Annelid

<h2 id="classification-and-diversity">Classification and diversity</h2>
<p>There are over 22,000 living annelid species,<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a><a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> ranging in size from microscopic to the Australian <a href="/facts/Giant_Gippsland_earthworm/84Rmkaof">giant Gippsland earthworm</a> and <i><a href="/facts/Amynthas_mekongianus/ErIIyaJX">Amynthas mekongianus</a></i>, which can both grow up to 3 meters (9.8 ft) long <a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a><a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a><a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> to the largest annelid, <i><a href="/facts/Microchaetus_rappi/DDAXwKpe">Microchaetus rappi</a></i> which can grow up to 6.7 m (22 ft). Although research since 1997 has radically changed scientists' views about the evolutionary family tree of the annelids,<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a><a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> most textbooks use the traditional classification into the following sub-groups:<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a><a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a>
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<ul><li><a href="/facts/Polychaete/Uo2YIINM">Polychaetes</a> (about 12,000 species<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a>). As their name suggests, they have multiple chetae ("hairs") per segment. Polychaetes have <a href="/facts/Parapodia/QjzQVE3y">parapodia</a> that function as limbs, and <a href="/facts/Nuchal_organ/2Gmzq3kg">nuchal organs</a> that are thought to be <a href="/facts/Chemoreceptor/KC6IDxzf">chemosensors</a>.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> Most are marine animals, although a few species live in fresh water and even fewer on land.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a></li></ul>

<ul><li><a href="/facts/Clitellate/2lD2UWIu">Clitellates</a> (about 10,000 species <a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a>). These have few or no chetae per segment, and no <a href="/facts/Nuchal_organ/2Gmzq3kg">nuchal organs</a> or parapodia. However, they have a unique reproductive organ, the ring-shaped  <a href="/facts/Clitellum/9gpJQ29l">clitellum</a> ("<a href="/facts/Pack_saddle/ub5pDYhG">pack saddle</a>") around their bodies, which produces a <a href="/facts/Pupa/0u7NE8Cq">cocoon</a> that stores and nourishes fertilized eggs until they hatch <a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a><a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> or, in moniligastrids, yolky eggs that provide nutrition for the embryos.<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> The clitellates are sub-divided into:<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a>
<ul><li><a href="/facts/Oligochaete/08sBMVpF">Oligochaetes</a> ("with few hairs"), which includes <a href="/facts/Earthworms/V7IUvWgQ">earthworms</a>. Oligochaetes have a sticky pad in the roof of the mouth.<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> Most are burrowers that feed on wholly or partly decomposed <a href="/facts/Organic_material/t17tVCvk">organic materials</a>.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a></li>
<li><a href="/facts/Hirudinea/eYSMx97F">Hirudinea</a>, whose name means "<a href="/facts/Leech/eYSMx97F">leech</a>-shaped" and whose best known members are leeches.<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> Marine species are mostly blood-sucking <a href="/facts/Parasite/HWPcgky7">parasites</a>, mainly on fish, while most freshwater species are predators.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> They have suckers at both ends of their bodies, and use these to move rather like <a href="/facts/Geometer_moth/ces6X6JW">inchworms</a>.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a></li></ul></li></ul>
<p>The <a href="/facts/Archiannelida/8606BEVq">Archiannelida</a>, minute annelids that live in the spaces between grains of <a href="/facts/Marine_sediment/0HfKDgDI">marine sediment</a>, were treated as a separate <a href="/facts/Class_(biology)/a7MrgFu3">class</a> because of their simple body structure, but are now regarded as polychaetes.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> Some other groups of animals have been classified in various ways, but are now widely regarded as annelids:
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<ul><li>Pogonophora / <a href="/facts/Siboglinidae/2clzTfUY">Siboglinidae</a> were first discovered in 1914, and their lack of a recognizable gut made it difficult to classify them. They have been classified as a separate <a href="/facts/Phylum/ELT3Jyax">phylum</a>, Pogonophora, or as two phyla, Pogonophora and <a href="/facts/Vestimentifera/2clzTfUY">Vestimentifera</a>. More recently they have been re-classified as a <a href="/facts/Family_(biology)/qTrn6E8E">family</a>, Siboglinidae, within the polychaetes.<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a><a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a></li>
<li>The <a href="/facts/Echiura/CYxsCTIc">Echiura</a> have a checkered <a href="/facts/Taxonomy_(biology)/Qkg8wuyw">taxonomic</a> history: in the 19th century they were assigned to the phylum "Gephyrea", which is now empty as its members have been assigned to other phyla; the Echiura were next regarded as annelids until the 1940s, when they were classified as a phylum in their own right; but a <a href="/facts/Molecular_phylogenetics/FysplHP5">molecular phylogenetics</a> analysis in 1997 concluded that echiurans are annelids.<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a><a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a><a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a></li>
<li><a href="/facts/Myzostomida/Jguck1aa">Myzostomida</a> live on <a href="/facts/Crinoid/RlhFVcdX">crinoids</a> and other <a href="/facts/Echinoderm/j2rd8qwt">echinoderms</a>, mainly as parasites. In the past they have been regarded as close relatives of the <a href="/facts/Trematode/d3J87oDn">trematode</a> <a href="/facts/Flatworm/lvqbrb2V">flatworms</a> or of the <a href="/facts/Tardigrade/WQUg0IS9">tardigrades</a>, but in 1998 it was suggested that they are a sub-group of polychaetes.<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> However, another analysis in 2002 suggested that myzostomids are more closely related to <a href="/facts/Flatworms/lvqbrb2V">flatworms</a> or to <a href="/facts/Rotifer/Gy8DKrFd">rotifers</a> and <a href="/facts/Acanthocephala/dJqMq426">acanthocephales</a>.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a></li>
