ANIMAL

<h1 class="firstHeading" id="firstHeading" lang="en"> Animal</h1> <div id="siteSub"> From Wikipedia, the free encyclopedia</div> <div class="hatnote"> For other uses, see <a class="mw-disambig" href="http://en.wikipedia.org/wiki/Animal_%28disambiguation%29" title="Animal (disambiguation)">Animal (disambiguation)</a>.</div> <div class="hatnote"> "Animalia" redirects here. For other uses, see <a class="mw-disambig" href="http://en.wikipedia.org/wiki/Animalia_%28disambiguation%29" title="Animalia (disambiguation)">Animalia (disambiguation)</a>.</div> <div> <table class="infobox biota" style="font-size: 100%; text-align: left; width: 200px;"> <tbody> <tr> <th colspan="2" style="background-color: #d3d3a4; text-align: center;">Animals<br /> <small>Temporal range: <a href="http://en.wikipedia.org/wiki/Cryogenian" title="Cryogenian">Cryogenian</a> – Recent <span style="display: inline-block;">670–0Ma</span></small> <div id="Timeline-row" style="border-style: solid none; border: 1px #666; clear: both; font-size: small; height: 18px; margin: 4px auto 0; overflow: visible; padding: 0px; position: relative; width: 250px; z-index: 0;"> <div style="background-color: #cb0381; height: 6px; left: 0px; position: absolute; text-align: center; width: 32.608695652174px;"> <small><a href="http://en.wikipedia.org/wiki/Hadean" title="Hadean">Had'n</a></small></div> <div style="background-color: #fe007c; height: 6px; left: 32.608695652174px; position: absolute; text-align: center; width: 81.521739130435px;"> <small><a href="http://en.wikipedia.org/wiki/Archean" title="Archean">Archean</a></small></div> <div style="background-color: #fe4c68; height: 6px; left: 114.13043478261px; position: absolute; text-align: center; width: 106.46739130435px;"> <small><a href="http://en.wikipedia.org/wiki/Proterozoic" title="Proterozoic">Proterozoic</a></small></div> <div style="background-color: #6fdaed; height: 6px; left: 220.59782608696px; position: absolute; text-align: center; width: 29.402173913043px;"> <small><a href="http://en.wikipedia.org/wiki/Phanerozoic" title="Phanerozoic">Pha.</a></small></div> <div style="background-color: transparent; clear: both; height: 8px; line-height: 0; margin: 0 auto; overflow: visible; padding: 0px; position: relative; top: -4px; width: 250px; z-index: 100;"> </div> </div> </th> </tr> <tr> <td colspan="2" style="text-align: center;"><a class="image" href="http://en.wikipedia.org/wiki/File:Animal_diversity.png"><img alt="Animal diversity.png" data-file-height="518" data-file-width="354" height="366" src="http://upload.wikimedia.org/wikipedia/commons/thumb/1/14/Animal_diversity.png/250px-Animal_diversity.png" width="250" /></a></td> </tr> <tr> <th colspan="2" style="background-color: #d3d3a4; text-align: center;"><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Biological_classification" title="Biological classification">Scientific classification</a></th> </tr> <tr> <td>Domain:</td> <td><span class="domain" style="white-space: nowrap;"><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Eukaryota" title="Eukaryota">Eukaryota</a></span></td> </tr> <tr style="vertical-align: top;"> <td>(unranked)</td> <td><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Unikonta" title="Unikonta">Unikonta</a></td> </tr> <tr style="vertical-align: top;"> <td>(unranked)</td> <td><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Opisthokonta" title="Opisthokonta">Opisthokonta</a></td> </tr> <tr style="vertical-align: top;"> <td>(unranked)</td> <td><a href="http://en.wikipedia.org/wiki/Holozoa" title="Holozoa">Holozoa</a></td> </tr> <tr style="vertical-align: top;"> <td>(unranked)</td> <td><a href="http://en.wikipedia.org/wiki/Filozoa" title="Filozoa">Filozoa</a></td> </tr> <tr> <td>Kingdom:</td> <td><span class="kingdom" style="white-space: nowrap;"><b>Animalia</b></span><br /> <small><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Carolus_Linnaeus" title="Carolus Linnaeus">Linnaeus</a>, <a href="http://en.wikipedia.org/wiki/Systema_Naturae" title="Systema Naturae">1758</a></small></td> </tr> <tr> <th colspan="2" style="background-color: #d3d3a4; text-align: center;"><a href="http://en.wikipedia.org/wiki/Phylum" title="Phylum">Phyla</a></th> </tr> <tr> <td colspan="2" style="text-align: left;"> <ul> <li><b>Subkingdom <a href="http://en.wikipedia.org/wiki/Parazoa" title="Parazoa">Parazoa</a></b> <ul> <li><a href="http://en.wikipedia.org/wiki/Sponge" title="Sponge">Porifera</a></li> <li><a href="http://en.wikipedia.org/wiki/Placozoa" title="Placozoa">Placozoa</a></li> </ul> </li> <li><b>Subkingdom <a href="http://en.wikipedia.org/wiki/Eumetazoa" title="Eumetazoa">Eumetazoa</a></b> <ul> <li><b><a href="http://en.wikipedia.org/wiki/Radiata" title="Radiata">Radiata</a> (unranked)</b> <ul> <li><a href="http://en.wikipedia.org/wiki/Ctenophora" title="Ctenophora">Ctenophora</a></li> <li><a href="http://en.wikipedia.org/wiki/Cnidaria" title="Cnidaria">Cnidaria</a></li> <li><a href="http://en.wikipedia.org/wiki/Trilobozoa" title="Trilobozoa">Trilobozoa</a> <a href="http://en.wikipedia.org/wiki/Extinction" title="Extinction">†</a></li> </ul> </li> <li><b><a href="http://en.wikipedia.org/wiki/Bilateria" title="Bilateria">Bilateria</a> (unranked)</b> <ul> <li><a href="http://en.wikipedia.org/wiki/Acoelomorpha" title="Acoelomorpha">Acoelomorpha</a></li> <li><a href="http://en.wikipedia.org/wiki/Tullimonstrum" title="Tullimonstrum">Tullimonstrum</a> <a href="http://en.wikipedia.org/wiki/Extinction" title="Extinction">†</a></li> <li><a href="http://en.wikipedia.org/wiki/Proarticulata" title="Proarticulata">Proarticulata</a> <a href="http://en.wikipedia.org/wiki/Extinction" title="Extinction">†</a></li> <li><b><a href="http://en.wikipedia.org/wiki/Mesozoa" title="Mesozoa">Mesozoa</a> (unranked)</b> <ul> <li><a href="http://en.wikipedia.org/wiki/Orthonectida" title="Orthonectida">Orthonectida</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Rhombozoa" title="Rhombozoa">Rhombozoa</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Monoblastozoa" title="Monoblastozoa">Monoblastozoa</a></li> </ul> </li> <li><b><a href="http://en.wikipedia.org/wiki/Nephrozoa" title="Nephrozoa">Nephrozoa</a> (unranked)</b> <ul> <li><a href="http://en.wikipedia.org/wiki/Chaetognatha" title="Chaetognatha">Chaetognatha</a></li> <li><b>Superphylum <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Deuterostomia" title="Deuterostomia">Deuterostomia</a></b> <ul> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Chordata" title="Chordata">Chordata</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Hemichordata" title="Hemichordata">Hemichordata</a></li> <li><a href="http://en.wikipedia.org/wiki/Echinoderm" title="Echinoderm">Echinodermata</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Xenoturbellida" title="Xenoturbellida">Xenoturbellida</a></li> <li><a href="http://en.wikipedia.org/wiki/Vetulicolia" title="Vetulicolia">Vetulicolia</a> <a href="http://en.wikipedia.org/wiki/Extinction" title="Extinction">†</a></li> </ul> </li> <li><b><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Protostomia" title="Protostomia">Protostomia</a> (unranked)</b> <ul> <li><b>Superphylum <a href="http://en.wikipedia.org/wiki/Ecdysozoa" title="Ecdysozoa">Ecdysozoa</a></b> <ul> <li><a href="http://en.wikipedia.org/wiki/Kinorhyncha" title="Kinorhyncha">Kinorhyncha</a></li> <li><a href="http://en.wikipedia.org/wiki/Loricifera" title="Loricifera">Loricifera</a></li> <li><a href="http://en.wikipedia.org/wiki/Priapulida" title="Priapulida">Priapulida</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Nematoda" title="Nematoda">Nematoda</a></li> <li><a href="http://en.wikipedia.org/wiki/Nematomorpha" title="Nematomorpha">Nematomorpha</a></li> <li><a href="http://en.wikipedia.org/wiki/Onychophora" title="Onychophora">Onychophora</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Tardigrada" title="Tardigrada">Tardigrada</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Arthropoda" title="Arthropoda">Arthropoda</a></li> </ul> </li> <li><b>Superphylum <a href="http://en.wikipedia.org/wiki/Platyzoa" title="Platyzoa">Platyzoa</a></b> <ul> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Platyhelminthes" title="Platyhelminthes">Platyhelminthes</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Gastrotricha" title="Gastrotricha">Gastrotricha</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Rotifera" title="Rotifera">Rotifera</a></li> <li><a href="http://en.wikipedia.org/wiki/Acanthocephala" title="Acanthocephala">Acanthocephala</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Gnathostomulida" title="Gnathostomulida">Gnathostomulida</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Micrognathozoa" title="Micrognathozoa">Micrognathozoa</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Cycliophora" title="Cycliophora">Cycliophora</a></li> </ul> </li> <li><b>Superphylum <a href="http://en.wikipedia.org/wiki/Lophotrochozoa" title="Lophotrochozoa">Lophotrochozoa</a></b> <ul> <li><a href="http://en.wikipedia.org/wiki/Sipuncula" title="Sipuncula">Sipuncula</a></li> <li><a href="http://en.wikipedia.org/wiki/Hyolitha" title="Hyolitha">Hyolitha</a> <a href="http://en.wikipedia.org/wiki/Extinction" title="Extinction">†</a></li> <li><a href="http://en.wikipedia.org/wiki/Nemertea" title="Nemertea">Nemertea</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Phoronida" title="Phoronida">Phoronida</a></li> <li><a href="http://en.wikipedia.org/wiki/Bryozoa" title="Bryozoa">Bryozoa</a></li> <li><a href="http://en.wikipedia.org/wiki/Entoprocta" title="Entoprocta">Entoprocta</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Brachiopoda" title="Brachiopoda">Brachiopoda</a></li> <li><a href="http://en.wikipedia.org/wiki/Mollusca" title="Mollusca">Mollusca</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Annelida" title="Annelida">Annelida</a></li> </ul> </li> </ul> </li> </ul> </li> </ul> </li> </ul> </li> </ul> </td> </tr> <tr> <th colspan="2" style="background-color: #d3d3a4; text-align: center;"><a href="http://en.wikipedia.org/wiki/Synonym_%28taxonomy%29" title="Synonym (taxonomy)">Synonyms</a></th> </tr> <tr> <td colspan="2" style="text-align: left;"> <ul> <li>Metazoa <small><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Haeckel" title="Haeckel">Haeckel</a>, 1874</small></li> </ul> </td> </tr> </tbody></table> </div> <b>Animals</b> are <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Multicellular" title="Multicellular">multicellular</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Eukaryotic" title="Eukaryotic">eukaryotic</a> <a href="http://en.wikipedia.org/wiki/Organism" title="Organism">organisms</a> of the <a href="http://en.wikipedia.org/wiki/Kingdom_%28biology%29" title="Kingdom (biology)">kingdom</a> <b>Animalia</b> (also called <b>Metazoa</b>). Their <a href="http://en.wikipedia.org/wiki/Body_plan" title="Body plan">body plan</a> eventually becomes fixed as they <a href="http://en.wikipedia.org/wiki/Developmental_biology" title="Developmental biology">develop</a>, although some undergo a process of <a href="http://en.wikipedia.org/wiki/Metamorphosis" title="Metamorphosis">metamorphosis</a> later on in their <a href="http://en.wikipedia.org/wiki/Life" title="Life">lives</a>. All animals are <a href="http://en.wikipedia.org/wiki/Motility" title="Motility">motile</a>, meaning they can move spontaneously and independently, at some point in their lives. All animals are <a href="http://en.wikipedia.org/wiki/Heterotroph" title="Heterotroph">heterotrophs</a>: they must ingest other organisms or their products for <a href="http://en.wikipedia.org/wiki/Sustenance" title="Sustenance">sustenance</a>.<br /> Most known animal <a href="http://en.wikipedia.org/wiki/Phylum" title="Phylum">phyla</a> appeared in the fossil record as marine species during the <a href="http://en.wikipedia.org/wiki/Cambrian_explosion" title="Cambrian explosion">Cambrian explosion</a>, about 542 million years ago. Animals are divided into various sub-groups, some of which are: <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Vertebrates" title="Vertebrates">vertebrates</a> (<a class="mw-redirect" href="http://en.wikipedia.org/wiki/Birds" title="Birds">birds</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Mammals" title="Mammals">mammals</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Amphibians" title="Amphibians">amphibians</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Reptiles" title="Reptiles">reptiles</a>, <a href="http://en.wikipedia.org/wiki/Fish" title="Fish">fish</a>); <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Molluscs" title="Molluscs">molluscs</a> (<a class="mw-redirect" href="http://en.wikipedia.org/wiki/Clams" title="Clams">clams</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Oysters" title="Oysters">oysters</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Octopuses" title="Octopuses">octopuses</a>, <a href="http://en.wikipedia.org/wiki/Squid" title="Squid">squid</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Snails" title="Snails">snails</a>); <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Arthropods" title="Arthropods">arthropods</a> (<a class="mw-redirect" href="http://en.wikipedia.org/wiki/Millipedes" title="Millipedes">millipedes</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Centipedes" title="Centipedes">centipedes</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Insects" title="Insects">insects</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Spiders" title="Spiders">spiders</a>, <a href="http://en.wikipedia.org/wiki/Scorpion" title="Scorpion">scorpions</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Crabs" title="Crabs">crabs</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Lobsters" title="Lobsters">lobsters</a>, <a href="http://en.wikipedia.org/wiki/Shrimp" title="Shrimp">shrimp</a>); <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Annelids" title="Annelids">annelids</a> (<a class="mw-redirect" href="http://en.wikipedia.org/wiki/Earthworms" title="Earthworms">earthworms</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Leeches" title="Leeches">leeches</a>); <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Sponges" title="Sponges">sponges</a>; and <a href="http://en.wikipedia.org/wiki/Jellyfish" title="Jellyfish">jellyfish</a>.