earth, and upon every fowl of the air, upon all that moveth upon the earth, and upon all the fishes of the sea; into your hand are they delivered. Every moving thing that liveth shall be meat for you.… But flesh with the life thereof, which is the blood thereof, shall ye not eat.
—Genesis 9:2-4, KJV

An early attempt to classify life came in 350 BCE from Aristotle, who put living things into two groups: "plants" and "animals".[1] Aristotle used methods based on superficial characteristics, like habitat, means of movement, and whether or not subjects have red blood.

Aristotle's natural history was mostly forgotten in the Dark Ages, mainly retained by the Church to specify what good Christians can or can't eat during Lent.

Some circles have tended to characterise animals using the word "creature" (implying a sense of things created), which has the advantage of grouping humans with other animals; whereas the term "beast" emphasises the "sub-human" characteristics of large vertebrates.

In the Enlightenment, due to mounting importance of observation-based and evidence-based knowledge, the need for a better system of classification reemerged. John RayWikipedia first coined the term "species" biologically and classified plants in Historia Plantarum (published in 1686), using a system based on similarities and differences on close observation rather than predefined "kinds" or "types". Ray's work would later influence Carl Linnaeus,Wikipedia who published the first edition of his Systema Naturae in 1735, which established "Animalia" and "Vegetablia" as the two "kingdoms" of life.[note 2] Linnaeus initially divided his animal kingdom into six classes:

  • Quadrupedia (all mammals excluding the whales and dolphins, initially classified as fish)
  • Aves
  • Amphibia (including reptiles)
  • Pisces (remaining vertebrates [sensu stricto])[note 3]
  • Insecta (= Arthropoda [sensu praesens])[note 4]
  • Vermes (the rest of the modern animal kingdom not in the above groups, plus amoebae)

The tenth edition of the Linnean Systema, published in 1758, also saw the true establishment of binomial nomenclature still used today to name species. As the decades passed, biologists added the taxonomic levels of family and phylum, split up Vermes into various groups, and made many other such changes. As of c. 1980, zoology divided "animals" into the following subkingdoms:

  • Protozoa ("first animals"), undifferentiated eukaryotes that do not have rigid cell walls or produce filamentous threads. Members have no embryology, so the MyxozoaWikipedia were included in their ranks.
  • Mesozoa ("middle animals", believed to be midway between protozoa and "true animals"), simple many-celled vermiform parasites of cephalopods and others. Members have a very simple embryology.
  • Parazoa ("near/beside animals", believed to be the closest relatives of "true" animals), including the sponges (phylum Porifera) and the Trichoplax adhaerans;Wikipedia these simple animals had many cells and an embryology including a gastrula,Wikipedia but no true differentiated tissues.
  • Metazoa ("following animals", because they followed the parazoa in chronological order) included all modern animals except sponges and trichoplaxes (see Parazoa, above). Now considered equal to the modern Eumetazoa.

Since about 1990, evolutionary relationships have become an integral part of taxonomy. The field of phylogenetics has taken over, which led to the Protozoa and Metazoa being dissolved and split into various groups. The modern kingdom Metazoa includes 1980s Metazoa, Mesozoa, Myxozoa, and Parazoa. The word "protozoan" is still sometimes used as an informal group, or evolutionary "grade".

Modern definition[edit]

As stated in the introduction, the modern Kingdom Metazoa includes permanently (excluding the zygote) many-celled organisms that undergo development (including at least one motile stage) in their life history. All the Metazoa and only the Metazoa produce a structural protein, collagen (this criterion prevents creatures like the Volvox from being animals as Linnaeus classified them). Typically, the "animals" are listed as being heterotrophic (i.e, needing to organic material to survive), although lineages like the beard worms,Wikipedia emerald sea slugs,Wikipedia and possibly the Ediacaran biota (see below) house bacteria that produce their nutrients from within so these lineages do not need to eat other organisms.

Today, animals are classed into roughly 36 phyla,[note 5] although only nine of them contain the majority of living animals.[2] These are:

  • Porifera:Wikipedia This phylum contains sponges. Sponges remain stationary throughout their life and rely on flowing water rather than a complex nervous, digestive, or circulatory system, making them often considered the "simplest" animals alive today.[3]
  • Cnidaria:Wikipedia Derived from the Greek word for "stinging nettle", this phylum contains coral, jellyfish, sea anemones, and other similar animals that are distinguished by their stinging cells. Cnidarians have either the polypWikipedia or medusaWikipedia form, with stinging cells on their tentacles.[4]
  • Arthropoda:Wikipedia By far the most diverse animal phylum, comprising over 80% of known extant animal species, it contains insects, arachnids, crustaceans, millipedes, trilobites, and more. Arthropods have exoskeletons made of chitinWikipedia and grow by molting.[5]
  • Nematoda:Wikipedia This phylum contains roundworms, which are very adaptable animals that can be found almost anywhere on Earth. They are distinguished from other worms by their tubelike digestive system.[6]
  • Annelida:Wikipedia These are the segmented worms, which include earthworms and leeches, among others. Unlike the nematodes, their bodies are made of segments which each contain the same organs.[7]
  • Mollusca: This phylum contains bivalves such as clams, gastropods such as snails, and cephalopods such as squids, alongside a few other groups like chitons. They are distinguished by their mantles, which in most mollusc species produces a hard shell.[8]
  • Platyhelminthes:Wikipedia This phylum contains flatworms, which include our friends the tapeworms and flukes alongside a few non-parasitic ones as well. Unlike the other worms, they don't have an internal body cavity containing their organs.[9]
  • Echinodermata:Wikipedia These include sea stars, sea urchins, sea cucumbers, sea lilies, and more, and are more closely related to vertebrates than any of the previously mentioned phyla. They are distinguished by their radial symmetryWikipedia and skeletons composed of ossicles.Wikipedia[10]
  • Chordata: The phylum that includes us, as well as all other vertebrates and a couple of invertebrates like lancelets and sea squirts. All chordates have a notochord,Wikipedia a hollow dorsal nerve cord,Wikipedia pharyngeal slits,Wikipedia a post-anal tail, and an endostyleWikipedia or thyroid.Wikipedia The main group is vertebrates, which contains mammals, birds, reptiles, amphibians, and fish.[11]