<li><a href="/facts/Sipuncula/uJxuIwvB">Sipuncula</a> was originally classified as annelids, despite the complete lack of segmentation, <a href="/facts/Bristle/66z9yNmB">bristles</a> and other annelid characters. The phylum Sipuncula was later allied with the <a href="/facts/Mollusca/1elgknsQ">Mollusca</a>, mostly on the basis of <a href="/facts/Morphogenesis/zHJh8Eae">developmental</a> and <a href="/facts/Larva/GKo2FIew">larval</a> characters. Phylogenetic analyses based on 79 ribosomal proteins indicated a position of Sipuncula within Annelida.<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a> Subsequent analysis of the <a href="/facts/Mitochondrion/ErHqrdJ1">mitochondrion</a>'s DNA has confirmed their close relationship to the <a href="/facts/Myzostomida/Jguck1aa">Myzostomida</a> and Annelida (including <a href="/facts/Echiura/CYxsCTIc">echiurans</a> and <a href="/facts/Siboglinidae/2clzTfUY">pogonophorans</a>).<a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a> It has also been shown that a rudimentary neural segmentation similar to that of annelids occurs in the early larval stage, even if these traits are absent in the adults.<a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a></li></ul>
<p>Mitogenomic and phylogenomic analysis also implies that <a href="/facts/Orthonectida/WxNDRxly">Orthonectida</a>, a group of extremely simplified parasites traditionally placed in <a href="/facts/Mesozoa/jNvUg9Yq">Mesozoa</a>, are actually reduced annelids.<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a> Research suggest that also <a href="/facts/Nemertea/D1HKC5NT">nemerteans</a> are annelids, with <a href="/facts/Oweniidae/eHFy72hg">Oweniidae</a> and <a href="/facts/Magelonidae/4VLUDLm2">Magelonidae</a> as their closest relatives.<a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a>
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<h2 id="distinguishing-features">Distinguishing features</h2>
<p>No single feature distinguishes annelids from other <a href="/facts/Invertebrate/3CyqCVnN">invertebrate</a> phyla, but they have a distinctive combination of features. Their bodies are long, with <a href="/facts/Segmentation_(biology)/S4Cw0UcN">segments</a> that are divided externally by shallow ring-like constrictions called annuli and internally by septa ("partitions") at the same points, although in some species the septa are incomplete and in a few cases missing. Most of the segments contain the same sets of <a href="/facts/Organ_(biology)/FUYsLaxz">organs</a>, although sharing a common <a href="/facts/Gut_(zoology)/nK2zwqBS">gut</a>, <a href="/facts/Circulatory_system/WXwudzzc">circulatory system</a> and <a href="/facts/Nervous_system/CDxRKU2U">nervous system</a> makes them inter-dependent.<a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a><a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a> Their bodies are covered by a <a href="/facts/Cuticle/LWaIDBIG">cuticle</a> (outer covering) that does not contain <a href="/facts/Cell_(biology)/bLM9UQvy">cells</a> but is <a href="/facts/Secretion/VHk9eLAV">secreted</a> by cells in the skin underneath, is made of tough but flexible <a href="/facts/Collagen/KsyAb72K">collagen</a><a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a> and does not <a href="/facts/Molt/2wdfTSL5">molt</a><a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a> – on the other hand <a href="/facts/Arthropod/JKJLTFzp">arthropods</a>' cuticles are made of the more rigid α-<a href="/facts/Chitin/Fo1f8b86">chitin</a>,<a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a><a class="footnote-ref" id="fnref:44" href="#fn:44"><sup>44</sup></a> and molt until the arthropods reach their full size.<a class="footnote-ref" id="fnref:45" href="#fn:45"><sup>45</sup></a> Most annelids have closed circulatory systems, where the blood makes its entire circuit via <a href="/facts/Blood_vessel/FG3GjPNW">blood vessels</a>.<a class="footnote-ref" id="fnref:46" href="#fn:46"><sup>46</sup></a>
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Summary of distinguishing features<table><tbody><tr><th rowspan="2"> </th><th rowspan="2">Annelida<a class="footnote-ref" id="fnref:47" href="#fn:47"><sup>47</sup></a></th><th colspan="2">Recently merged into Annelida<a class="footnote-ref" id="fnref:48" href="#fn:48"><sup>48</sup></a></th><th>Closely related</th><th colspan="2">Similar-looking phyla</th></tr><tr><th><a href="/facts/Echiura/CYxsCTIc">Echiura</a><a class="footnote-ref" id="fnref:49" href="#fn:49"><sup>49</sup></a></th><th><a href="/facts/Sipuncula/uJxuIwvB">Sipuncula</a><a class="footnote-ref" id="fnref:50" href="#fn:50"><sup>50</sup></a></th><th><a href="/facts/Nemertea/D1HKC5NT">Nemertea</a><a class="footnote-ref" id="fnref:51" href="#fn:51"><sup>51</sup></a></th><th><a href="/facts/Arthropoda/JKJLTFzp">Arthropoda</a><a class="footnote-ref" id="fnref:52" href="#fn:52"><sup>52</sup></a></th><th><a href="/facts/Onychophora/CVn4wdwx">Onychophora</a><a class="footnote-ref" id="fnref:53" href="#fn:53"><sup>53</sup></a></th></tr><tr><th>External segmentation</th><td>Yes</td><td colspan="2">No</td><td>Only in a few species</td><td>Yes, except in <a href="/facts/Mite/M52qFjlE">mites</a></td><td>No</td></tr><tr><th>Repetition of internal organs</th><td>Yes</td><td colspan="2">No</td><td>Yes</td><td>In primitive forms</td><td>Yes</td></tr><tr><th>Septa between segments</th><td>In most species</td><td colspan="5">No</td></tr><tr><th>Cuticle material</th><td colspan="3">Collagen</td><td>None</td><td colspan="2">α-chitin</td></tr><tr><th>Molting</th><td>Generally no;<a class="footnote-ref" id="fnref:54" href="#fn:54"><sup>54</sup></a> but some <a href="/facts/Polychaete/Uo2YIINM">polychaetes</a> molt their jaws, and <a href="/facts/Leech/eYSMx97F">leeches</a> molt their skins<a class="footnote-ref" id="fnref:55" href="#fn:55"><sup>55</sup></a></td><td colspan="3">No<a class="footnote-ref" id="fnref:56" href="#fn:56"><sup>56</sup></a></td><td colspan="2">Yes<a class="footnote-ref" id="fnref:57" href="#fn:57"><sup>57</sup></a></td></tr><tr><th>Body cavity</th><td>Coelom; but this is reduced or missing in many leeches and some small polychaetes<a class="footnote-ref" id="fnref:58" href="#fn:58"><sup>58</sup></a></td><td>Two coelomata, main and in <a href="/facts/Proboscis/Ym7nscSW">proboscis</a></td><td>Two coelomata, main and in tentacles</td><td>Coelom only in proboscis</td><td colspan="2"><a href="/facts/Hemocoel/WXwudzzc">Hemocoel</a></td></tr><tr><th>Circulatory system</th><td>Closed in most species</td><td>Open outflow, return via branched <a href="/facts/Vein/I6ycr6gF">vein</a></td><td>Open</td><td>Closed</td><td colspan="2">Open</td></tr></tbody></table>