<br /> <div class="toc" id="toc"> <div id="toctitle"> <h2> Contents</h2> </div> <ul> <li class="toclevel-1 tocsection-1"><a href="http://en.wikipedia.org/wiki/Animal#Etymology"><span class="tocnumber">1</span> <span class="toctext">Etymology</span></a></li> <li class="toclevel-1 tocsection-2"><a href="http://en.wikipedia.org/wiki/Animal#Characteristics"><span class="tocnumber">2</span> <span class="toctext">Characteristics</span></a> <ul> <li class="toclevel-2 tocsection-3"><a href="http://en.wikipedia.org/wiki/Animal#Structure"><span class="tocnumber">2.1</span> <span class="toctext">Structure</span></a></li> <li class="toclevel-2 tocsection-4"><a href="http://en.wikipedia.org/wiki/Animal#Reproduction_and_development"><span class="tocnumber">2.2</span> <span class="toctext">Reproduction and development</span></a></li> <li class="toclevel-2 tocsection-5"><a href="http://en.wikipedia.org/wiki/Animal#Food_and_energy_sourcing"><span class="tocnumber">2.3</span> <span class="toctext">Food and energy sourcing</span></a></li> </ul> </li> <li class="toclevel-1 tocsection-6"><a href="http://en.wikipedia.org/wiki/Animal#Origin_and_fossil_record"><span class="tocnumber">3</span> <span class="toctext">Origin and fossil record</span></a></li> <li class="toclevel-1 tocsection-7"><a href="http://en.wikipedia.org/wiki/Animal#Groups_of_animals"><span class="tocnumber">4</span> <span class="toctext">Groups of animals</span></a> <ul> <li class="toclevel-2 tocsection-8"><a href="http://en.wikipedia.org/wiki/Animal#Ctenophora.2C_Porifera.2C_Placozoa.2C_Cnidaria_and_Bilateria"><span class="tocnumber">4.1</span> <span class="toctext">Ctenophora, Porifera, Placozoa, Cnidaria and Bilateria</span></a></li> <li class="toclevel-2 tocsection-9"><a href="http://en.wikipedia.org/wiki/Animal#Deuterostomes"><span class="tocnumber">4.2</span> <span class="toctext">Deuterostomes</span></a></li> <li class="toclevel-2 tocsection-10"><a href="http://en.wikipedia.org/wiki/Animal#Ecdysozoa"><span class="tocnumber">4.3</span> <span class="toctext">Ecdysozoa</span></a></li> <li class="toclevel-2 tocsection-11"><a href="http://en.wikipedia.org/wiki/Animal#Platyzoa"><span class="tocnumber">4.4</span> <span class="toctext">Platyzoa</span></a></li> <li class="toclevel-2 tocsection-12"><a href="http://en.wikipedia.org/wiki/Animal#Lophotrochozoa"><span class="tocnumber">4.5</span> <span class="toctext">Lophotrochozoa</span></a></li> </ul> </li> <li class="toclevel-1 tocsection-13"><a href="http://en.wikipedia.org/wiki/Animal#Model_organisms"><span class="tocnumber">5</span> <span class="toctext">Model organisms</span></a></li> <li class="toclevel-1 tocsection-14"><a href="http://en.wikipedia.org/wiki/Animal#History_of_classification"><span class="tocnumber">6</span> <span class="toctext">History of classification</span></a></li> <li class="toclevel-1 tocsection-15"><a href="http://en.wikipedia.org/wiki/Animal#See_also"><span class="tocnumber">7</span> <span class="toctext">See also</span></a></li> <li class="toclevel-1 tocsection-16"><a href="http://en.wikipedia.org/wiki/Animal#References"><span class="tocnumber">8</span> <span class="toctext">References</span></a></li> <li class="toclevel-1 tocsection-17"><a href="http://en.wikipedia.org/wiki/Animal#Bibliography"><span class="tocnumber">9</span> <span class="toctext">Bibliography</span></a></li> <li class="toclevel-1 tocsection-18"><a href="http://en.wikipedia.org/wiki/Animal#External_links"><span class="tocnumber">10</span> <span class="toctext">External links</span></a></li> </ul> </div> <h2> <span class="mw-headline" id="Etymology">Etymology</span></h2> The word "animal" comes from the <a href="http://en.wikipedia.org/wiki/Latin" title="Latin">Latin</a> word <i>animalis</i>, meaning "having <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Breath" title="Breath">breath</a>".<sup class="reference" id="cite_ref-1"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-1"><span>[</span>1<span>]</span></a></sup> In everyday non-scientific usage the word excludes humans – that is, "animal" is often used to refer only to non-human members of the kingdom Animalia; often, only closer relatives of humans such as <a href="http://en.wikipedia.org/wiki/Mammal" title="Mammal">mammals</a>, or mammals and other <a href="http://en.wikipedia.org/wiki/Vertebrate" title="Vertebrate">vertebrates</a>, are meant.<sup class="reference" id="cite_ref-2"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-2"><span>[</span>2<span>]</span></a></sup> The biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as <a href="http://en.wikipedia.org/wiki/Sponge" title="Sponge">sponges</a>, <a href="http://en.wikipedia.org/wiki/Jellyfish" title="Jellyfish">jellyfish</a>, <a href="http://en.wikipedia.org/wiki/Insect" title="Insect">insects</a>, and <a href="http://en.wikipedia.org/wiki/Human" title="Human">humans</a>.<sup class="reference" id="cite_ref-americanheritage_animal_3-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-americanheritage_animal-3"><span>[</span>3<span>]</span></a></sup><br /> <h2> <span class="mw-headline" id="Characteristics">Characteristics</span></h2> Animals have several characteristics that set them apart from other living things. Animals are <a href="http://en.wikipedia.org/wiki/Eukaryote" title="Eukaryote">eukaryotic</a> and <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Multicellular" title="Multicellular">multicellular</a>,<sup class="reference" id="cite_ref-NationalZoo_4-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-NationalZoo-4"><span>[</span>4<span>]</span></a></sup> which separates them from <a href="http://en.wikipedia.org/wiki/Bacteria" title="Bacteria">bacteria</a> and most <a href="http://en.wikipedia.org/wiki/Protist" title="Protist">protists</a>. They are <a href="http://en.wikipedia.org/wiki/Heterotroph" title="Heterotroph">heterotrophic</a>,<sup class="reference" id="cite_ref-Windows_5-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-Windows-5"><span>[</span>5<span>]</span></a></sup> generally digesting food in an internal chamber, which separates them from plants and <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Alga" title="Alga">algae</a>.<sup class="reference" id="cite_ref-6"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-6"><span>[</span>6<span>]</span></a></sup> They are also distinguished from plants, algae, and <a href="http://en.wikipedia.org/wiki/Fungus" title="Fungus">fungi</a> by lacking rigid <a href="http://en.wikipedia.org/wiki/Cell_wall" title="Cell wall">cell walls</a>.<sup class="reference" id="cite_ref-AnimalCells_7-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-AnimalCells-7"><span>[</span>7<span>]</span></a></sup> All animals are <a href="http://en.wikipedia.org/wiki/Motility" title="Motility">motile</a>,<sup class="reference" id="cite_ref-Concepts_8-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-Concepts-8"><span>[</span>8<span>]</span></a></sup> if only at certain life stages. In most animals, <a href="http://en.wikipedia.org/wiki/Embryo" title="Embryo">embryos</a> pass through a <a href="http://en.wikipedia.org/wiki/Blastula" title="Blastula">blastula stage</a>,<sup class="reference" id="cite_ref-9"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-9"><span>[</span>9<span>]</span></a></sup> which is a characteristic exclusive to animals.<br /> <h3> <span class="mw-headline" id="Structure">Structure</span></h3> With a few exceptions, most notably the <a href="http://en.wikipedia.org/wiki/Sponge" title="Sponge">sponges</a> (Phylum <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Porifera" title="Porifera">Porifera</a>) and <a href="http://en.wikipedia.org/wiki/Placozoa" title="Placozoa">Placozoa</a>, animals have <a href="http://en.wikipedia.org/wiki/Anatomy" title="Anatomy">bodies</a> differentiated into separate <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Biological_tissue" title="Biological tissue">tissues</a>. These include <a href="http://en.wikipedia.org/wiki/Muscle" title="Muscle">muscles</a>, which are able to contract and control locomotion, and <a href="http://en.wikipedia.org/wiki/Nervous_system" title="Nervous system">nerve tissues</a>, which send and process signals. Typically, there is also an internal <a href="http://en.wikipedia.org/wiki/Digestion" title="Digestion">digestive</a> chamber, with one or two openings.<sup class="reference" id="cite_ref-scienceperspectives10_10-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-scienceperspectives10-10"><span>[</span>10<span>]</span></a></sup> Animals with this sort of organization are called metazoans, or <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Eumetazoan" title="Eumetazoan">eumetazoans</a> when the former is used for animals in general.<sup class="reference" id="cite_ref-11"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-11"><span>[</span>11<span>]</span></a></sup><br /> All animals have <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Eukaryotic" title="Eukaryotic">eukaryotic</a> cells, surrounded by a characteristic <a href="http://en.wikipedia.org/wiki/Extracellular_matrix" title="Extracellular matrix">extracellular matrix</a> composed of <a href="http://en.wikipedia.org/wiki/Collagen" title="Collagen">collagen</a> and elastic <a href="http://en.wikipedia.org/wiki/Glycoprotein" title="Glycoprotein">glycoproteins</a>.<sup class="reference" id="cite_ref-12"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-12"><span>[</span>12<span>]</span></a></sup> This may be calcified to form structures like <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Animal_shell" title="Animal shell">shells</a>, <a href="http://en.wikipedia.org/wiki/Bone" title="Bone">bones</a>, and <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Spicule_%28sponge%29" title="Spicule (sponge)">spicules</a>.<sup class="reference" id="cite_ref-13"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-13"><span>[</span>13<span>]</span></a></sup> During development, it forms a relatively flexible framework<sup class="reference" id="cite_ref-14"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-14"><span>[</span>14<span>]</span></a></sup> upon which cells can move about and be reorganized, making complex structures possible. In contrast, other <a href="http://en.wikipedia.org/wiki/Multicellular_organism" title="Multicellular organism">multicellular organisms</a>, like plants and fungi, have cells held in place by cell walls, and so develop by progressive growth.<sup class="reference" id="cite_ref-scienceperspectives10_10-1"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-scienceperspectives10-10"><span>[</span>10<span>]</span></a></sup> Also, unique to animal cells are the following intercellular junctions: <a href="http://en.wikipedia.org/wiki/Tight_junction" title="Tight junction">tight junctions</a>, <a href="http://en.wikipedia.org/wiki/Gap_junction" title="Gap junction">gap junctions</a>, and <a href="http://en.wikipedia.org/wiki/Desmosome" title="Desmosome">desmosomes</a>.<sup class="reference" id="cite_ref-15"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-15"><span>[</span>15<span>]</span></a></sup><br /> <h3> <span class="mw-headline" id="Reproduction_and_development">Reproduction and development</span></h3> <div class="hatnote boilerplate seealso"> See also: <a href="http://en.wikipedia.org/wiki/Sexual_reproduction#Animals" title="Sexual reproduction">Sexual reproduction § Animals</a> and <a href="http://en.wikipedia.org/wiki/Asexual_reproduction#Examples_in_animals" title="Asexual reproduction">Asexual reproduction § Examples in animals</a></div> <div class="thumb tleft"> <div class="thumbinner" style="width: 222px;"> <a class="image" href="http://en.wikipedia.org/wiki/File:Mitosis-fluorescent.jpg"><img alt="" class="thumbimage" data-file-height="264" data-file-width="300" height="194" src="http://upload.wikimedia.org/wikipedia/commons/thumb/5/5b/Mitosis-fluorescent.jpg/220px-Mitosis-fluorescent.jpg" width="220" /></a> <div class="thumbcaption"> A <a href="http://en.wikipedia.org/wiki/Newt" title="Newt">newt</a> <a href="http://en.wikipedia.org/wiki/Lung" title="Lung">lung</a> <a href="http://en.wikipedia.org/wiki/Cell_%28biology%29" title="Cell (biology)">cell</a> <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Staining_%28biology%29" title="Staining (biology)">stained</a> with <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Fluorescent" title="Fluorescent">fluorescent</a> <a href="http://en.wikipedia.org/wiki/Dye" title="Dye">dyes</a> undergoing the early <a href="http://en.wikipedia.org/wiki/Anaphase" title="Anaphase">anaphase</a> stage of <a href="http://en.wikipedia.org/wiki/Mitosis" title="Mitosis">mitosis</a></div> </div> </div> Nearly all animals undergo some form of <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Sexual_reproduction_in_animals" title="Sexual reproduction in animals">sexual reproduction</a>.<sup class="reference" id="cite_ref-16"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-16"><span>[</span>16<span>]</span></a></sup> They have a few specialized <a href="http://en.wikipedia.org/wiki/Gamete" title="Gamete">reproductive cells</a>, which undergo <a href="http://en.wikipedia.org/wiki/Meiosis" title="Meiosis">meiosis</a> to produce smaller, motile <a href="http://en.wikipedia.org/wiki/Spermatozoon" title="Spermatozoon">spermatozoa</a> or larger, non-motile <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Ovum" title="Ovum">ova</a>.<sup class="reference" id="cite_ref-17"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-17"><span>[</span>17<span>]</span></a></sup> These fuse to form <a href="http://en.wikipedia.org/wiki/Zygote" title="Zygote">zygotes</a>, which develop into new individuals.<sup class="reference" id="cite_ref-18"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-18"><span>[</span>18<span>]</span></a></sup><br /> Many animals are also capable of <a href="http://en.wikipedia.org/wiki/Asexual_reproduction" title="Asexual reproduction">asexual reproduction</a>.