A few smaller, but nonetheless important phyla include:

  • Bryozoa:Wikipedia These are tiny filter feeders called zooids which build large colonies of thousands, leaving behind a skeleton known as a zooecia. They are common fossils and are still decently widespread today, even if they are frequently overlooked.[12]
  • Brachiopoda:Wikipedia This contains brachiopods, which look superficially similar to clams but differ greatly in terms of their internal structure. They were widespread throughout the Paleozoic, but are uncommon today.[13]
  • Hemichordata:Wikipedia Containing acorn worms, pterobranchs, and a few others, this phylum was thought to be the most closely related to chordates but genetic analysis revealed that these are more closely related to echinoderms. Their bodies have three parts, a preoral lobe, a collar, and a trunk, and they have a few structures reminiscent of chordates, such as a stomochord,Wikipedia which resembles a notochord.[14]

At law[edit]

Animals are chattel property. Wild animals freely roaming are considered the common property of all, and those who manage to capture or kill one acquire it as their property.[15] Given animals' status as items of chattel property, the human owners of animals are entitled to kill them, for food, sport, or any reasons; these rights, however, are subject to the police power of the state, which may restrict them in order to guard public health, welfare, or morality.[16]


There are two leading opinions regarding the origins of animals: the explanation supported by hundreds of thousands of peer-reviewed papers by thousands of scientists over 150+ years from all walks of life, gathered and refined through the most rigourous methods possible and the bullshit explanation.


The kingdom Metazoa's oldest fossil record appears in the Ediacaran period, 630 million to 542 million years ago. As indicated by genetic and fossil records, the first animals were invertebrates not unlike sponges, jellyfish, and sea pens. The very first animals with a distinct head and tail appeared 600 million years ago.Wikipedia Over a period of time lasting up to 80 million years, these bilaterians diversified into various body plans and gained hard parts like armour, shells, and bone. This important evolutionary process has been named the Cambrian explosion because initially, before soft-bodied fossils were discovered, it appeared several major groups of animals appeared to appear from nothing. Appearances are deceiving, however. Animals have since diversified from then on, eventually branching out into the groups we know today.


Some people believe with no empirical evidence to support their views that some old psychopath poofed every living thing into existence over a period of six days. Animals are said to have been created over the fifth and sixth days of this week. Humans are also believed to be distinct from animals because humans have the ability to be "born again" and accept God into their hearts. These claims are with absolutely no supporting evidence, and have massive refuting evidence, although they somehow gain quite a following.

See also[edit]


  1. See Online Etymology Dictionary discussion. The Latin root-word anima can mean "breath" or "soul", hence some scope for theological confusion over whether non-human animals can have souls and whether humans alone have divinely-imparted souls/breath. Compare concepts like inspirationWikipedia — literally "breathing in" or "spirit".
  2. Linnaeus also included a "mineral kingdom" to classify the inorganic natural world, but science has since abandoned it.
  3. "Vertebrata" is considered distinct from "Craniata", as supported by some (not all) genetic and morphological analyses. (The other hypothesis is that lampreys and hagfish are closely related, forming one group, Cyclostomata. This is supported by other, mostly genetic, analyses. You can argue either way with fair levels of support.) The hagfish are included in the latter but not the former. When Linnaeus added the hagfish, he included them in "Vermes".
  4. Latin, "in the present sense".
  5. As there are many debates as to whether certain extinct creatures belong to their own phyla, the exact number depends on who you ask.


  1. Read the book in English online
  2. Animal Phyla, Fossil Museum
  3. What is a sponge?, National Ocean and Atmospheric Administration
  4. Introduction to Cnidaria, Hydrozoan Society
  5. Phylum Arthropoda, Exploring our Fluid Earth
  6. Introduction to the Nematoda, University of California Berkeley
  7. Annelida - Segmented Worms, PBS
  8. Phylum Mollusca, Exploring our Fluid Earth
  9. Introduction to the Platyhelminthes, University of California Berkeley
  10. Echinoderms, Danielle Hall Smithsonian Ocean
  11. Phylum Chordata, Exploring our Fluid Earth
  12. Bryozoans (Moss Animals), Missouri Department of Conservation
  13. Brachiopods, British Geological Survey
  14. Introduction to the Hemichordata, University of California Museum of Paleontology
  15. Bostock-Ferari Amusement Co. v. Brocksmith, 34 Ind. App. 566, 73 N.E. 281 (1905)
  16. Youngstown v. Traylor, 123 Ohio St. 3d 132, 914 N.E.2d 1026 (2009)