<h2 id="description">Description</h2>
<h3>Segmentation</h3>

<p>In addition to Sipuncula and Echiura, also lineages like Lobatocerebrum, <a href="/facts/Diurodrilus/4jRz4MpG">Diurodrilus</a> and Polygordius have lost their segmentation, but these are the exceptions from the rule.<a class="footnote-ref" id="fnref:59" href="#fn:59"><sup>59</sup></a> Most of an annelid's body consists of segments that are practically identical, having the same sets of internal organs and external <a href="/facts/Chaeta/hcc9Kb7v">chaetae</a> (Greek χαιτη, meaning "hair") and, in some species, appendages. The frontmost and rearmost sections are not regarded as true segments as they do not contain the standard sets of organs and do not develop in the same way as the true segments. The frontmost section, called the <a href="/facts/Prostomium/Kjc826WN">prostomium</a> (Greek προ- meaning "in front of" and στομα meaning "mouth") contains the brain and sense organs, while the rearmost, called the <a href="/facts/Pygidium/1cQh2lS0">pygidium</a> (Greek πυγιδιον, meaning "little tail") or <a href="/facts/Periproct/oYcrKb1P">periproct</a> contains the <a href="/facts/Anus/KoCBptr5">anus</a>, generally on the underside. The first section behind the prostomium, called the <a href="/facts/Peristomium/8RI7gXf0">peristomium</a> (Greek περι- meaning "around" and στομα meaning "mouth"), is regarded by some zoologists as not a true segment, but in some <a href="/facts/Polychaete/Uo2YIINM">polychaetes</a> the peristomium has chetae and appendages like those of other segments.<a class="footnote-ref" id="fnref:60" href="#fn:60"><sup>60</sup></a>
</p><p>The segments develop one at a time from a growth zone just ahead of the pygidium, so that an annelid's youngest segment is just in front of the growth zone while the peristomium is the oldest. This pattern is called <a href="/facts/Teloblast/LNU8v09c">teloblastic growth</a>.<a class="footnote-ref" id="fnref:61" href="#fn:61"><sup>61</sup></a> Some groups of annelids, including all <a href="/facts/Leech/eYSMx97F">leeches</a>,<a class="footnote-ref" id="fnref:62" href="#fn:62"><sup>62</sup></a> have fixed maximum numbers of segments, while others add segments throughout their lives.<a class="footnote-ref" id="fnref:63" href="#fn:63"><sup>63</sup></a>
</p><p>
The phylum's name is derived from the <a href="/facts/Latin/pM3Kge85">Latin</a> word <i>annelus</i>, meaning "little ring".<a class="footnote-ref" id="fnref:64" href="#fn:64"><sup>64</sup></a></p>
<h3>Body wall, chaetae and parapodia</h3>

<p>Annelids' cuticles are made of <a href="/facts/Collagen/KsyAb72K">collagen</a> fibers, usually in layers that spiral in alternating directions so that the fibers cross each other. These are secreted by the one-cell deep epidermis (outermost skin layer). A few marine annelids that live in tubes lack cuticles, but their tubes have a similar structure, and <a href="/facts/Mucus/hnlBuHAy">mucus</a>-secreting <a href="/facts/Gland/gfq2of7k">glands</a> in the epidermis protect their skins.<a class="footnote-ref" id="fnref:65" href="#fn:65"><sup>65</sup></a> Under the epidermis is the <a href="/facts/Dermis/WAm7Fd6P">dermis</a>, which is made of <a href="/facts/Connective_tissue/mal3fc7k">connective tissue</a>, in other words a combination of cells and non-cellular materials such as collagen. Below this are two layers of muscles, which develop from the lining of the <a href="/facts/Coelom/PD25dd6m">coelom</a> (body cavity): circular muscles make a segment longer and slimmer when they contract, while under them are longitudinal muscles, usually four distinct strips,<a class="footnote-ref" id="fnref:66" href="#fn:66"><sup>66</sup></a>  whose contractions make the segment shorter and fatter.<a class="footnote-ref" id="fnref:67" href="#fn:67"><sup>67</sup></a> But several families have lost the circular muscles, and it has been suggested that the lack of circular muscles is a <a href="/facts/Plesiomorphic/n5K8vSKd">plesiomorphic</a> character in Annelida.<a class="footnote-ref" id="fnref:68" href="#fn:68"><sup>68</sup></a> Some annelids also have oblique internal muscles that connect the underside of the body to each side.<a class="footnote-ref" id="fnref:69" href="#fn:69"><sup>69</sup></a>
</p><p>The <a href="/facts/Seta/jUmts6C2">setae</a> ("hairs") of annelids project out from the epidermis to provide <a href="/facts/Traction_(engineering)/Aynjhy6n">traction</a> and other capabilities. The simplest are unjointed and form paired bundles near the top and bottom of each side of each segment. The <a href="/facts/Parapodia/QjzQVE3y">parapodia</a> ("limbs") of annelids that have them often bear more complex chetae at their tips – for example jointed, comb-like or hooked.<a class="footnote-ref" id="fnref:70" href="#fn:70"><sup>70</sup></a> Chetae are made of moderately flexible β-<a href="/facts/Chitin/Fo1f8b86">chitin</a> and are formed by <a href="/facts/Follicle_(anatomy)/3KttrdjU">follicles</a>, each of which has a chetoblast ("hair-forming") cell at the bottom and muscles that can extend or retract the cheta. The chetoblasts produce chetae by forming <a href="/facts/Microvilli/tlI7DwCh">microvilli</a>, fine hair-like extensions that increase the area available for secreting the cheta. When the cheta is complete, the microvilli withdraw into the chetoblast, leaving parallel tunnels that run almost the full length of the cheta.<a class="footnote-ref" id="fnref:71" href="#fn:71"><sup>71</sup></a> Hence annelids' chetae are structurally different from the <a href="/facts/Seta/jUmts6C2">setae</a> ("bristles") of <a href="/facts/Arthropod/JKJLTFzp">arthropods</a>, which are made of the more rigid α-chitin, have a single internal cavity, and are mounted on flexible joints in shallow pits in the cuticle.<a class="footnote-ref" id="fnref:72" href="#fn:72"><sup>72</sup></a>