<sup class="reference" id="cite_ref-19"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-19"><span>[</span>19<span>]</span></a></sup> This may take place through <a href="http://en.wikipedia.org/wiki/Parthenogenesis" title="Parthenogenesis">parthenogenesis</a>, where fertile eggs are produced without mating, budding, or <a href="http://en.wikipedia.org/wiki/Fragmentation_%28reproduction%29" title="Fragmentation (reproduction)">fragmentation</a>.<sup class="reference" id="cite_ref-20"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-20"><span>[</span>20<span>]</span></a></sup><br /> A <a href="http://en.wikipedia.org/wiki/Zygote" title="Zygote">zygote</a> initially develops into a hollow sphere, called a <a href="http://en.wikipedia.org/wiki/Blastula" title="Blastula">blastula</a>,<sup class="reference" id="cite_ref-21"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-21"><span>[</span>21<span>]</span></a></sup> which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge.<sup class="reference" id="cite_ref-22"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-22"><span>[</span>22<span>]</span></a></sup> In most other groups, the blastula undergoes more complicated rearrangement.<sup class="reference" id="cite_ref-23"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-23"><span>[</span>23<span>]</span></a></sup> It first <a href="http://en.wikipedia.org/wiki/Invagination" title="Invagination">invaginates</a> to form a <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Gastrula" title="Gastrula">gastrula</a> with a digestive chamber, and two separate <a href="http://en.wikipedia.org/wiki/Germ_layer" title="Germ layer">germ layers</a> — an external <a href="http://en.wikipedia.org/wiki/Ectoderm" title="Ectoderm">ectoderm</a> and an internal <a href="http://en.wikipedia.org/wiki/Endoderm" title="Endoderm">endoderm</a>.<sup class="reference" id="cite_ref-24"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-24"><span>[</span>24<span>]</span></a></sup> In most cases, a <a href="http://en.wikipedia.org/wiki/Mesoderm" title="Mesoderm">mesoderm</a> also develops between them.<sup class="reference" id="cite_ref-25"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-25"><span>[</span>25<span>]</span></a></sup> These germ layers then differentiate to form tissues and organs.<sup class="reference" id="cite_ref-26"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-26"><span>[</span>26<span>]</span></a></sup><br /> <h3> <span class="mw-headline" id="Food_and_energy_sourcing">Food and energy sourcing</span></h3> <div class="hatnote relarticle mainarticle"> Main article: <a href="http://en.wikipedia.org/wiki/Animal_nutrition" title="Animal nutrition">Animal nutrition</a></div> All animals are <a href="http://en.wikipedia.org/wiki/Heterotroph" title="Heterotroph">heterotrophs</a>, meaning that they feed directly or indirectly on other living things.<sup class="reference" id="cite_ref-27"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-27"><span>[</span>27<span>]</span></a></sup> They are often further subdivided into groups such as <a href="http://en.wikipedia.org/wiki/Carnivore" title="Carnivore">carnivores</a>, <a href="http://en.wikipedia.org/wiki/Herbivore" title="Herbivore">herbivores</a>, <a href="http://en.wikipedia.org/wiki/Omnivore" title="Omnivore">omnivores</a>, and <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Parasite" title="Parasite">parasites</a>.<sup class="reference" id="cite_ref-28"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-28"><span>[</span>28<span>]</span></a></sup><br /> <a href="http://en.wikipedia.org/wiki/Predation" title="Predation">Predation</a> is a <a href="http://en.wikipedia.org/wiki/Biological_interaction" title="Biological interaction">biological interaction</a> where a predator (a heterotroph that is hunting) feeds on its prey (the organism that is attacked).<sup class="reference" id="cite_ref-Ecology_29-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-Ecology-29"><span>[</span>29<span>]</span></a></sup> Predators may or may not kill their prey prior to feeding on them, but the act of predation almost always results in the death of the prey.<sup class="reference" id="cite_ref-30"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-30"><span>[</span>30<span>]</span></a></sup> The other main category of consumption is <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Detritivory" title="Detritivory">detritivory</a>, the consumption of dead <a href="http://en.wikipedia.org/wiki/Organic_matter" title="Organic matter">organic matter</a>.<sup class="reference" id="cite_ref-31"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-31"><span>[</span>31<span>]</span></a></sup> It can at times be difficult to separate the two <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Feeding_behaviour" title="Feeding behaviour">feeding behaviours</a>, for example, where <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Parasite" title="Parasite">parasitic species</a> prey on a host organism and then lay their eggs on it for their offspring to feed on its decaying corpse. Selective pressures imposed on one another has led to an <a href="http://en.wikipedia.org/wiki/Evolutionary_arms_race" title="Evolutionary arms race">evolutionary arms race</a> between prey and predator, resulting in various <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Antipredator_adaptations" title="Antipredator adaptations">antipredator adaptations</a>.<sup class="reference" id="cite_ref-32"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-32"><span>[</span>32<span>]</span></a></sup><br /> Most animals indirectly use the energy of <a href="http://en.wikipedia.org/wiki/Sunlight" title="Sunlight">sunlight</a> by eating plants or plant-eating animals. Most plants use light to convert <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Inorganic" title="Inorganic">inorganic</a> molecules in their environment into <a href="http://en.wikipedia.org/wiki/Carbohydrate" title="Carbohydrate">carbohydrates</a>, <a href="http://en.wikipedia.org/wiki/Fat" title="Fat">fats</a>, <a href="http://en.wikipedia.org/wiki/Protein" title="Protein">proteins</a> and other biomolecules, characteristically containing <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Reduction_%28chemistry%29" title="Reduction (chemistry)">reduced</a> carbon in the form of carbon-hydrogen bonds. Starting with <a href="http://en.wikipedia.org/wiki/Carbon_dioxide" title="Carbon dioxide">carbon dioxide</a> (CO₂) and water (H₂O), photosynthesis converts the energy of sunlight into chemical energy in the form of simple sugars (e.g., <a href="http://en.wikipedia.org/wiki/Glucose" title="Glucose">glucose</a>), with the release of molecular <a href="http://en.wikipedia.org/wiki/Oxygen" title="Oxygen">oxygen</a>. These sugars are then used as the building blocks for plant growth, including the production of other biomolecules.<sup class="reference" id="cite_ref-scienceperspectives10_10-2"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-scienceperspectives10-10"><span>[</span>10<span>]</span></a></sup> When an animal eats plants (or eats other animals which have eaten plants), the reduced carbon compounds in the food become a source of energy and building materials for the animal.<sup class="reference" id="cite_ref-33"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-33"><span>[</span>33<span>]</span></a></sup> They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion.<sup class="reference" id="cite_ref-34"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-34"><span>[</span>34<span>]</span></a></sup><sup class="reference" id="cite_ref-35"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-35"><span>[</span>35<span>]</span></a></sup><br /> Animals living close to <a href="http://en.wikipedia.org/wiki/Hydrothermal_vent" title="Hydrothermal vent">hydrothermal vents</a> and <a href="http://en.wikipedia.org/wiki/Cold_seep" title="Cold seep">cold seeps</a> on the <a href="http://en.wikipedia.org/wiki/Seabed" title="Seabed">ocean floor</a> are not dependent on the energy of sunlight.<sup class="reference" id="cite_ref-36"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-36"><span>[</span>36<span>]</span></a></sup> Instead <a href="http://en.wikipedia.org/wiki/Chemosynthesis" title="Chemosynthesis">chemosynthetic</a> <a href="http://en.wikipedia.org/wiki/Archaea" title="Archaea">archaea</a> and bacteria form the base of the <a href="http://en.wikipedia.org/wiki/Food_chain" title="Food chain">food chain</a>.<sup class="reference" id="cite_ref-37"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-37"><span>[</span>37<span>]</span></a></sup><br /> <h2> <span class="mw-headline" id="Origin_and_fossil_record">Origin and fossil record</span></h2> <div class="hatnote"> Further information: <a href="http://en.wikipedia.org/wiki/Urmetazoan" title="Urmetazoan">Urmetazoan</a></div> <div class="thumb tright"> <div class="thumbinner" style="width: 222px;"> <a class="image" href="http://en.wikipedia.org/wiki/File:Dunkleosteus_BW.jpg"><img alt="" class="thumbimage" data-file-height="539" data-file-width="800" height="148" src="http://upload.wikimedia.org/wikipedia/commons/thumb/9/95/Dunkleosteus_BW.jpg/220px-Dunkleosteus_BW.jpg" width="220" /></a> <div class="thumbcaption"> <i><a href="http://en.wikipedia.org/wiki/Dunkleosteus" title="Dunkleosteus">Dunkleosteus</a></i> was a 10-metre-long (33 ft) <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Prehistoric_fish" title="Prehistoric fish">prehistoric fish</a>.<sup class="reference" id="cite_ref-38"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-38"><span>[</span>38<span>]</span></a></sup></div> </div> </div> Animals are generally considered to have <a href="http://en.wikipedia.org/wiki/Evolution" title="Evolution">evolved</a> from a <a href="http://en.wikipedia.org/wiki/Flagellate" title="Flagellate">flagellated</a> eukaryote.<sup class="reference" id="cite_ref-39"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-39"><span>[</span>39<span>]</span></a></sup> Their closest known living relatives are the <a href="http://en.wikipedia.org/wiki/Choanoflagellate" title="Choanoflagellate">choanoflagellates</a>, collared flagellates that have a morphology similar to the choanocytes of certain sponges.<sup class="reference" id="cite_ref-40"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-40"><span>[</span>40<span>]</span></a></sup> <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Molecular" title="Molecular">Molecular</a> studies place animals in a supergroup called the <a href="http://en.wikipedia.org/wiki/Opisthokont" title="Opisthokont">opisthokonts</a>, which also include the choanoflagellates, <a href="http://en.wikipedia.org/wiki/Fungus" title="Fungus">fungi</a> and a few small parasitic <a href="http://en.wikipedia.org/wiki/Protist" title="Protist">protists</a>.<sup class="reference" id="cite_ref-41"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-41"><span>[</span>41<span>]</span></a></sup> The name comes from the posterior location of the <a href="http://en.wikipedia.org/wiki/Flagellum" title="Flagellum">flagellum</a> in motile cells, such as most animal spermatozoa, whereas other <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Eukaryotes" title="Eukaryotes">eukaryotes</a> tend to have anterior flagella.<sup class="reference" id="cite_ref-42"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-42"><span>[</span>42<span>]</span></a></sup><br /> The first fossils that might represent animals appear in the <a href="http://en.wikipedia.org/wiki/Trezona_Formation" title="Trezona Formation">Trezona Formation</a> at Trezona Bore, West Central Flinders, <a href="http://en.wikipedia.org/wiki/South_Australia" title="South Australia">South Australia</a>.<sup class="reference" id="cite_ref-roseMaloof_43-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-roseMaloof-43"><span>[</span>43<span>]</span></a></sup> These fossils are interpreted as being early sponges. They were found in 665-million-year-old rock.<sup class="reference" id="cite_ref-roseMaloof_43-1"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-roseMaloof-43"><span>[</span>43<span>]</span></a></sup><br /> The next oldest possible animal fossils are found towards the end of the <a href="http://en.wikipedia.org/wiki/Precambrian" title="Precambrian">Precambrian</a>, around 610 million years ago, and are known as the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Ediacaran_biota" title="Ediacaran biota">Ediacaran or Vendian biota</a>.<sup class="reference" id="cite_ref-44"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-44"><span>[</span>44<span>]</span></a></sup> These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all.<sup class="reference" id="cite_ref-45"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-45"><span>[</span>45<span>]</span></a></sup><br /> Aside from them, most known animal phyla make a more or less simultaneous appearance during the <a href="http://en.wikipedia.org/wiki/Cambrian" title="Cambrian">Cambrian</a> period, about 542 million years ago.<sup class="reference" id="cite_ref-46"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-46"><span>[</span>46<span>]</span></a></sup> It is still disputed whether this event, called the <a href="http://en.wikipedia.org/wiki/Cambrian_explosion" title="Cambrian explosion">Cambrian explosion</a>, is due to a rapid divergence between different groups or due to a change in conditions that made fossilization possible.<br /> Some paleontologists suggest that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.<sup class="reference" id="cite_ref-47"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-47"><span>[</span>47<span>]</span></a></sup> <a href="http://en.wikipedia.org/wiki/Trace_fossil" title="Trace fossil">Trace fossils</a> such as tracks and burrows found in the <a href="http://en.wikipedia.org/wiki/Tonian" title="Tonian">Tonian</a> period indicate the presence of <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Triploblastic" title="Triploblastic">triploblastic</a> worms, like metazoans, roughly as large (about 5 mm wide) and complex as <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Earthworms" title="Earthworms">earthworms</a>.<sup class="reference" id="cite_ref-Seilacher1998_48-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-Seilacher1998-48"><span>[</span>48<span>]</span></a></sup> During the beginning of the Tonian period around 1 billion years ago, there was a decrease in <a href="http://en.wikipedia.org/wiki/Stromatolite" title="Stromatolite">Stromatolite</a> diversity, which may indicate the appearance of grazing animals, since stromatolite diversity increased when grazing animals went extinct at the <a href="http://en.wikipedia.org/wiki/Permian%E2%80%93Triassic_extinction_event" title="Permian–Triassic extinction event">End Permian</a> and <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_extinction_event" title="Ordovician–Silurian extinction event">End Ordovician</a> extinction events, and decreased shortly after the grazer populations recovered. However the discovery that tracks very similar to these early trace fossils are produced today by the giant single-celled protist <i><a href="http://en.wikipedia.org/wiki/Gromia_sphaerica" title="Gromia sphaerica">Gromia sphaerica</a></i> casts doubt on their interpretation as evidence of early animal evolution.<sup class="reference" id="cite_ref-Matz2008_49-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-Matz2008-49"><span>[</span>49<span>]</span></a></sup><sup class="reference" id="cite_ref-MSNBC200811_50-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-MSNBC200811-50"><span>[</span>50<span>]</span></a></sup><br /> <h2> <span class="mw-headline" id="Groups_of_animals">Groups of animals</span></h2> <div class="thumb tright"> <div class="thumbinner" style="width: 303px;"> <a class="image" href="http://en.wikipedia.org/wiki/File:AnimalsRelativeNumbers.png"><img alt="" class="thumbimage" data-file-height="750" data-file-width="1031" height="219" src="http://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/AnimalsRelativeNumbers.png/301px-AnimalsRelativeNumbers.png" width="301" /></a> <div class="thumbcaption"> The relative number of species contributed to the total by each phylum of animals.</div> </div> </div> <h3> <span class="mw-headline" id="Ctenophora.2C_Porifera.2C_Placozoa.2C_Cnidaria_and_Bilateria">Ctenophora, Porifera, Placozoa, Cnidaria and Bilateria</span></h3> Phylogenetic analysis suggests that the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Porifera" title="Porifera">Porifera</a> and <a href="http://en.wikipedia.org/wiki/Ctenophora" title="Ctenophora">Ctenophora</a> diverged before a clade (<a href="http://en.wikipedia.org/wiki/ParaHoxozoa" title="ParaHoxozoa">ParaHoxozoa</a>) that gave rise to the <a href="http://en.wikipedia.org/wiki/Bilateria" title="Bilateria">Bilateria</a>, <a href="http://en.wikipedia.org/wiki/Cnidaria" title="Cnidaria">Cnidaria</a> and <a href="http://en.wikipedia.org/wiki/Placozoa" title="Placozoa">Placozoa</a>.<sup class="reference" id="cite_ref-Ryan2010_51-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-Ryan2010-51"><span>[</span>51<span>]</span></a></sup> Another study based on the presence or absence of <a href="http://en.wikipedia.org/wiki/Intron" title="Intron">introns</a> suggests that Cnidaria, Porifera and Placozoa may be a sister group of Bilateria and Ctenophora.<sup class="reference" id="cite_ref-Lehmann2012_52-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-Lehmann2012-52"><span>[</span>52<span>]</span></a></sup> A December, 2013, study,<sup class="reference" id="cite_ref-Ryan2013_53-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-Ryan2013-53"><span>[</span>53<span>]</span></a></sup> and a June, 2014, study<sup class="reference" id="cite_ref-kohn_54-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-kohn-54"><span>[</span>54<span>]</span></a></sup> both concluded, using entirely distinct methodologies, that the <a href="http://en.wikipedia.org/wiki/Cladogram" title="Cladogram">cladogram</a> of animals is:<br /> <table style="border-spacing: 0px; border-width: 0px; font-size: 100%; line-height: 100%;"> <tbody> <tr> <td style="border-bottom: 1px solid black; border-left: 0; border-right: 0; border-top: 0; padding: 0 0.2em; text-align: center; vertical-align: bottom; width: 1.5em;"><span class="nowrap">  </span></td> <td rowspan="2" style="border: 0; padding: 0;"> <table style="border-spacing: 0px; border-width: 0px;"> <tbody> <tr> <td style="border-bottom: 1px solid black; border-left: 0; border-right: 0; border-top: 0; padding: 0 0.2em; text-align: center; vertical-align: bottom; width: 1.5em;"><br /></td> <td rowspan="2" style="border: 0; padding: 0;"> <table style="border-spacing: 0px; border-width: 0px;"> <tbody> <tr> <td style="border-bottom: 1px solid black; border-left: 0; border-right: 0; border-top: 0; padding: 0 0.2em; text-align: center; vertical-align: bottom; width: 1.5em;"><br /></td> <td rowspan="2" style="border: 0; padding: 0;"> <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Choanoflagellata" title="Choanoflagellata">Choanoflagellata</a><br /> </td> </tr> <tr> <td style="border: 0; padding: 0; vertical-align: top;"><br /></td> </tr> </tbody></table> </td> </tr> <tr> <td style="border-bottom: 0; border-left: 1px solid black; border-right: 0; border-top: 0; vertical-align: top;"><br /></td> </tr> <tr> <td style="border-bottom: 1px solid black; border-left: 1px solid black; border-right: 0; border-top: 0; padding: 0 0.2em; text-align: center; vertical-align: bottom;"><span class="nowrap">Animal</span></td> <td rowspan="2" style="border: 0; padding: 0;"> <table style="border-spacing: 0px; border-width: 0px;"> <tbody> <tr> <td style="border-bottom: 1px solid black; border-left: 0; border-right: 0; border-top: 0; padding: 0 0.2em; text-align: center; vertical-align: bottom; width: 1.5em;"><br /></td> <td rowspan="2" style="border: 0; padding: 0;"> <a href="http://en.wikipedia.org/wiki/Ctenophora" title="Ctenophora">Ctenophora</a><br /> </td> </tr> <tr> <td style="border-bottom: 0; border-left: 1px solid black; border-right: 0; border-top: 0; vertical-align: top;"><br /></td> </tr> <tr> <td style="border-bottom: 1px solid black; border-left: 1px solid black; border-right: 0; border-top: 0; padding: 0 0.2em; text-align: center; vertical-align: bottom;"><span class="nowrap">  </span></td> <td rowspan="2" style="border: 0; padding: 0;"> <table style="border-spacing: 0px; border-width: 0px;"> <tbody> <tr> <td style="border-bottom: 1px solid black; border-left: 0; border-right: 0; border-top: 0; padding: 0 0.2em; text-align: center; vertical-align: bottom; width: 1.5em;"><br /></td> <td rowspan="2" style="border: 0; padding: 0;"> <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Porifera" title="Porifera">Porifera</a><br /> </td> </tr> <tr> <td style="border-bottom: 0; border-left: 1px solid black; border-right: 0; border-top: 0; vertical-align: top;"><br /></td> </tr> <tr> <td style="border-bottom: 1px solid black; border-left: 1px solid black; border-right: 0; border-top: 0; padding: 0 0.2em; text-align: center; vertical-align: bottom;"><br /></td> <td rowspan="2" style="border: 0; padding: 0;"> <table style="border-spacing: 0px; border-width: 0px;"> <tbody> <tr> <td style="border-bottom: 1px solid black; border-left: 0; border-right: 0; border-top: 0; padding: 0 0.2em; text-align: center; vertical-align: bottom; width: 1.5em;"><br /></td> <td rowspan="2" style="border: 0; padding: 0;"> <a href="http://en.wikipedia.org/wiki/Placozoa" title="Placozoa">Placozoa</a><br /> </td> </tr> <tr> <td style="border-bottom: 0; border-left: 1px solid black; border-right: 0; border-top: 0; vertical-align: top;"><br /></td> </tr> <tr> <td style="border-bottom: 1px solid black; border-left: 1px solid black; border-right: 0; border-top: 0; padding: 0 0.2em; text-align: center; vertical-align: bottom;"><br /></td> <td rowspan="2" style="border: 0; padding: 0;"> <table style="border-spacing: 0px; border-width: 0px;"> <tbody> <tr> <td style="border-bottom: 1px solid black; border-left: 0; border-right: 0; border-top: 0; padding: 0 0.2em; text-align: center; vertical-align: bottom; width: 1.5em;"><br /></td> <td rowspan="2" style="border: 0; padding: 0;"> <a href="http://en.wikipedia.org/wiki/Cnidaria" title="Cnidaria">Cnidaria</a><br /> </td> </tr> <tr> <td style="border-bottom: 0; border-left: 1px solid black; border-right: 0; border-top: 0; vertical-align: top;"><br /></td> </tr> <tr> <td style="border-bottom: 1px solid black; border-left: 1px solid black; border-right: 0; border-top: 0; padding: 0 0.2em; text-align: center; vertical-align: bottom;"><br /></td> <td rowspan="2" style="border: 0; padding: 0;"> <a href="http://en.wikipedia.org/wiki/Bilateria" title="Bilateria">Bilateria</a><br /> </td> </tr> <tr> <td style="border: 0; padding: 0; vertical-align: top;"><br /></td> </tr> </tbody></table> </td> </tr> <tr> <td style="border: 0; padding: 0; vertical-align: top;"><br /></td> </tr> </tbody></table> </td> </tr> <tr> <td style="border: 0; padding: 0; vertical-align: top;"><br /></td> </tr> </tbody></table> </td> </tr> <tr> <td style="border: 0; padding: 0; vertical-align: top;"><br /></td> </tr> </tbody></table> </td> </tr> <tr> <td style="border: 0; padding: 0; vertical-align: top;"><br /></td> </tr> </tbody></table> </td> </tr> <tr> <td style="border: 0; padding: 0; vertical-align: top;"><br /></td> </tr> </tbody></table> <br /> <div class="thumb tleft"> <div class="thumbinner" style="width: 172px;"> <a class="image" href="http://en.wikipedia.org/wiki/File:Elephant-ear-sponge.jpg"><img alt="" class="thumbimage" data-file-height="1214" data-file-width="835" height="247" src="http://upload.wikimedia.org/wikipedia/commons/thumb/f/f8/Elephant-ear-sponge.jpg/170px-Elephant-ear-sponge.jpg" width="170" /></a> <div class="thumbcaption"> Orange elephant ear sponge, <i><a href="http://en.wikipedia.org/wiki/Agelas_clathrodes" title="Agelas clathrodes">Agelas clathrodes</a></i>, in foreground. Two corals in the background: a <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Sea_fan" title="Sea fan">sea fan</a>, <i><a href="http://en.wikipedia.org/wiki/Iciligorgia_schrammi" title="Iciligorgia schrammi">Iciligorgia schrammi</a></i>, and a sea rod, <i><a href="http://en.wikipedia.org/wiki/Plexaurella_nutans" title="Plexaurella nutans">Plexaurella nutans</a></i>.</div> </div> </div> The sponges (<a class="mw-redirect" href="http://en.wikipedia.org/wiki/Porifera" title="Porifera">Porifera</a>) were long thought to have diverged from other animals early.<sup class="reference" id="cite_ref-55"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-55"><span>[</span>55<span>]</span></a></sup> They lack the complex organization found in most other phyla.<sup class="reference" id="cite_ref-56"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-56"><span>[</span>56<span>]</span></a></sup> Their cells are differentiated, but in most cases not organized into distinct tissues.<sup class="reference" id="cite_ref-57"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-57"><span>[</span>57<span>]</span></a></sup> Sponges typically feed by drawing in water through pores.<sup class="reference" id="cite_ref-58"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-58"><span>[</span>58<span>]</span></a></sup> <a href="http://en.wikipedia.org/wiki/Archaeocyatha" title="Archaeocyatha">Archaeocyatha</a>, which have fused skeletons, may represent sponges or a separate phylum.<sup class="reference" id="cite_ref-59"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-59"><span>[</span>59<span>]</span></a></sup> However, a phylogenomic study in 2008 of 150 genes in 29 animals across 21 phyla revealed that it is the <a href="http://en.wikipedia.org/wiki/Ctenophora" title="Ctenophora">Ctenophora</a> or comb jellies which are the basal lineage of animals, at least among those 21 phyla. The authors speculate that sponges—or at least those lines of sponges they investigated—are not so primitive, but may instead be secondarily simplified.<sup class="reference" id="cite_ref-class_60-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-class-60"><span>[</span>60<span>]</span></a></sup><br /> Among the other phyla, the Ctenophora and the <a href="http://en.wikipedia.org/wiki/Cnidaria" title="Cnidaria">Cnidaria</a>, which includes <a href="http://en.wikipedia.org/wiki/Sea_anemone" title="Sea anemone">sea anemones</a>, <a href="http://en.wikipedia.org/wiki/Coral" title="Coral">corals</a>, and <a href="http://en.wikipedia.org/wiki/Jellyfish" title="Jellyfish">jellyfish</a>, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus.<sup class="reference" id="cite_ref-61"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-61"><span>[</span>61<span>]</span></a></sup> Both have distinct tissues, but they are not organized into <a href="http://en.wikipedia.org/wiki/Organ_%28anatomy%29" title="Organ (anatomy)">organs</a>.<sup class="reference" id="cite_ref-62"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-62"><span>[</span>62<span>]</span></a></sup> There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Diploblastic" title="Diploblastic">diploblastic</a>.<sup class="reference" id="cite_ref-63"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-63"><span>[</span>63<span>]</span></a></sup> The tiny <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Placozoan" title="Placozoan">placozoans</a> are similar, but they do not have a permanent digestive chamber.<br /> A new family of animals, the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Dendrogrammatidae" title="Dendrogrammatidae">Dendrogrammatidae</a>, was discovered in Australian water. Further DNA testing is required, but scientists suspects they could represent a whole new phylum, possible descendants of the <a href="http://en.wikipedia.org/wiki/Ediacara_biota" title="Ediacara biota">Ediacaran fauna</a>.