</p><p>Nearly all polychaetes have parapodia that function as limbs, while other major annelid groups lack them. Parapodia are unjointed paired extensions of the body wall, and their muscles are derived from the circular muscles of the body. They are often supported internally by one or more large, thick chetae. The parapodia of burrowing and tube-dwelling polychaetes are often just ridges whose tips bear hooked chetae. In active crawlers and swimmers the parapodia are often divided into large upper and lower paddles on a very short trunk, and the paddles are generally fringed with chetae and sometimes with <a href="/facts/Cirrus_(biology)/INVUascX">cirri</a> (fused bundles of <a href="/facts/Cilia/C9l436HM">cilia</a>) and <a href="/facts/Gill/FWgM3Jp8">gills</a>.<a class="footnote-ref" id="fnref:73" href="#fn:73"><sup>73</sup></a>
</p>

<h3>Nervous system and senses</h3>
<p>The <a href="/facts/Brain/EKla9NkJ">brain</a> generally forms a ring round the <a href="/facts/Pharynx/51F3eVms">pharynx</a> (throat), consisting of a pair of <a href="/facts/Ganglia/Q5rx1UlU">ganglia</a> (local control centers) above and in front of the pharynx, linked by nerve cords either side of the pharynx to another pair of ganglia just below and behind it.<a class="footnote-ref" id="fnref:74" href="#fn:74"><sup>74</sup></a> The brains of <a href="/facts/Polychaete/Uo2YIINM">polychaetes</a> are generally in the prostomium, while those of clitellates are in the peristomium or sometimes the first segment behind the prostomium.<a class="footnote-ref" id="fnref:75" href="#fn:75"><sup>75</sup></a> In some very mobile and active <a href="/facts/Polychaete/Uo2YIINM">polychaetes</a> the brain is enlarged and more complex, with visible hindbrain, midbrain and forebrain sections.<a class="footnote-ref" id="fnref:76" href="#fn:76"><sup>76</sup></a> The rest of the <a href="/facts/Central_nervous_system/k8I6To97">central nervous system</a>, the <a href="/facts/Ventral_nerve_cord/s7Q7toBB">ventral nerve cord</a>, is generally "ladder-like", consisting of a pair of nerve cords that run through the bottom part of the body and have in each segment paired ganglia linked by a transverse connection. From each <a href="/facts/Segmental_ganglion/482haoAp">segmental ganglion</a> a branching system of local nerves runs into the body wall and then encircles the body.<a class="footnote-ref" id="fnref:77" href="#fn:77"><sup>77</sup></a> However, in most polychaetes the two main nerve cords are fused, and in the tube-dwelling <a href="/facts/Genus/m8EFjSms">genus</a> <i>Owenia</i> the single nerve chord has no ganglia and is located in the <a href="/facts/Epidermis_(skin)/JvRABubP">epidermis</a>.<a class="footnote-ref" id="fnref:78" href="#fn:78"><sup>78</sup></a><a class="footnote-ref" id="fnref:79" href="#fn:79"><sup>79</sup></a>
</p><p>As in <a href="/facts/Arthropod/JKJLTFzp">arthropods</a>, each muscle fiber (cell) is controlled by more than one <a href="/facts/Neuron/G7CyTVdH">neuron</a>, and the speed and power of the fiber's contractions depends on the combined effects of all its neurons. <a href="/facts/Vertebrate/tT54FSTT">Vertebrates</a> have a different system, in which one neuron controls a group of muscle fibers.<a class="footnote-ref" id="fnref:80" href="#fn:80"><sup>80</sup></a> Most annelids' longitudinal nerve trunks include giant <a href="/facts/Axon/5DCd4wLo">axons</a> (the output signal lines of nerve cells). Their large diameter decreases their resistance, which allows them to transmit signals exceptionally fast. This enables these worms to withdraw rapidly from danger by shortening their bodies. Experiments have shown that cutting the giant axons prevents this escape response but does not affect normal movement.<a class="footnote-ref" id="fnref:81" href="#fn:81"><sup>81</sup></a>
</p><p>The sensors are primarily single cells that detect light, chemicals, pressure waves and contact, and are present on the head, appendages (if any) and other parts of the body.<a class="footnote-ref" id="fnref:82" href="#fn:82"><sup>82</sup></a> Nuchal ("on the neck") organs are paired, <a href="/facts/Cilia/C9l436HM">ciliated</a> structures found only in polychaetes, and are thought to be <a href="/facts/Chemoreceptor/KC6IDxzf">chemosensors</a>.<a class="footnote-ref" id="fnref:83" href="#fn:83"><sup>83</sup></a> Some polychaetes also have various combinations of <a href="/facts/Ocelli/zV5g5Eqo">ocelli</a> ("little eyes") that detect the direction from which light is coming and <a href="/facts/Camera_eye/h3lbYU55">camera eyes</a> or <a href="/facts/Compound_eye/yoRE9HWY">compound eyes</a> that can probably form images.<a class="footnote-ref" id="fnref:84" href="#fn:84"><sup>84</sup></a><a class="footnote-ref" id="fnref:85" href="#fn:85"><sup>85</sup></a> The compound eyes probably <a href="/facts/Convergent_evolution/dd0L8z4R">evolved independently</a> of arthropods' eyes.<a class="footnote-ref" id="fnref:86" href="#fn:86"><sup>86</sup></a> Some tube-worms use ocelli widely spread over their bodies to detect the shadows of fish, so that they can quickly withdraw into their tubes.<a class="footnote-ref" id="fnref:87" href="#fn:87"><sup>87</sup></a> Some burrowing and tube-dwelling polychaetes have <a href="/facts/Statocyst/yzPr5Ey5">statocysts</a> (tilt and balance sensors) that indicate which way is down.<a class="footnote-ref" id="fnref:88" href="#fn:88"><sup>88</sup></a> A few polychaete <a href="/facts/Genus/m8EFjSms">genera</a> have on the undersides of their heads palps that are used both in feeding and as "feelers", and some of these also have antennae that are structurally similar but probably are used mainly as "feelers".<a class="footnote-ref" id="fnref:89" href="#fn:89"><sup>89</sup></a>
</p>
<h3>Coelom, locomotion and circulatory system</h3>