<sup class="reference" id="cite_ref-64"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-64"><span>[</span>64<span>]</span></a></sup><br /> The remaining animals form a <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Monophyletic" title="Monophyletic">monophyletic</a> group called the <a href="http://en.wikipedia.org/wiki/Bilateria" title="Bilateria">Bilateria</a>. For the most part, they are <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Symmetry_%28biology%29" title="Symmetry (biology)">bilaterally symmetric</a>, and often have a specialized head with feeding and sensory organs. The body is <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Triploblastic" title="Triploblastic">triploblastic</a>, i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is also an internal body cavity called a <a href="http://en.wikipedia.org/wiki/Coelom" title="Coelom">coelom</a> or pseudocoelom. There are exceptions to each of these characteristics, however — for instance adult <a href="http://en.wikipedia.org/wiki/Echinoderm" title="Echinoderm">echinoderms</a> are radially symmetric, and certain <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Parasitic_worm" title="Parasitic worm">parasitic worms</a> have extremely simplified body structures.<br /> Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to two major lineages: the <a href="http://en.wikipedia.org/wiki/Deuterostome" title="Deuterostome">deuterostomes</a> and the <a href="http://en.wikipedia.org/wiki/Protostome" title="Protostome">protostomes</a>, the latter of which includes the <a href="http://en.wikipedia.org/wiki/Ecdysozoa" title="Ecdysozoa">Ecdysozoa</a>, <a href="http://en.wikipedia.org/wiki/Platyzoa" title="Platyzoa">Platyzoa</a>, and <a href="http://en.wikipedia.org/wiki/Lophotrochozoa" title="Lophotrochozoa">Lophotrochozoa</a>. In addition, there are a few small groups of bilaterians with relatively similar structure that appear to have diverged before these major groups. These include the <a href="http://en.wikipedia.org/wiki/Acoelomorpha" title="Acoelomorpha">Acoelomorpha</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Rhombozoa" title="Rhombozoa">Rhombozoa</a>, and <a href="http://en.wikipedia.org/wiki/Orthonectida" title="Orthonectida">Orthonectida</a>. The <a href="http://en.wikipedia.org/wiki/Myxozoa" title="Myxozoa">Myxozoa</a>, single-celled parasites that were originally considered Protozoa, are now believed to have developed from the Medusozoa as well.<br /> <h3> <span class="mw-headline" id="Deuterostomes">Deuterostomes</span></h3> <div class="thumb tright"> <div class="thumbinner" style="width: 222px;"> <a class="image" href="http://en.wikipedia.org/wiki/File:Superbfairywrenscropped.jpeg"><img alt="" class="thumbimage" data-file-height="813" data-file-width="1220" height="147" src="http://upload.wikimedia.org/wikipedia/commons/thumb/c/ce/Superbfairywrenscropped.jpeg/220px-Superbfairywrenscropped.jpeg" width="220" /></a> <div class="thumbcaption"> <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Superb_Fairy-wren" title="Superb Fairy-wren">Superb Fairy-wren</a>, <i>Malurus cyaneus</i></div> </div> </div> <a href="http://en.wikipedia.org/wiki/Deuterostome" title="Deuterostome">Deuterostomes</a> differ from the other Bilateria, called <a href="http://en.wikipedia.org/wiki/Protostome" title="Protostome">protostomes</a>, in several ways. In both cases there is a complete digestive tract. However, in protostomes, the first opening of the gut to appear in embryological development (the <a href="http://en.wikipedia.org/wiki/Archenteron" title="Archenteron">archenteron</a>) develops into the mouth, with the anus forming secondarily. In deuterostomes the anus forms first, with the mouth developing secondarily.<sup class="reference" id="cite_ref-65"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-65"><span>[</span>65<span>]</span></a></sup> In most protostomes, cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes, it forms through <a href="http://en.wikipedia.org/wiki/Invagination" title="Invagination">invagination</a> of the endoderm, called enterocoelic pouching.<sup class="reference" id="cite_ref-66"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-66"><span>[</span>66<span>]</span></a></sup> Deuterostome embryos undergo radial <a href="http://en.wikipedia.org/wiki/Cleavage_%28embryo%29" title="Cleavage (embryo)">cleavage</a> during cell division, while protostomes undergo spiral cleavage.<sup class="reference" id="cite_ref-67"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-67"><span>[</span>67<span>]</span></a></sup><br /> All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Echinodermata" title="Echinodermata">Echinodermata</a> and <a href="http://en.wikipedia.org/wiki/Chordate" title="Chordate">Chordata</a>.<sup class="reference" id="cite_ref-68"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-68"><span>[</span>68<span>]</span></a></sup> The former are radially symmetric and exclusively marine, such as <a href="http://en.wikipedia.org/wiki/Starfish" title="Starfish">starfish</a>, <a href="http://en.wikipedia.org/wiki/Sea_urchin" title="Sea urchin">sea urchins</a>, and <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Holothuroidea" title="Holothuroidea">sea cucumbers</a>.<sup class="reference" id="cite_ref-69"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-69"><span>[</span>69<span>]</span></a></sup> The latter are dominated by the <a href="http://en.wikipedia.org/wiki/Vertebrate" title="Vertebrate">vertebrates</a>, animals with backbones.<sup class="reference" id="cite_ref-70"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-70"><span>[</span>70<span>]</span></a></sup> These include <a href="http://en.wikipedia.org/wiki/Fish" title="Fish">fish</a>, <a href="http://en.wikipedia.org/wiki/Amphibian" title="Amphibian">amphibians</a>, <a href="http://en.wikipedia.org/wiki/Reptile" title="Reptile">reptiles</a>, <a href="http://en.wikipedia.org/wiki/Bird" title="Bird">birds</a>, and <a href="http://en.wikipedia.org/wiki/Mammal" title="Mammal">mammals</a>.<sup class="reference" id="cite_ref-71"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-71"><span>[</span>71<span>]</span></a></sup><br /> In addition to these, the deuterostomes also include the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Hemichordata" title="Hemichordata">Hemichordata</a>, or acorn worms.<sup class="reference" id="cite_ref-72"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-72"><span>[</span>72<span>]</span></a></sup> Although they are not especially prominent today, the important fossil <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Graptolite" title="Graptolite">graptolites</a> may belong to this group.<sup class="reference" id="cite_ref-73"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-73"><span>[</span>73<span>]</span></a></sup><br /> The <a href="http://en.wikipedia.org/wiki/Chaetognatha" title="Chaetognatha">Chaetognatha</a> or arrow worms may also be deuterostomes, but more recent studies suggest protostome affinities.<br /> <h3> <span class="mw-headline" id="Ecdysozoa">Ecdysozoa</span></h3> <div class="thumb tright"> <div class="thumbinner" style="width: 222px;"> <a class="image" href="http://en.wikipedia.org/wiki/File:Sympetrum_flaveolum_-_side_%28aka%29.jpg"><img alt="" class="thumbimage" data-file-height="1574" data-file-width="2567" height="135" src="http://upload.wikimedia.org/wikipedia/commons/thumb/0/03/Sympetrum_flaveolum_-_side_%28aka%29.jpg/220px-Sympetrum_flaveolum_-_side_%28aka%29.jpg" width="220" /></a> <div class="thumbcaption"> <a href="http://en.wikipedia.org/wiki/Yellow-winged_darter" title="Yellow-winged darter">Yellow-winged darter</a>, <i>Sympetrum flaveolum</i></div> </div> </div> The <a href="http://en.wikipedia.org/wiki/Ecdysozoa" title="Ecdysozoa">Ecdysozoa</a> are protostomes, named after the common <a href="http://en.wikipedia.org/wiki/Phenotypic_trait" title="Phenotypic trait">trait</a> of growth by <a href="http://en.wikipedia.org/wiki/Moulting" title="Moulting">moulting</a> or <a href="http://en.wikipedia.org/wiki/Ecdysis" title="Ecdysis">ecdysis</a>.<sup class="reference" id="cite_ref-74"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-74"><span>[</span>74<span>]</span></a></sup> The largest animal phylum belongs here, the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Arthropoda" title="Arthropoda">Arthropoda</a>, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the <a href="http://en.wikipedia.org/wiki/Onychophora" title="Onychophora">Onychophora</a> and <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Tardigrada" title="Tardigrada">Tardigrada</a>, are close relatives of the arthropods and share these traits.<br /> The ecdysozoans also include the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Nematoda" title="Nematoda">Nematoda</a> or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water.<sup class="reference" id="cite_ref-75"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-75"><span>[</span>75<span>]</span></a></sup> A number are important parasites.<sup class="reference" id="cite_ref-76"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-76"><span>[</span>76<span>]</span></a></sup> Smaller phyla related to them are the <a href="http://en.wikipedia.org/wiki/Nematomorpha" title="Nematomorpha">Nematomorpha</a> or horsehair worms, and the <a href="http://en.wikipedia.org/wiki/Kinorhyncha" title="Kinorhyncha">Kinorhyncha</a>, <a href="http://en.wikipedia.org/wiki/Priapulida" title="Priapulida">Priapulida</a>, and <a href="http://en.wikipedia.org/wiki/Loricifera" title="Loricifera">Loricifera</a>. These groups have a reduced coelom, called a pseudocoelom.<br /> The remaining two groups of protostomes are sometimes grouped together as the <a href="http://en.wikipedia.org/wiki/Spiralia" title="Spiralia">Spiralia</a>, since in both embryos develop with <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Spiral_cleavage" title="Spiral cleavage">spiral cleavage</a>.<br /> <h3> <span class="mw-headline" id="Platyzoa">Platyzoa</span></h3> <div class="thumb tright"> <div class="thumbinner" style="width: 222px;"> <a class="image" href="http://en.wikipedia.org/wiki/File:Bedford%27s_Flatworm.jpg"><img alt="" class="thumbimage" data-file-height="768" data-file-width="1024" height="165" src="http://upload.wikimedia.org/wikipedia/commons/thumb/7/76/Bedford%27s_Flatworm.jpg/220px-Bedford%27s_Flatworm.jpg" width="220" /></a> <div class="thumbcaption"> <i><a href="http://en.wikipedia.org/wiki/Pseudobiceros_bedfordi" title="Pseudobiceros bedfordi">Pseudobiceros bedfordi</a></i>, (Bedford's flatworm)</div> </div> </div> The <a href="http://en.wikipedia.org/wiki/Platyzoa" title="Platyzoa">Platyzoa</a> include the phylum <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Platyhelminthes" title="Platyhelminthes">Platyhelminthes</a>, the flatworms.<sup class="reference" id="cite_ref-Gilson_77-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-Gilson-77"><span>[</span>77<span>]</span></a></sup> These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors.<sup class="reference" id="cite_ref-78"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-78"><span>[</span>78<span>]</span></a></sup> A number of <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Parasites" title="Parasites">parasites</a> are included in this group, such as the <a href="http://en.wikipedia.org/wiki/Trematoda" title="Trematoda">flukes</a> and <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Tapeworm" title="Tapeworm">tapeworms</a>.<sup class="reference" id="cite_ref-Gilson_77-1"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-Gilson-77"><span>[</span>77<span>]</span></a></sup> Flatworms are <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Aceolomate" title="Aceolomate">acoelomates</a>, lacking a body cavity, as are their closest relatives, the microscopic <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Gastrotricha" title="Gastrotricha">Gastrotricha</a>.<sup class="reference" id="cite_ref-umodena_79-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-umodena-79"><span>[</span>79<span>]</span></a></sup><br /> The other platyzoan phyla are mostly microscopic and <a href="http://en.wikipedia.org/wiki/Body_cavity#Pseudocoelomates" title="Body cavity">pseudocoelomate</a>. The most prominent are the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Rotifera" title="Rotifera">Rotifera</a> or rotifers, which are common in aqueous environments. They also include the <a href="http://en.wikipedia.org/wiki/Acanthocephala" title="Acanthocephala">Acanthocephala</a> or spiny-headed worms, the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Gnathostomulida" title="Gnathostomulida">Gnathostomulida</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Micrognathozoa" title="Micrognathozoa">Micrognathozoa</a>, and possibly the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Cycliophora" title="Cycliophora">Cycliophora</a>.<sup class="reference" id="cite_ref-IntroCyclio_80-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-IntroCyclio-80"><span>[</span>80<span>]</span></a></sup> These groups share the presence of complex jaws, from which they are called the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Gnathifera_%28phylum%29" title="Gnathifera (phylum)">Gnathifera</a>.<br /> <h3> <span class="mw-headline" id="Lophotrochozoa">Lophotrochozoa</span></h3> <div class="thumb tright"> <div class="thumbinner" style="width: 222px;"> <a class="image" href="http://en.wikipedia.org/wiki/File:Grapevinesnail_01.jpg"><img alt="" class="thumbimage" data-file-height="604" data-file-width="1024" height="130" src="http://upload.wikimedia.org/wikipedia/commons/thumb/6/69/Grapevinesnail_01.jpg/220px-Grapevinesnail_01.jpg" width="220" /></a> <div class="thumbcaption"> <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Roman_snail" title="Roman snail">Roman snail</a>, <i>Helix pomatia</i></div> </div> </div> The <a href="http://en.wikipedia.org/wiki/Lophotrochozoa" title="Lophotrochozoa">Lophotrochozoa</a>, evolved within Protostomia, include two of the most successful animal phyla, the <a href="http://en.wikipedia.org/wiki/Mollusca" title="Mollusca">Mollusca</a> and <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Annelida" title="Annelida">Annelida</a>.<sup class="reference" id="cite_ref-81"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-81"><span>[</span>81<span>]</span></a></sup><sup class="reference" id="cite_ref-82"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-82"><span>[</span>82<span>]</span></a></sup> The former, which is the second-largest animal phylum by number of described species, includes animals such as <a href="http://en.wikipedia.org/wiki/Snail" title="Snail">snails</a>, <a href="http://en.wikipedia.org/wiki/Clam" title="Clam">clams</a>, and <a href="http://en.wikipedia.org/wiki/Squid" title="Squid">squids</a>, and the latter comprises the segmented worms, such as <a href="http://en.wikipedia.org/wiki/Earthworm" title="Earthworm">earthworms</a> and <a href="http://en.wikipedia.org/wiki/Leech" title="Leech">leeches</a>. These two groups have long been considered close relatives because of the common presence of <a href="http://en.wikipedia.org/wiki/Trochophore" title="Trochophore">trochophore</a> larvae, but the annelids were considered closer to the arthropods because they are both segmented.<sup class="reference" id="cite_ref-83"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-83"><span>[</span>83<span>]</span></a></sup> Now, this is generally considered <a href="http://en.wikipedia.org/wiki/Convergent_evolution" title="Convergent evolution">convergent evolution</a>, owing to many morphological and genetic differences between the two phyla.<sup class="reference" id="cite_ref-84"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-84"><span>[</span>84<span>]</span></a></sup><br /> The Lophotrochozoa also include the <a href="http://en.wikipedia.org/wiki/Nemertea" title="Nemertea">Nemertea</a> or ribbon worms, the <a href="http://en.wikipedia.org/wiki/Sipuncula" title="Sipuncula">Sipuncula</a>, and several phyla that have a ring of ciliated tentacles around the mouth, called a <a href="http://en.wikipedia.org/wiki/Lophophore" title="Lophophore">lophophore</a>.<sup class="reference" id="cite_ref-85"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-85"><span>[</span>85<span>]</span></a></sup> These were traditionally grouped together as the lophophorates.<sup class="reference" id="cite_ref-86"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-86"><span>[</span>86<span>]</span></a></sup> but it now appears that the lophophorate group may be <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Paraphyletic" title="Paraphyletic">paraphyletic</a>,<sup class="reference" id="cite_ref-87"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-87"><span>[</span>87<span>]</span></a></sup> with some closer to the nemerteans and some to the molluscs and annelids.<sup class="reference" id="cite_ref-88"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-88"><span>[</span>88<span>]</span></a></sup><sup class="reference" id="cite_ref-89"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-89"><span>[</span>89<span>]</span></a></sup> They include the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Brachiopoda" title="Brachiopoda">Brachiopoda</a> or lamp shells, which are prominent in the fossil record, the <a href="http://en.wikipedia.org/wiki/Entoprocta" title="Entoprocta">Entoprocta</a>, the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Phoronida" title="Phoronida">Phoronida</a>, and possibly the <a href="http://en.wikipedia.org/wiki/Bryozoa" title="Bryozoa">Bryozoa</a> or moss animals.<sup class="reference" id="cite_ref-90"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-90"><span>[</span>90<span>]</span></a></sup><br /> <h2> <span class="mw-headline" id="Model_organisms">Model organisms</span></h2> <div class="hatnote relarticle mainarticle"> Main articles: <a href="http://en.wikipedia.org/wiki/Model_organism" title="Model organism">Model organism</a> and <a href="http://en.wikipedia.org/wiki/Animal_testing" title="Animal testing">Animal testing</a></div> Because of the great diversity found in animals, it is more economical for scientists to study a small number of chosen species so that connections can be drawn from their work and conclusions extrapolated about how animals function in general. Because they are easy to keep and breed, the fruit fly <i><a href="http://en.wikipedia.org/wiki/Drosophila_melanogaster" title="Drosophila melanogaster">Drosophila melanogaster</a></i> and the nematode <i><a href="http://en.wikipedia.org/wiki/Caenorhabditis_elegans" title="Caenorhabditis elegans">Caenorhabditis elegans</a></i> have long been the most intensively studied metazoan <a href="http://en.wikipedia.org/wiki/Model_organism" title="Model organism">model organisms</a>, and were among the first life-forms to be genetically sequenced. This was facilitated by the severely reduced state of their <a href="http://en.wikipedia.org/wiki/Genome" title="Genome">genomes</a>, but as many <a href="http://en.wikipedia.org/wiki/Gene" title="Gene">genes</a>, <a href="http://en.wikipedia.org/wiki/Intron" title="Intron">introns</a>, and <a href="http://en.wikipedia.org/wiki/Genetic_linkage" title="Genetic linkage">linkages</a> lost, these ecdysozoans can teach us little about the origins of animals in general. The extent of this type of evolution within the superphylum will be revealed by the crustacean, annelid, and molluscan <a href="http://en.wikipedia.org/wiki/Genome_project" title="Genome project">genome projects</a> currently in progress. Analysis of the <a href="http://en.wikipedia.org/wiki/Starlet_sea_anemone" title="Starlet sea anemone">starlet sea anemone</a> genome has emphasised the importance of sponges, placozoans, and <a href="http://en.wikipedia.org/wiki/Choanoflagellate" title="Choanoflagellate">choanoflagellates</a>, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to the Eumetazoa.<sup class="reference" id="cite_ref-91"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-91"><span>[</span>91<span>]</span></a></sup><br /> An analysis of the <a href="http://en.wikipedia.org/wiki/Homoscleromorpha" title="Homoscleromorpha">homoscleromorph sponge</a> <i>Oscarella carmela</i> also suggests that the last common ancestor of sponges and the eumetazoan animals was more complex than previously assumed.<sup class="reference" id="cite_ref-92"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-92"><span>[</span>92<span>]</span></a></sup><br /> Other model organisms belonging to the animal kingdom include the house mouse (<i><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Mus_musculus" title="Mus musculus">Mus musculus</a></i>) and zebrafish (<i><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Danio_rerio" title="Danio rerio">Danio rerio</a></i>).<br /> <h2> <span class="mw-headline" id="History_of_classification">History of classification</span></h2> <div class="thumb tright"> <div class="thumbinner" style="width: 172px;"> <a class="image" href="http://en.wikipedia.org/wiki/File:Carl_von_Linn%C3%A9.jpg"><img alt="" class="thumbimage" data-file-height="2342" data-file-width="1938" height="205" src="http://upload.wikimedia.org/wikipedia/commons/thumb/6/68/Carl_von_Linn%C3%A9.jpg/170px-Carl_von_Linn%C3%A9.jpg" width="170" /></a> <div class="thumbcaption"> <a href="http://en.wikipedia.org/wiki/Carl_Linnaeus" title="Carl Linnaeus">Carl Linnaeus</a>, known as the father of modern <a href="http://en.wikipedia.org/wiki/Taxonomy_%28biology%29" title="Taxonomy (biology)">taxonomy</a></div> </div> </div> <a href="http://en.wikipedia.org/wiki/Aristotle" title="Aristotle">Aristotle</a> divided the living world between animals and plants, and this was followed by <a href="http://en.wikipedia.org/wiki/Carl_Linnaeus" title="Carl Linnaeus">Carl Linnaeus</a>, in the first hierarchical classification.<sup class="reference" id="cite_ref-Linn1758_93-0"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-Linn1758-93"><span>[</span>93<span>]</span></a></sup> In Linnaeus's original scheme, the animals were one of three kingdoms, divided into the classes of <a href="http://en.wikipedia.org/wiki/Vermes_in_the_10th_edition_of_Systema_Naturae" title="Vermes in the 10th edition of Systema Naturae">Vermes</a>, <a href="http://en.wikipedia.org/wiki/Insecta_in_the_10th_edition_of_Systema_Naturae" title="Insecta in the 10th edition of Systema Naturae">Insecta</a>, <a href="http://en.wikipedia.org/wiki/Pisces_in_the_10th_edition_of_Systema_Naturae" title="Pisces in the 10th edition of Systema Naturae">Pisces</a>, <a href="http://en.wikipedia.org/wiki/Amphibia_in_the_10th_edition_of_Systema_Naturae" title="Amphibia in the 10th edition of Systema Naturae">Amphibia</a>, <a href="http://en.wikipedia.org/wiki/Aves_in_the_10th_edition_of_Systema_Naturae" title="Aves in the 10th edition of Systema Naturae">Aves</a>, and <a href="http://en.wikipedia.org/wiki/Mammalia_in_the_10th_edition_of_Systema_Naturae" title="Mammalia in the 10th edition of Systema Naturae">Mammalia</a>. Since then the last four have all been subsumed into a single phylum, the <a href="http://en.wikipedia.org/wiki/Chordate" title="Chordate">Chordata</a>, whereas the various other forms have been separated out.<br /> In 1874, <a href="http://en.wikipedia.org/wiki/Ernst_Haeckel" title="Ernst Haeckel">Ernst Haeckel</a> divided the animal kingdom into two subkingdoms: Metazoa (multicellular animals) and <a href="http://en.wikipedia.org/wiki/Protozoa" title="Protozoa">Protozoa</a> (single-celled animals).<sup class="reference" id="cite_ref-94"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-94"><span>[</span>94<span>]</span></a></sup> The protozoa were later moved to the kingdom <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Protista" title="Protista">Protista</a>, leaving only the metazoa. Thus Metazoa is now considered a synonym of Animalia.<sup class="reference" id="cite_ref-95"><a href="http://en.wikipedia.org/wiki/Animal#cite_note-95"><span>[</span>95<span>]</span></a></sup><br /> <h2> <span class="mw-headline" id="See_also">See also</span></h2> <table class="metadata mbox-small" style="background-color: #f9f9f9; border: 1px solid #aaa;"> <tbody> <tr> <td class="mbox-image"><img alt="Book icon" data-file-height="48" data-file-width="48" height="30" src="http://upload.wikimedia.org/wikipedia/commons/thumb/a/a8/Office-book.svg/30px-Office-book.svg.png" width="30" /></td> <td class="mbox-text plainlist"> <ul style="font-weight: bold;"> <li><a href="http://en.wikipedia.org/wiki/Book:Animal" title="Book:Animal">Book: Animal</a></li> </ul> </td> </tr> </tbody></table> <ul> <li><a href="http://en.wikipedia.org/wiki/Animal_coloration" title="Animal coloration">Animal coloration</a></li> <li><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Biological_classification" title="Biological classification">Biological classification</a></li> <li><a href="http://en.wikipedia.org/wiki/Ethology" title="Ethology">Ethology</a></li> <li><a href="http://en.wikipedia.org/wiki/Fauna" title="Fauna">Fauna</a></li> <li><a href="http://en.wikipedia.org/wiki/List_of_animal_names" title="List of animal names">List of animal names</a></li> <li><a href="http://en.wikipedia.org/wiki/List_of_animals_by_number_of_neurons" title="List of animals by number of neurons">List of animals by number of neurons</a></li> <li><a href="http://en.wikipedia.org/wiki/Lists_of_animals" title="Lists of animals">Lists of animals</a></li> <li><a href="http://en.wikipedia.org/wiki/Lists_of_organisms_by_population" title="Lists of organisms by population">Lists of organisms by population</a></li> <li><a href="http://en.wikipedia.org/wiki/Zoology" title="Zoology">Zoology</a></li> <li><a href="http://en.wikipedia.org/wiki/Animal_attacks" title="Animal attacks">Animal attacks</a></li> </ul>

ANIMAL

Animal From Wikipedia, the free encyclopedia For other uses, see Animal (disambiguation) . "Animalia" redirects here. For o...

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PATHOGENS

<h1 class="firstHeading" id="firstHeading" lang="en"> Pathogen</h1> <div id="siteSub"> From Wikipedia, the free encyclopedia</div> <div class="mw-content-ltr" dir="ltr" id="mw-content-text" lang="en"> <div class="hatnote"> For other uses, see <a class="mw-disambig" href="http://en.wikipedia.org/wiki/Pathogen_%28disambiguation%29" title="Pathogen (disambiguation)">Pathogen (disambiguation)</a>.</div> A <b>pathogen</b> (<a href="http://en.wikipedia.org/wiki/Greek_language" title="Greek language">Greek</a>: <span lang="el">πάθος</span> <i>pathos</i> “suffering, passion” and -γενής <i>-genēs</i> “producer of”) in the oldest and broadest sense is anything that can produce <a href="http://en.wikipedia.org/wiki/Disease" title="Disease">disease</a>, a term which came into use in the 1880s.<sup class="reference" id="cite_ref-1"><a href="http://en.wikipedia.org/wiki/Pathogen#cite_note-1"><span>[</span>1<span>]</span></a></sup><sup class="reference" id="cite_ref-casadevall2014_2-0"><a href="http://en.wikipedia.org/wiki/Pathogen#cite_note-casadevall2014-2"><span>[</span>2<span>]</span></a></sup> Typically the term is used to describe an infectious agent such as a <a href="http://en.wikipedia.org/wiki/Virus" title="Virus">virus</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Bacterium" title="Bacterium">bacterium</a>, <a href="http://en.wikipedia.org/wiki/Prion" title="Prion">prion</a>, <a href="http://en.wikipedia.org/wiki/Fungus" title="Fungus">fungus</a>, <a href="http://en.wikipedia.org/wiki/Viroid" title="Viroid">viroid</a>, or <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Parasite" title="Parasite">parasite</a> that causes disease in its <a href="http://en.wikipedia.org/wiki/Host_%28biology%29" title="Host (biology)">host</a>. The host may be an <a href="http://en.wikipedia.org/wiki/Animal" title="Animal">animal</a>, a <a href="http://en.wikipedia.org/wiki/Plant" title="Plant">plant</a>, a <a href="http://en.wikipedia.org/wiki/Fungus" title="Fungus">fungus</a> or even another microorganism.