<p>Most annelids have a pair of <a href="/facts/Coelom/PD25dd6m">coelomata</a> (body cavities) in each segment, separated from other segments by <a href="/facts/Septum/lPNxGG9u">septa</a> and from each other by vertical <a href="/facts/Mesentery_(zoology)/7Ac2v224">mesenteries</a>. Each septum forms a sandwich with <a href="/facts/Connective_tissue/mal3fc7k">connective tissue</a> in the middle and <a href="/facts/Mesothelium/0PDq8h0o">mesothelium</a> (<a href="/facts/Mucous_membrane/oIw0vyel">membrane</a> that serves as a lining) from the preceding and following segments on either side. Each mesentery is similar except that the mesothelium is the lining of each of the pair of coelomata, and the blood vessels and, in polychaetes, the main nerve cords are embedded in it.<a class="footnote-ref" id="fnref:90" href="#fn:90"><sup>90</sup></a>  The mesothelium is made of modified epitheliomuscular cells;<a class="footnote-ref" id="fnref:91" href="#fn:91"><sup>91</sup></a> in other words, their bodies form part of the epithelium but their bases extend to form <a href="/facts/Muscle/dmxrL8pH">muscle</a> fibers in the body wall.<a class="footnote-ref" id="fnref:92" href="#fn:92"><sup>92</sup></a> The mesothelium may also form radial and circular muscles on the septa, and circular muscles around the blood vessels and gut. Parts of the mesothelium, especially on the outside of the gut, may also form <a href="/facts/Chloragogen_cell/qBfU9TwW">chloragogen cells</a> that perform similar functions to the <a href="/facts/Liver/Rtp4Fk2b">livers</a> of vertebrates: producing and storing <a href="/facts/Glycogen/kWa8fPFE">glycogen</a> and <a href="/facts/Fat/87HcP11s">fat</a>; producing the <a href="/facts/Oxygen/acyn6o8r">oxygen</a>-carrier <a href="/facts/Hemoglobin/RsD1MzYA">hemoglobin</a>; breaking down <a href="/facts/Protein/Jxmagu3E">proteins</a>; and turning <a href="/facts/Nitrogen/Nf1QpGDz">nitrogenous</a> waste products into <a href="/facts/Ammonia/MtLtJFtz">ammonia</a> and <a href="/facts/Urea/6j84AoCG">urea</a> to be <a href="/facts/Excretion/mspsTfnv">excreted</a>.<a class="footnote-ref" id="fnref:93" href="#fn:93"><sup>93</sup></a>
</p>

<p>Many annelids move by <a href="/facts/Peristalsis/R6zzQb1d">peristalsis</a> (waves of contraction and expansion that sweep along the body),<a class="footnote-ref" id="fnref:94" href="#fn:94"><sup>94</sup></a> or flex the body while using <a href="/facts/Parapodia/QjzQVE3y">parapodia</a>  to crawl or swim.<a class="footnote-ref" id="fnref:95" href="#fn:95"><sup>95</sup></a> In these animals the septa enable the circular and longitudinal muscles to change the shape of individual segments, by making each segment a separate fluid-filled "balloon".<a class="footnote-ref" id="fnref:96" href="#fn:96"><sup>96</sup></a> However, the septa are often incomplete in annelids that are semi-<a href="/facts/Sessility_(zoology)/f0xV4VV1">sessile</a> or that do not move by peristalsis or by movements of parapodia – for example some move by whipping movements of the body, some small marine species move by means of <a href="/facts/Cilia/C9l436HM">cilia</a> (fine muscle-powered hairs) and some burrowers turn their pharynges (throats) inside out to penetrate the sea-floor and drag themselves into it.<a class="footnote-ref" id="fnref:97" href="#fn:97"><sup>97</sup></a>
</p><p>The fluid in the coelomata contains coelomocyte cells that defend the animals against parasites and infections. In some species coelomocytes may also contain a <a href="/facts/Respiratory_pigment/gtwTJS3M">respiratory pigment</a> – red <a href="/facts/Hemoglobin/RsD1MzYA">hemoglobin</a> in some species, green <a href="/facts/Chlorocruorin/Loj6M1W8">chlorocruorin</a> in others (dissolved in the plasma)<a class="footnote-ref" id="fnref:98" href="#fn:98"><sup>98</sup></a> – and provide oxygen transport within their segments. Respiratory pigment is also dissolved in the <a href="/facts/Blood_plasma/oHuwwugG">blood plasma</a>. Species with well-developed septa generally also have blood vessels running all long their bodies above and below the gut, the upper one carrying blood forwards while the lower one carries it backwards. Networks of <a href="/facts/Capillary/4BIzKJAe">capillaries</a> in the body wall and around the gut transfer blood between the main blood vessels and to parts of the segment that need oxygen and nutrients. Both of the major vessels, especially the upper one, can pump blood by contracting. In some annelids the forward end of the upper blood vessel is enlarged with muscles to form a heart, while in the forward ends of many <a href="/facts/Earthworm/V7IUvWgQ">earthworms</a> some of the vessels that connect the upper and lower main vessels function as hearts. Species with poorly developed or no septa generally have no blood vessels and rely on the circulation within the coelom for delivering nutrients and oxygen.<a class="footnote-ref" id="fnref:99" href="#fn:99"><sup>99</sup></a>
</p><p>However, <a href="/facts/Leech/eYSMx97F">leeches</a> and their closest relatives have a body structure that is very uniform within the group but significantly different from that of other annelids, including other members of the Clitellata.<a class="footnote-ref" id="fnref:100" href="#fn:100"><sup>100</sup></a> In leeches there are no septa, the connective tissue layer of the body wall is so thick that it occupies much of the body, and the two coelomata are widely separated and run the length of the body. They function as the main blood vessels, although they are side-by-side rather than upper and lower. However, they are lined with mesothelium, like the coelomata and unlike the blood vessels of other annelids. Leeches generally use suckers at their front and rear ends to move like <a href="/facts/Geometer_moth/ces6X6JW">inchworms</a>. The anus is on the upper surface of the pygidium.<a class="footnote-ref" id="fnref:101" href="#fn:101"><sup>101</sup></a>
</p>
<h3>Respiration</h3>
<p>In some annelids, including <a href="/facts/Earthworm/V7IUvWgQ">earthworms</a>, all <a href="/facts/Respiration_(physiology)/KKWAJIbd">respiration</a> is via the skin. However, many <a href="/facts/Polychaete/Uo2YIINM">polychaetes</a> and some <a href="/facts/Clitellate/2lD2UWIu">clitellates</a> (the group to which earthworms belong) have <a href="/facts/Gill/FWgM3Jp8">gills</a> associated with most segments, often as extensions of the <a href="/facts/Parapodia/QjzQVE3y">parapodia</a> in polychaetes. The gills of tube-dwellers and burrowers usually cluster around whichever end has the stronger water flow.<a class="footnote-ref" id="fnref:102" href="#fn:102"><sup>102</sup></a>
</p>
<h3>Feeding and excretion</h3>

<p>Feeding structures in the mouth region vary widely, and have little correlation with the animals' diets. Many polychaetes have a muscular <a href="/facts/Pharynx/51F3eVms">pharynx</a> that can be everted (turned inside out to extend it). In these animals the foremost few segments often lack septa so that, when the muscles in these segments contract, the sharp increase in fluid pressure from all these segments everts the pharynx very quickly. Two <a href="/facts/Family_(biology)/qTrn6E8E">families</a>, the <a href="/facts/Eunicidae/oblkIIf3">Eunicidae</a> and <a href="/facts/Phyllodocidae/WlGRRB7y">Phyllodocidae</a>, have evolved jaws, which can be used for seizing prey, biting off pieces of vegetation, or grasping dead and decaying matter. On the other hand, some predatory polychaetes have neither jaws nor eversible pharynges.  Selective deposit feeders generally live in tubes on the sea-floor and use palps to find food particles in the sediment and then wipe them into their mouths. <a href="/facts/Filter_feeding/hmVLwWB5">Filter feeders</a> use "crowns" of palps covered in <a href="/facts/Cilia/C9l436HM">cilia</a> that wash food particles towards their mouths. Non-selective deposit feeders ingest soil or marine <a href="/facts/Sediment/zsIH1A6U">sediments</a> via mouths that are generally unspecialized. Some <a href="/facts/Clitellata/2lD2UWIu">clitellates</a> have sticky pads in the roofs of their mouths, and some of these can evert the pads to capture prey. Leeches often have an eversible proboscis, or a muscular pharynx with two or three teeth.<a class="footnote-ref" id="fnref:103" href="#fn:103"><sup>103</sup></a>
</p><p>The gut is generally an almost straight tube supported by the mesenteries (vertical partitions within segments), and ends with the <a href="/facts/Anus/KoCBptr5">anus</a> on the underside of the pygidium.<a class="footnote-ref" id="fnref:104" href="#fn:104"><sup>104</sup></a> However, in members of the tube-dwelling family <a href="/facts/Siboglinidae/2clzTfUY">Siboglinidae</a> the gut is blocked by a swollen lining that houses <a href="/facts/Symbiotic/MrBArRGa">symbiotic</a> <a href="/facts/Bacteria/wC8xSCsU">bacteria</a>, which can make up 15% of the worms' total weight. The bacteria convert <a href="/facts/Inorganic/TfFfzxSz">inorganic</a> matter – such as <a href="/facts/Hydrogen_sulfide/UbCqjMMk">hydrogen sulfide</a> and <a href="/facts/Carbon_dioxide/xGzpppEp">carbon dioxide</a> from <a href="/facts/Hydrothermal_vents/0xPqNUkY">hydrothermal vents</a>, or <a href="/facts/Methane/kBHneYnh">methane</a> from <a href="/facts/Petroleum_seep/h4MIL1wy">seeps</a> – to organic matter that feeds themselves and their hosts, while the worms extend their palps into the gas flows to absorb the gases needed by the bacteria.<a class="footnote-ref" id="fnref:105" href="#fn:105"><sup>105</sup></a>
</p><p>Annelids with blood vessels use <a href="/facts/Metanephridia/YdMaNPHn">metanephridia</a> to remove soluble waste products, while those without use <a href="/facts/Protonephridia/YdMaNPHn">protonephridia</a>.<a class="footnote-ref" id="fnref:106" href="#fn:106"><sup>106</sup></a> Both of these systems use a two-stage filtration process, in which fluid and waste products are first extracted and these are filtered again to re-absorb any re-usable materials while dumping toxic and spent materials as <a href="/facts/Urine/oNUYVtvX">urine</a>. The difference is that protonephridia combine both filtration stages in the same organ, while metanephridia perform only the second filtration and rely on other mechanisms for the first – in annelids special filter cells in the walls of the blood vessels let fluids and other small molecules pass into the coelomic fluid, where it circulates to the metanephridia.<a class="footnote-ref" id="fnref:107" href="#fn:107"><sup>107</sup></a> In annelids the points at which fluid enters the protonephridia or metanephridia are on the forward side of a septum while the second-stage filter and the nephridiopore (exit opening in the body wall) are in the following segment. As a result, the hindmost segment (before the growth zone and pygidium) has no structure that extracts its wastes, as there is no following segment to filter and discharge them, while the first segment contains an extraction structure that passes wastes to the second, but does not contain the structures that re-filter and discharge urine.<a class="footnote-ref" id="fnref:108" href="#fn:108"><sup>108</sup></a>
</p>
<h3>Reproduction and life cycle</h3>
<h4>Asexual reproduction</h4>

<p><a href="/facts/Polychaete/Uo2YIINM">Polychaetes</a> can reproduce asexually, by dividing into two or more pieces or by <a href="/facts/Budding/TEB3zXxB">budding</a> off a new individual while the parent remains a complete organism.<a class="footnote-ref" id="fnref:109" href="#fn:109"><sup>109</sup></a><a class="footnote-ref" id="fnref:110" href="#fn:110"><sup>110</sup></a> Some <a href="/facts/Oligochaete/08sBMVpF">oligochaetes</a>, such as <i><a href="/facts/Aulophorus/Amr2hUg4">Aulophorus</a> furcatus</i>, seem to reproduce entirely asexually, while others reproduce asexually in summer and sexually in autumn. Asexual reproduction in oligochaetes is always by dividing into two or more pieces, rather than by budding.<a class="footnote-ref" id="fnref:111" href="#fn:111"><sup>111</sup></a><a class="footnote-ref" id="fnref:112" href="#fn:112"><sup>112</sup></a> However, <a href="/facts/Leech/eYSMx97F">leeches</a> have never been seen reproducing asexually.<a class="footnote-ref" id="fnref:113" href="#fn:113"><sup>113</sup></a><a class="footnote-ref" id="fnref:114" href="#fn:114"><sup>114</sup></a>
</p><p>Most polychaetes and oligochaetes also use similar mechanisms to regenerate after suffering damage. Two polychaete <a href="/facts/Genus/m8EFjSms">genera</a>, <i><a href="/facts/Chaetopterus/ISUWdNtR">Chaetopterus</a></i> and <i><a href="/facts/Dodecaceria/bCIVx5vu">Dodecaceria</a></i>, can regenerate from a single segment, and others can regenerate even if their heads are removed.<a class="footnote-ref" id="fnref:115" href="#fn:115"><sup>115</sup></a><a class="footnote-ref" id="fnref:116" href="#fn:116"><sup>116</sup></a> Annelids are the most complex animals that can regenerate after such severe damage.<a class="footnote-ref" id="fnref:117" href="#fn:117"><sup>117</sup></a> On the other hand, leeches cannot regenerate.<a class="footnote-ref" id="fnref:118" href="#fn:118"><sup>118</sup></a>
</p>
<h4>Sexual reproduction</h4>