<sup class="reference" id="cite_ref-3"><a href="http://en.wikipedia.org/wiki/Pathogen#cite_note-3"><span>[</span>3<span>]</span></a></sup><sup class="reference" id="cite_ref-4"><a href="http://en.wikipedia.org/wiki/Pathogen#cite_note-4"><span>[</span>4<span>]</span></a></sup><br /> There are several substrates including <i>pathways</i> whereby pathogens can invade a host. The principal pathways have different episodic time frames, but <a href="http://en.wikipedia.org/wiki/Soil_contamination" title="Soil contamination">soil contamination</a> has the longest or most persistent potential for harboring a pathogen. Diseases caused by organisms in humans are known as pathogenic diseases. Some of the diseases that are caused by viral pathogens include <a href="http://en.wikipedia.org/wiki/Smallpox" title="Smallpox">smallpox</a>, <a href="http://en.wikipedia.org/wiki/Influenza" title="Influenza">influenza</a>, <a href="http://en.wikipedia.org/wiki/Mumps" title="Mumps">mumps</a>, <a href="http://en.wikipedia.org/wiki/Measles" title="Measles">measles</a>, <a href="http://en.wikipedia.org/wiki/Chickenpox" title="Chickenpox">chickenpox</a>, <a href="http://en.wikipedia.org/wiki/Ebola_virus_disease" title="Ebola virus disease">ebola</a> and <a href="http://en.wikipedia.org/wiki/Rubella" title="Rubella">rubella</a>.<br /> Not all pathogens are necessarily undesirable to humans. In <a href="http://en.wikipedia.org/wiki/Entomology" title="Entomology">entomology</a>, pathogens are one of the "three P's" (<a class="mw-redirect" href="http://en.wikipedia.org/wiki/Predator" title="Predator">predators</a>, pathogens and <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Parasitoids" title="Parasitoids">parasitoids</a>) that serve as natural or introduced <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Biological_control" title="Biological control">biological controls</a> to suppress <a href="http://en.wikipedia.org/wiki/Arthropod" title="Arthropod">arthropod</a> <a href="http://en.wikipedia.org/wiki/Pest_%28organism%29" title="Pest (organism)">pest</a> populations.<br /> <div class="toc" id="toc"> <div id="toctitle"> <h2> Contents</h2> </div> <ul> <li class="toclevel-1 tocsection-1"><a href="http://en.wikipedia.org/wiki/Pathogen#Types_of_pathogen"><span class="tocnumber">1</span> <span class="toctext">Types of pathogen</span></a> <ul> <li class="toclevel-2 tocsection-2"><a href="http://en.wikipedia.org/wiki/Pathogen#Subcellular_infectious_objects"><span class="tocnumber">1.1</span> <span class="toctext">Subcellular infectious objects</span></a> <ul> <li class="toclevel-3 tocsection-3"><a href="http://en.wikipedia.org/wiki/Pathogen#Prionic"><span class="tocnumber">1.1.1</span> <span class="toctext">Prionic</span></a></li> <li class="toclevel-3 tocsection-4"><a href="http://en.wikipedia.org/wiki/Pathogen#Viral"><span class="tocnumber">1.1.2</span> <span class="toctext">Viral</span></a></li> </ul> </li> <li class="toclevel-2 tocsection-5"><a href="http://en.wikipedia.org/wiki/Pathogen#Prokaryotes"><span class="tocnumber">1.2</span> <span class="toctext">Prokaryotes</span></a> <ul> <li class="toclevel-3 tocsection-6"><a href="http://en.wikipedia.org/wiki/Pathogen#Bacterial"><span class="tocnumber">1.2.1</span> <span class="toctext">Bacterial</span></a></li> </ul> </li> <li class="toclevel-2 tocsection-7"><a href="http://en.wikipedia.org/wiki/Pathogen#Eukaryotes"><span class="tocnumber">1.3</span> <span class="toctext">Eukaryotes</span></a> <ul> <li class="toclevel-3 tocsection-8"><a href="http://en.wikipedia.org/wiki/Pathogen#Fungal"><span class="tocnumber">1.3.1</span> <span class="toctext">Fungal</span></a></li> </ul> </li> <li class="toclevel-2 tocsection-9"><a href="http://en.wikipedia.org/wiki/Pathogen#Other_parasites"><span class="tocnumber">1.4</span> <span class="toctext">Other parasites</span></a></li> </ul> </li> <li class="toclevel-1 tocsection-10"><a href="http://en.wikipedia.org/wiki/Pathogen#Treatment_and_health_care"><span class="tocnumber">2</span> <span class="toctext">Treatment and health care</span></a></li> <li class="toclevel-1 tocsection-11"><a href="http://en.wikipedia.org/wiki/Pathogen#Virulence"><span class="tocnumber">3</span> <span class="toctext">Virulence</span></a></li> <li class="toclevel-1 tocsection-12"><a href="http://en.wikipedia.org/wiki/Pathogen#Transmission"><span class="tocnumber">4</span> <span class="toctext">Transmission</span></a></li> <li class="toclevel-1 tocsection-13"><a href="http://en.wikipedia.org/wiki/Pathogen#See_also"><span class="tocnumber">5</span> <span class="toctext">See also</span></a></li> <li class="toclevel-1 tocsection-14"><a href="http://en.wikipedia.org/wiki/Pathogen#References"><span class="tocnumber">6</span> <span class="toctext">References</span></a></li> <li class="toclevel-1 tocsection-15"><a href="http://en.wikipedia.org/wiki/Pathogen#External_links"><span class="tocnumber">7</span> <span class="toctext">External links</span></a></li> </ul> </div> <h2> <span class="mw-headline" id="Types_of_pathogen">Types of pathogen</span></h2> <h3> <span class="mw-headline" id="Subcellular_infectious_objects">Subcellular infectious objects</span></h3> <h4> <span class="mw-headline" id="Prionic">Prionic</span></h4> <div class="hatnote relarticle mainarticle"> Main article: <a href="http://en.wikipedia.org/wiki/Prion" title="Prion">Prion</a></div> According to the prion theory, prions are infectious pathogens that do not contain <a href="http://en.wikipedia.org/wiki/Nucleic_acid" title="Nucleic acid">nucleic acids</a>. These abnormally folded proteins are found characteristically in some diseases such as <a href="http://en.wikipedia.org/wiki/Scrapie" title="Scrapie">scrapie</a>, <a href="http://en.wikipedia.org/wiki/Bovine_spongiform_encephalopathy" title="Bovine spongiform encephalopathy">bovine spongiform encephalopathy</a> (mad cow disease) and <a href="http://en.wikipedia.org/wiki/Creutzfeldt%E2%80%93Jakob_disease" title="Creutzfeldt–Jakob disease">Creutzfeldt–Jakob disease</a>.<sup class="reference" id="cite_ref-5"><a href="http://en.wikipedia.org/wiki/Pathogen#cite_note-5"><span>[</span>5<span>]</span></a></sup> Although prions fail to meet the requirements laid out by <a href="http://en.wikipedia.org/wiki/Koch%27s_postulates" title="Koch's postulates">Koch's postulates</a>, the hypothesis of prions as a new class of pathogen led <a href="http://en.wikipedia.org/wiki/Stanley_B._Prusiner" title="Stanley B. Prusiner">Stanley B. Prusiner</a> to receive the <a href="http://en.wikipedia.org/wiki/Nobel_Prize_in_Physiology_or_Medicine" title="Nobel Prize in Physiology or Medicine">Nobel Prize in Physiology or Medicine</a> in 1997.<br /> <h4> <span class="mw-headline" id="Viral">Viral</span></h4> <div class="hatnote"> Further information: <a href="http://en.wikipedia.org/wiki/Viral_disease" title="Viral disease">Viral disease</a></div> Pathogenic viruses are diseases mainly those of the families of: <a href="http://en.wikipedia.org/wiki/Adenoviridae" title="Adenoviridae">Adenoviridae</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Picornaviridae" title="Picornaviridae">Picornaviridae</a>, <a href="http://en.wikipedia.org/wiki/Herpesviridae" title="Herpesviridae">Herpesviridae</a>, <a href="http://en.wikipedia.org/wiki/Hepadnaviridae" title="Hepadnaviridae">Hepadnaviridae</a>, <a href="http://en.wikipedia.org/wiki/Flaviviridae" title="Flaviviridae">Flaviviridae</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Retroviridae" title="Retroviridae">Retroviridae</a>, <a href="http://en.wikipedia.org/wiki/Orthomyxoviridae" title="Orthomyxoviridae">Orthomyxoviridae</a>, <a href="http://en.wikipedia.org/wiki/Paramyxoviridae" title="Paramyxoviridae">Paramyxoviridae</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Papovaviridae" title="Papovaviridae">Papovaviridae</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Polyomavirus" title="Polyomavirus">Polyomavirus</a>, <a href="http://en.wikipedia.org/wiki/Rhabdoviridae" title="Rhabdoviridae">Rhabdoviridae</a>, <a href="http://en.wikipedia.org/wiki/Togaviridae" title="Togaviridae">Togaviridae</a>. Viruses typically range between 20-300 nanometers in length. <sup class="reference" id="cite_ref-6"><a href="http://en.wikipedia.org/wiki/Pathogen#cite_note-6"><span>[</span>6<span>]</span></a></sup><br /> <h3> <span class="mw-headline" id="Prokaryotes">Prokaryotes</span></h3> <h4> <span class="mw-headline" id="Bacterial">Bacterial</span></h4> <div class="hatnote relarticle mainarticle"> Main article: <a href="http://en.wikipedia.org/wiki/Pathogenic_bacteria" title="Pathogenic bacteria">Pathogenic bacteria</a></div> Although the vast majority of bacteria are harmless or beneficial, a few pathogenic bacteria can cause infectious diseases. Bacteria can often be killed by antibiotics because the cell wall on the outside is destroyed, expelling the DNA out of the body of the pathogen, therefore making the pathogen incapable of producing proteins and dies. They typically range between 1 and 5 micrometers in length.<br /> <h3> <span class="mw-headline" id="Eukaryotes">Eukaryotes</span></h3> <h4> <span class="mw-headline" id="Fungal">Fungal</span></h4> <div class="hatnote relarticle mainarticle"> Main article: <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Pathogenic_fungi" title="Pathogenic fungi">Pathogenic fungi</a></div> Fungi comprise a eukaryotic kingdom of microbes that are usually <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Saprophyte" title="Saprophyte">saprophytes</a> (consume dead organisms) but can cause diseases in humans, animals and plants. Fungi are the most common cause of diseases in crops and other plants. The typical fungal spore size is 1-40 micrometers in length.<br /> <h3> <span class="mw-headline" id="Other_parasites">Other parasites</span></h3> <div class="hatnote relarticle mainarticle"> Main article: <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Human_parasites" title="Human parasites">Human parasites</a></div> Some eukaryotic organisms, such as <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Protists" title="Protists">protists</a> and <a href="http://en.wikipedia.org/wiki/Helminths" title="Helminths">helminths</a>, cause disease.<br /> <h2> <span class="mw-headline" id="Treatment_and_health_care">Treatment and health care</span></h2> Bacteria are usually treated with <a href="http://en.wikipedia.org/wiki/Antibiotics" title="Antibiotics">antibiotics</a> while viruses are treated with <a href="http://en.wikipedia.org/wiki/Antiviral_drug" title="Antiviral drug">antiviral</a> compounds. Eukaryotic pathogens are typically not susceptible to antibiotics and thus need specific drugs. Infection with many pathogens can be prevented by <a href="http://en.wikipedia.org/wiki/Immunization" title="Immunization">immunization</a>. A small amount of pathogens are used in <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Vaccines" title="Vaccines">vaccines</a> to make immunity stay alert and strengthen defense on the insides to prepare for a larger quantity of the virus ever getting inside. <a href="http://en.wikipedia.org/wiki/Hygiene" title="Hygiene">Hygiene</a> is critical for the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Prevention_%28medical%29" title="Prevention (medical)">prevention</a> of <a href="http://en.wikipedia.org/wiki/Infection" title="Infection">infection</a> by pathogens.<br /> <h2> <span class="mw-headline" id="Virulence">Virulence</span></h2> <a href="http://en.wikipedia.org/wiki/Virulence" title="Virulence">Virulence</a> (the tendency of a pathogen to cause damage to a host's <a href="http://en.wikipedia.org/wiki/Physical_fitness" title="Physical fitness">fitness</a>) evolves when that pathogen can spread from a diseased host, despite that host being very debilitated. <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Horizontal_transmission" title="Horizontal transmission">Horizontal transmission</a> occurs between hosts of the same species, in contrast to <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Vertical_transmission" title="Vertical transmission">vertical transmission</a>, which tends to evolve symbiosis (after a period of high morbidity and mortality in the population) by linking the pathogen's evolutionary success to the evolutionary success of the host organism.<br /> <a href="http://en.wikipedia.org/wiki/Evolutionary_medicine" title="Evolutionary medicine">Evolutionary medicine</a> has found that under horizontal transmission, <a href="http://en.wikipedia.org/wiki/Optimal_virulence" title="Optimal virulence">the host population might never develop tolerance to the pathogen</a>.<br /> <h2> <span class="mw-headline" id="Transmission">Transmission</span></h2> <div class="hatnote relarticle mainarticle"> Main article: <a href="http://en.wikipedia.org/wiki/Transmission_%28medicine%29" title="Transmission (medicine)">Transmission (medicine)</a></div> Transmission of pathogens occurs through many different routes, including airborne, direct or indirect contact, sexual contact, through blood, breast milk, or other body fluids, and through the <a href="http://en.wikipedia.org/wiki/Fecal-oral_route" title="Fecal-oral route">fecal-oral route</a>.<br /> <h2> <span class="mw-headline" id="See_also">See also</span></h2> <ul> <li><a href="http://en.wikipedia.org/wiki/Ecological_competence" title="Ecological competence">Ecological competence</a></li> <li><a href="http://en.wikipedia.org/wiki/Emerging_Pathogens_Institute" title="Emerging Pathogens Institute">Emerging Pathogens Institute</a></li> <li><a href="http://en.wikipedia.org/wiki/Human_pathogen" title="Human pathogen">Human pathogen</a></li> <li><a href="http://en.wikipedia.org/wiki/List_of_human_diseases_associated_with_infectious_pathogens" title="List of human diseases associated with infectious pathogens">List of human diseases associated with infectious pathogens</a></li> <li><a href="http://en.wikipedia.org/wiki/PHI-base" title="PHI-base">Pathogen-Host Interaction Database (PHI-base)</a></li> </ul> <h2> <span class="mw-headline" id="References">References</span></h2> <div class="reflist columns references-column-width" style="-moz-column-width: 33em; -webkit-column-width: 33em; column-width: 33em; list-style-type: decimal;"> <ol class="references"> <li id="cite_note-1"> <span class="reference-text"><span class="citation web"><a class="external text" href="http://dictionary.reference.com/browse/Pathogen" rel="nofollow">"Pathogen"</a>. <i><a href="http://en.wikipedia.org/wiki/Dictionary.com" title="Dictionary.com">Dictionary.com</a> Unabridged</i>. <a href="http://en.wikipedia.org/wiki/Random_House" title="Random House">Random House</a><span class="reference-accessdate">. Retrieved <span class="nowrap">August 17,</span> 2013</span>.