<p>It is thought that annelids were originally animals with two separate <a href="/facts/Sex/ssBol2pP">sexes</a>, which released <a href="/facts/Ovum/gJyW3W3W">ova</a> and <a href="/facts/Sperm/RCxpXygQ">sperm</a> into the water via their <a href="/facts/Nephridia/YdMaNPHn">nephridia</a>.<a class="footnote-ref" id="fnref:119" href="#fn:119"><sup>119</sup></a> The fertilized eggs develop into <a href="/facts/Trochophore/bSNMnxRv">trochophore</a> <a href="/facts/Larva/GKo2FIew">larvae</a>, which live as <a href="/facts/Plankton/WGo3LU9T">plankton</a>.<a class="footnote-ref" id="fnref:120" href="#fn:120"><sup>120</sup></a> Later they sink to the sea-floor and <a href="/facts/Metamorphose/nKDTSjsK">metamorphose</a> into miniature adults: the part of the trochophore between the apical tuft and the prototroch becomes the prostomium (head);  a small area round the trochophore's <a href="/facts/Anus/KoCBptr5">anus</a> becomes the pygidium (tail-piece); a narrow band immediately in front of that becomes the growth zone that produces new segments; and the rest of the trochophore becomes the peristomium (the segment that contains the mouth).<a class="footnote-ref" id="fnref:121" href="#fn:121"><sup>121</sup></a>
</p><p>However, the lifecycles of most living <a href="/facts/Polychaete/Uo2YIINM">polychaetes</a>, which are almost all marine animals, are unknown, and only about 25% of the 300+ species whose lifecycles are known follow this pattern. About 14% use a similar <a href="/facts/External_fertilization/PB4mUeJx">external fertilization</a> but produce <a href="/facts/Yolk/IcvGBDFJ">yolk</a>-rich eggs, which reduce the time the larva needs to spend among the plankton, or eggs from which miniature adults emerge rather than larvae. The rest care for the fertilized eggs until they hatch – some by producing jelly-covered masses of eggs which they tend, some by attaching the eggs to their bodies and a few species by keeping the eggs within their bodies until they hatch. These species use a variety of methods for sperm transfer; for example, in some the females collect sperm released into the water, while in others the males have a <a href="/facts/Penis/s9zpC49k">penis</a> that inject sperm into the female.<a class="footnote-ref" id="fnref:122" href="#fn:122"><sup>122</sup></a> There is no guarantee that this is a representative sample of polychaetes' reproductive patterns, and it simply reflects scientists' current knowledge.<a class="footnote-ref" id="fnref:123" href="#fn:123"><sup>123</sup></a>
</p><p>Some polychaetes breed only once in their lives, while others breed almost continuously or through several breeding seasons. While most polychaetes remain of one sex all their lives, a significant percentage of species are full <a href="/facts/Hermaphrodite/C4aYIk4n">hermaphrodites</a> or change sex during their lives. Most polychaetes whose reproduction has been studied lack permanent <a href="/facts/Gonad/36BUam2x">gonads</a>, and it is uncertain how they produce ova and sperm. In a few species the rear of the body splits off and becomes a separate individual that lives just long enough to swim to a suitable environment, usually near the surface, and spawn.<a class="footnote-ref" id="fnref:124" href="#fn:124"><sup>124</sup></a>
</p><p>Most mature <a href="/facts/Clitellata/2lD2UWIu">clitellates</a> (the group that includes <a href="/facts/Earthworm/V7IUvWgQ">earthworms</a> and <a href="/facts/Leeches/eYSMx97F">leeches</a>) are full hermaphrodites, although in a few leech species younger adults function as males and become female at maturity. All have well-developed gonads, and all <a href="/facts/Copulation_(zoology)/rYcheMjL">copulate</a>. Earthworms store their partners' sperm in <a href="/facts/Spermatheca/fkBuN7Ia">spermathecae</a> ("sperm stores") and then the <a href="/facts/Clitellum/9gpJQ29l">clitellum</a> produces a <a href="/facts/Pupa/0u7NE8Cq">cocoon</a> that collects ova from the <a href="/facts/Ovary/z2hG6bNy">ovaries</a> and then sperm from the spermathecae. Fertilization and development of earthworm eggs takes place in the cocoon. Leeches' eggs are fertilized in the ovaries, and then transferred to the cocoon. In all clitellates the cocoon also either produces yolk when the eggs are fertilized or nutrients while they are developing. All clitellates hatch as miniature adults rather than larvae.<a class="footnote-ref" id="fnref:125" href="#fn:125"><sup>125</sup></a>
</p>
<h2 id="ecological-significance">Ecological significance</h2>
<p><a href="/facts/Charles_Darwin/uwDLGItY">Charles Darwin</a>'s book <i><a href="/facts/The_Formation_of_Vegetable_Mould_Through_the_Action_of_Worms/omVVFN2f">The Formation of Vegetable Mould Through the Action of Worms</a></i> (1881) presented the first scientific analysis of earthworms' contributions to <a href="/facts/Soil_fertility/E7ofTY0f">soil fertility</a>.<a class="footnote-ref" id="fnref:126" href="#fn:126"><sup>126</sup></a> Some burrow while others live entirely on the surface, generally in moist <a href="/facts/Leaf_litter/Y0aJpt4P">leaf litter</a>. The burrowers loosen the <a href="/facts/Soil/5Ddpr3QM">soil</a> so that oxygen and water can penetrate it, and both surface and burrowing worms help to produce soil by mixing organic and mineral matter, by accelerating the <a href="/facts/Decomposition/6Bve4pgl">decomposition</a> of organic matter and thus making it more quickly available to other organisms, and by concentrating minerals and converting them to forms that plants can use more easily.<a class="footnote-ref" id="fnref:127" href="#fn:127"><sup>127</sup></a><a class="footnote-ref" id="fnref:128" href="#fn:128"><sup>128</sup></a> Earthworms are also important prey for birds ranging in size from <a href="/facts/American_robin/7qL16gSR">robins</a> to <a href="/facts/Stork/JwsYJyYd">storks</a>, and for mammals ranging from <a href="/facts/Shrew/FqIA64gX">shrews</a> to <a href="/facts/Badger/b7dIdTmM">badgers</a>, and in some cases conserving earthworms may be essential for conserving endangered birds.<a class="footnote-ref" id="fnref:129" href="#fn:129"><sup>129</sup></a>
</p><p>Terrestrial annelids can be invasive in some situations. In the glaciated areas of North America, for example, almost all native earthworms are thought to have been killed by the glaciers and the worms currently found in those areas are all introduced from other areas, primarily from Europe, and, more recently, from Asia.  Northern hardwood forests are especially negatively impacted by invasive worms through the loss of leaf duff, soil fertility, changes in soil chemistry and the loss of ecological diversity. Especially of concern is <i><a href="/facts/Amynthas_agrestis/GXR1Po90">Amynthas agrestis</a></i> and at least one state (Wisconsin) has listed it as a prohibited species.