</span><span class="Z3988" title="ctx_ver=Z39.88-2004&rfr_id=info%3Asid%2Fen.wikipedia.org%3APathogen&rft.atitle=Pathogen&rft.genre=article&rft_id=http%3A%2F%2Fdictionary.reference.com%2Fbrowse%2FPathogen&rft.jtitle=Dictionary.com+Unabridged&rft.pub=Random+House&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal"></span></span></li> <li id="cite_note-casadevall2014-2"> <span class="reference-text"><span class="citation journal">Casadevall, Arturo; Pirofski, Liise-anne (11 December 2014). "Ditch the term pathogen". Comment. <i><a href="http://en.wikipedia.org/wiki/Nature_%28journal%29" title="Nature (journal)">Nature</a></i> (paper) <b>516</b> (7530): 165–6. <a href="http://en.wikipedia.org/wiki/Digital_object_identifier" title="Digital object identifier">doi</a>:<a class="external text" href="http://dx.doi.org/10.1038%2F516165a" rel="nofollow">10.1038/516165a</a>.</span><span class="Z3988" title="ctx_ver=Z39.88-2004&rfr_id=info%3Asid%2Fen.wikipedia.org%3APathogen&rft.atitle=Ditch+the+term+pathogen&rft.au=Casadevall%2C+Arturo&rft.aulast=Casadevall%2C+Arturo&rft.au=Pirofski%2C+Liise-anne&rft.date=11+December+2014&rft.genre=article&rft_id=info%3Adoi%2F10.1038%2F516165a&rft.issue=7530&rft.jtitle=Nature&rft.pages=165-6&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.volume=516"></span></span></li> <li id="cite_note-3"> <span class="reference-text"><a class="external free" href="http://www.medterms.com/script/main/art" rel="nofollow">http://www.medterms.com/script/main/art</a>.<sup class="noprint Inline-Template"><span style="white-space: nowrap;">[<i><a href="http://en.wikipedia.org/wiki/Wikipedia:Link_rot" title="Wikipedia:Link rot"><span title=" since January 2015">dead link</span></a></i>]</span></sup></span></li> <li id="cite_note-4"> <span class="reference-text"><span class="citation web"><a class="external text" href="http://www.metapathogen.com/" rel="nofollow">"MetaPathogen - about various types of pathogenic organisms"</a><span class="reference-accessdate">. Retrieved <span class="nowrap">15 January</span> 2015</span>.</span><span class="Z3988" title="ctx_ver=Z39.88-2004&rfr_id=info%3Asid%2Fen.wikipedia.org%3APathogen&rft.btitle=MetaPathogen+-+about+various+types+of+pathogenic+organisms&rft.genre=book&rft_id=http%3A%2F%2Fwww.metapathogen.com&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook"></span></span></li> <li id="cite_note-5"> <span class="reference-text"><a class="external text" href="http://www.mad-cow.org/%7Etom/prionSP.html" rel="nofollow"><i>The prion diseases</i></a> STANLEY B. PRUSINER, Scientific American</span></li> <li id="cite_note-6"> <span class="reference-text"><a class="external text" href="http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/ebola/qa.htm" rel="nofollow">Viral Special Pathogens Branch | [26] Moved | CDC</a></span></li> </ol> </div> <h2> <span class="mw-headline" id="External_links">External links</span></h2> <ul> <li><a class="external text" href="http://d.umn.edu/%7Erhicks/genmicro/Pronunciation%20Guide.pdf" rel="nofollow">Pronunciation Guide to Microorganisms (1)</a></li> <li><a class="external text" href="http://rci.rutgers.edu/%7Emicrolab/CLASSINFO/IMAGESCI/pronunciation%20guide.pdf" rel="nofollow">Pronunciation Guide to Microorganisms (2)</a></li> </ul> <table class="navbox" style="border-spacing: 0;"> <tbody> <tr> <td style="padding: 2px;"> <table class="nowraplinks collapsible autocollapse navbox-inner" id="collapsibleTable0" style="background: transparent; border-spacing: 0; color: inherit;"> <tbody> <tr> <th class="navbox-title" colspan="2" scope="col"><span class="collapseButton">[<a href="http://en.wikipedia.org/wiki/Pathogen#" id="collapseButton0">hide</a>]</span> <div class="plainlinks hlist navbar mini"> <ul> <li class="nv-view"><a href="http://en.wikipedia.org/wiki/Template:Infectious_disease" title="Template:Infectious disease"><span style="background: none transparent; border: none;" title="View this template">v</span></a></li> <li class="nv-talk"><a href="http://en.wikipedia.org/wiki/Template_talk:Infectious_disease" title="Template talk:Infectious disease"><span style="background: none transparent; border: none;" title="Discuss this template">t</span></a></li> <li class="nv-edit"><a class="external text" href="http://en.wikipedia.org/w/index.php?title=Template:Infectious_disease&action=edit"><span style="background: none transparent; border: none;" title="Edit this template">e</span></a></li> </ul> </div> <div style="font-size: 110%;"> <a href="http://en.wikipedia.org/wiki/Infection" title="Infection">Infectious disease</a> and <a href="http://en.wikipedia.org/wiki/Microbiology" title="Microbiology">microbiology</a></div> </th> </tr> <tr style="height: 2px;"> <td colspan="2"><br /></td> </tr> <tr> <th class="navbox-group" scope="row">Disciplines<br /> (<strong class="selflink">pathogens</strong>)<br /> Major diseases</th> <td class="navbox-list navbox-odd hlist" style="border-left-style: solid; border-left-width: 2px; padding: 0px; text-align: left; width: 100%;"> <table class="nowraplinks navbox-subgroup" style="border-spacing: 0;"> <tbody> <tr> <th class="navbox-group" scope="row" style="text-align: left;"><a href="http://en.wikipedia.org/wiki/Microbiology" title="Microbiology">Bacteriology</a> <span style="font-weight: normal;">(<a href="http://en.wikipedia.org/wiki/Pathogenic_bacteria" title="Pathogenic bacteria">bacteria</a>)</span></th> <td class="navbox-list navbox-odd" style="border-left-style: solid; border-left-width: 2px; padding: 0px; text-align: left; width: 100%;"> <div style="padding: 0em 0.25em;"> <ul> <li><a href="http://en.wikipedia.org/wiki/Cholera" title="Cholera">Cholera</a></li> <li><a href="http://en.wikipedia.org/wiki/Diphtheria" title="Diphtheria">Diphtheria</a></li> <li><a href="http://en.wikipedia.org/wiki/Leprosy" title="Leprosy">Leprosy</a></li> <li><a href="http://en.wikipedia.org/wiki/Syphilis" title="Syphilis">Syphilis</a></li> <li><a href="http://en.wikipedia.org/wiki/Tuberculosis" title="Tuberculosis">Tuberculosis</a></li> </ul> </div> </td> </tr> <tr style="height: 2px;"> <td colspan="2"><br /></td> </tr> <tr> <th class="navbox-group" scope="row" style="text-align: left;"><a href="http://en.wikipedia.org/wiki/Virology" title="Virology">Virology</a> <span style="font-weight: normal;">(<a href="http://en.wikipedia.org/wiki/Viral_disease" title="Viral disease">viruses</a>, <a href="http://en.wikipedia.org/wiki/Prion" title="Prion">prions</a>)</span></th> <td class="navbox-list navbox-even" style="border-left-style: solid; border-left-width: 2px; padding: 0px; text-align: left; width: 100%;"> <div style="padding: 0em 0.25em;"> <ul> <li><a href="http://en.wikipedia.org/wiki/HIV/AIDS" title="HIV/AIDS">AIDS</a></li> <li><a href="http://en.wikipedia.org/wiki/Creutzfeldt%E2%80%93Jakob_disease" title="Creutzfeldt–Jakob disease">Creutzfeldt–Jakob disease</a></li> <li><a href="http://en.wikipedia.org/wiki/Influenza" title="Influenza">Influenza</a></li> <li><a href="http://en.wikipedia.org/wiki/Measles" title="Measles">Measles</a></li> <li><a href="http://en.wikipedia.org/wiki/Poliomyelitis" title="Poliomyelitis">Polio</a></li> <li><a href="http://en.wikipedia.org/wiki/Smallpox" title="Smallpox">Smallpox</a></li> </ul> </div> </td> </tr> <tr style="height: 2px;"> <td colspan="2"><br /></td> </tr> <tr> <th class="navbox-group" scope="row" style="text-align: left;"><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Pathogenic_fungi" title="Pathogenic fungi">Mycology</a> <span style="font-weight: normal;">(<a href="http://en.wikipedia.org/wiki/Mycosis" title="Mycosis">fungi</a>)</span></th> <td class="navbox-list navbox-odd" style="border-left-style: solid; border-left-width: 2px; padding: 0px; text-align: left; width: 100%;"> <div style="padding: 0em 0.25em;"> <ul> <li><a href="http://en.wikipedia.org/wiki/Aspergillosis" title="Aspergillosis">Aspergillosis</a></li> <li><a href="http://en.wikipedia.org/wiki/Candidiasis" title="Candidiasis">Candidiasis</a></li> <li><a href="http://en.wikipedia.org/wiki/Dermatophytosis" title="Dermatophytosis">Tinea</a></li> </ul> </div> </td> </tr> <tr style="height: 2px;"> <td colspan="2"><br /></td> </tr> <tr> <th class="navbox-group" scope="row" style="text-align: left;"><a href="http://en.wikipedia.org/wiki/Parasitology" title="Parasitology">Parasitology</a> <span style="font-weight: normal;">(<a href="http://en.wikipedia.org/wiki/Protozoa" title="Protozoa">protozoa</a>, <a href="http://en.wikipedia.org/wiki/Helminths" title="Helminths">helminths</a>)</span></th> <td class="navbox-list navbox-even" style="border-left-style: solid; border-left-width: 2px; padding: 0px; text-align: left; width: 100%;"> <div style="padding: 0em 0.25em;"> <ul> <li><a href="http://en.wikipedia.org/wiki/Amoebic_dysentery" title="Amoebic dysentery">Amoebic dysentery</a></li> <li><a href="http://en.wikipedia.org/wiki/Hookworm" title="Hookworm">Hookworm</a></li> <li><a href="http://en.wikipedia.org/wiki/Malaria" title="Malaria">Malaria</a></li> <li><a href="http://en.wikipedia.org/wiki/Schistosomiasis" title="Schistosomiasis">Schistosomiasis</a></li> </ul> </div> </td> </tr> <tr style="height: 2px;"> <td colspan="2"><br /></td> </tr> <tr> <th class="navbox-group" scope="row" style="text-align: left;"><a href="http://en.wikipedia.org/wiki/Medical_entomology" title="Medical entomology">Entomology</a> <span style="font-weight: normal;">(<a class="mw-redirect" href="http://en.wikipedia.org/wiki/Ectoparasite" title="Ectoparasite">ectoparasites</a>)</span></th> <td class="navbox-list navbox-odd" style="border-left-style: solid; border-left-width: 2px; padding: 0px; text-align: left; width: 100%;"> <div style="padding: 0em 0.25em;"> <ul> <li><a href="http://en.wikipedia.org/wiki/Louse" title="Louse">Lice</a></li> <li><a href="http://en.wikipedia.org/wiki/Scabies" title="Scabies">Scabies</a></li> </ul> </div> </td> </tr> </tbody></table> </td> </tr> <tr style="height: 2px;"> <td colspan="2"><br /></td> </tr> <tr> <th class="navbox-group" scope="row">People</th> <td class="navbox-list navbox-even hlist" style="border-left-style: solid; border-left-width: 2px; padding: 0px; text-align: left; width: 100%;"> <div style="padding: 0em 0.25em;"> <ul> <li><a href="http://en.wikipedia.org/wiki/Alexander_Fleming" title="Alexander Fleming">Alexander Fleming</a></li> <li><a href="http://en.wikipedia.org/wiki/Robert_Koch" title="Robert Koch">Robert Koch</a></li> <li><a href="http://en.wikipedia.org/wiki/Louis_Pasteur" title="Louis Pasteur">Louis Pasteur</a></li> <li><a href="http://en.wikipedia.org/wiki/Ignaz_Semmelweis" title="Ignaz Semmelweis">Ignaz Semmelweis</a></li> </ul> </div> </td> </tr> <tr style="height: 2px;"> <td colspan="2"><br /></td> </tr> <tr> <th class="navbox-group" scope="row">Related topics</th> <td class="navbox-list navbox-even hlist" style="border-left-style: solid; border-left-width: 2px; padding: 0px; text-align: left; width: 100%;"> <div style="padding: 0em 0.25em;"> <ul> <li><a href="http://en.wikipedia.org/wiki/Antibiotics" title="Antibiotics">Antibiotics</a></li> <li><a href="http://en.wikipedia.org/wiki/Eradication_of_infectious_diseases" title="Eradication of infectious diseases">Eradication</a></li> <li><a href="http://en.wikipedia.org/wiki/Pandemic" title="Pandemic">Pandemic</a></li> <li><a href="http://en.wikipedia.org/wiki/Transmission_%28medicine%29" title="Transmission (medicine)">Transmission</a> <ul> <li><a href="http://en.wikipedia.org/wiki/Horizontal_disease_transmission" title="Horizontal disease transmission">horizontal</a></li> <li><a href="http://en.wikipedia.org/wiki/Vertically_transmitted_infection" title="Vertically transmitted infection">vertical</a></li> </ul> </li> <li><a href="http://en.wikipedia.org/wiki/Vaccination" title="Vaccination">Vaccination</a></li> <li><a href="http://en.wikipedia.org/wiki/Zoonosis" title="Zoonosis">Zoonosis</a></li> </ul> </div> </td> </tr> </tbody></table> </td> </tr> </tbody></table> <table class="navbox" style="border-spacing: 0;"> <tbody> <tr> <td style="padding: 2px;"> <table class="nowraplinks noprint navbox-inner" style="background: transparent; border-spacing: 0; color: inherit;"> <tbody> <tr> <td class="navbox-list navbox-odd" colspan="2" style="padding: 0px; width: 100%;"> <div style="padding: 0em 0.25em;"> <ul> <li style="display: inline; white-space: nowrap;"><span style="margin: auto 0.5em;"><a class="image" href="http://en.wikipedia.org/wiki/File:Issoria_lathonia.jpg"><img alt="Issoria lathonia.jpg" data-file-height="445" data-file-width="629" height="17" src="http://upload.wikimedia.org/wikipedia/commons/thumb/2/2d/Issoria_lathonia.jpg/24px-Issoria_lathonia.jpg" width="24" /></a></span><span style="font-weight: bold;"><a href="http://en.wikipedia.org/wiki/Portal:Biology" title="Portal:Biology">Biology portal</a></span></li> <li style="display: inline; white-space: nowrap;"><span style="margin: auto 0.5em;"><a class="image" href="http://en.wikipedia.org/wiki/File:Rod_of_Asclepius2.svg"><img alt="Rod of Asclepius2.svg" data-file-height="804" data-file-width="251" height="21" src="http://upload.wikimedia.org/wikipedia/commons/thumb/e/e3/Rod_of_Asclepius2.svg/6px-Rod_of_Asclepius2.svg.png" width="6" /></a></span><span style="font-weight: bold;"><a href="http://en.wikipedia.org/wiki/Portal:Medicine" title="Portal:Medicine">Medicine portal</a></span></li> </ul> </div> </td> </tr> </tbody></table> </td> </tr> </tbody></table> </div> <div class="mw-normal-catlinks" id="mw-normal-catlinks"> <a href="http://en.wikipedia.org/wiki/Help:Category" title="Help:Category">Categories</a>: <ul> <li><a href="http://en.wikipedia.org/wiki/Category:Infectious_diseases" title="Category:Infectious diseases">Infectious diseases</a></li> </ul> </div>

PATHOGENS

Pathogen From Wikipedia, the free encyclopedia For other uses, see Pathogen (disambiguation) . A pathogen ( Greek : πάθος pathos “...

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