</p><p>Earthworms migrate only a limited distance annually on their own, and the spread of invasive worms is increased rapidly by anglers and from worms or their cocoons in the dirt on vehicle tires or footwear.
</p><p>Marine annelids may account for over one-third of bottom-dwelling animal species around <a href="/facts/Coral_reef/xzeK6yAp">coral reefs</a> and in <a href="/facts/Tidal_zone/grnBDVQ3">tidal zones</a>.<a class="footnote-ref" id="fnref:130" href="#fn:130"><sup>130</sup></a> Burrowing species increase the penetration of water and oxygen into the sea-floor <a href="/facts/Sediment/zsIH1A6U">sediment</a>, which encourages the growth of populations of <a href="/facts/Aerobic_organism/a629EQD7">aerobic bacteria</a> and small animals alongside their burrows.<a class="footnote-ref" id="fnref:131" href="#fn:131"><sup>131</sup></a>
</p><p>Although blood-sucking <a href="/facts/Leech/eYSMx97F">leeches</a> do little direct harm to their victims, some transmit <a href="/facts/Flagellate/0XWJTXX6">flagellates</a> that can be very dangerous to their hosts. Some small tube-dwelling <a href="/facts/Oligochaete/08sBMVpF">oligochaetes</a> transmit <a href="/facts/Myxosporea/V15GS41p">myxosporean</a> <a href="/facts/Parasite/HWPcgky7">parasites</a> that cause <a href="/facts/Whirling_disease/4Gc41C3g">whirling disease</a> in fish.<a class="footnote-ref" id="fnref:132" href="#fn:132"><sup>132</sup></a>
</p>
<h2 id="interaction-with-humans">Interaction with humans</h2>
<p>Earthworms make a significant contribution to <a href="/facts/Soil_fertility/E7ofTY0f">soil fertility</a>.<a class="footnote-ref" id="fnref:133" href="#fn:133"><sup>133</sup></a> The rear end of the <a href="/facts/Palolo_worm/r505PTpU">Palolo worm</a>, a marine <a href="/facts/Polychaete/Uo2YIINM">polychaete</a> that tunnels through coral, detaches in order to spawn at the surface, and the people of <a href="/facts/Samoa/lyQ1B9YV">Samoa</a> regard these spawning modules as a delicacy.<a class="footnote-ref" id="fnref:134" href="#fn:134"><sup>134</sup></a> <a href="/facts/Angling/FaWYQkUU">Anglers</a> sometimes find that worms are more effective bait than artificial flies, and worms can be kept for several days in a tin lined with damp moss.<a class="footnote-ref" id="fnref:135" href="#fn:135"><sup>135</sup></a>  <a href="/facts/Ragworm/AFzrnwE9">Ragworms</a> are commercially important as bait and as food sources for <a href="/facts/Aquaculture/MYdIMfbt">aquaculture</a>, and there have been proposals to farm them in order to reduce over-fishing of their natural populations.<a class="footnote-ref" id="fnref:136" href="#fn:136"><sup>136</sup></a> Some marine <a href="/facts/Polychaete/Uo2YIINM">polychaetes</a>' predation on molluscs causes serious losses to fishery and <a href="/facts/Aquaculture/MYdIMfbt">aquaculture</a> operations.<a class="footnote-ref" id="fnref:137" href="#fn:137"><sup>137</sup></a>
</p><p>Scientists study aquatic annelids to monitor the oxygen content, salinity and pollution levels in fresh and marine water.<a class="footnote-ref" id="fnref:138" href="#fn:138"><sup>138</sup></a>
</p><p>Accounts of the use of <a href="/facts/Leech/eYSMx97F">leeches</a> for the medically dubious practice of <a href="/facts/Blood-letting/rEgXWbCK">blood-letting</a> have come from China around 30 AD, India around 200 AD, ancient Rome around 50 AD and later throughout Europe. In the 19th century medical demand for leeches was so high that some areas' stocks were exhausted and other regions imposed restrictions or bans on exports, and <i><a href="/facts/Hirudo_medicinalis/RNUMEfdc">Hirudo medicinalis</a></i> is treated as an endangered species by both <a href="/facts/IUCN/QwV8JXup">IUCN</a> and <a href="/facts/CITES/hCahxMSr">CITES</a>. More recently leeches have been used to assist in <a href="/facts/Microsurgery/336jh7FH">microsurgery</a>, and their <a href="/facts/Saliva/gK4nvayc">saliva</a> has provided <a href="/facts/Anti-inflammatory/W27poSUd">anti-inflammatory</a> compounds and several important <a href="/facts/Anticoagulant/cARpjm0G">anticoagulants</a>, one of which also prevents <a href="/facts/Tumor/K0ytzmPz">tumors</a> from <a href="/facts/Metastasis/4dadbm6z">spreading</a>.<a class="footnote-ref" id="fnref:139" href="#fn:139"><sup>139</sup></a>
</p><p>Ragworms' jaws are strong but much lighter than the hard parts of many other organisms, which are <a href="/facts/Biomineralization/m5mGPaTM">biomineralized</a> with <a href="/facts/Calcium/hf252CC0">calcium</a> salts. These advantages have attracted the attention of engineers. Investigations showed that ragworm jaws are made of unusual  <a href="/facts/Protein/Jxmagu3E">proteins</a> that bind strongly to <a href="/facts/Zinc/80b1qgk3">zinc</a>.<a class="footnote-ref" id="fnref:140" href="#fn:140"><sup>140</sup></a>
</p>
<h2 id="evolutionary-history">Evolutionary history</h2>
<p class="note">See also: <a href="/facts/List_of_annelid_families/XSra4PYo">List of annelid families</a></p>
<h3>Fossil record</h3>

<p>Since annelids are <a href="/facts/Soft-bodied_organisms/V99XGUMp">soft-bodied</a>, their fossils are rare.<a class="footnote-ref" id="fnref:141" href="#fn:141"><sup>141</sup></a> <a href="/facts/Polychaete/Uo2YIINM">Polychaetes</a>' fossil record consists mainly of the jaws that some species had and the <a href="/facts/Mineral/LkUFbNmv">mineralized</a> tubes that some secreted.<a class="footnote-ref" id="fnref:142" href="#fn:142"><sup>142</sup></a> Some <a href="/facts/Ediacaran_biota/1lx1CXxT">Ediacaran</a> fossils such as <i><a href="/facts/Dickinsonia/Rs0Eqyco">Dickinsonia</a></i> in some ways resemble <a href="/facts/Polychaete/Uo2YIINM">polychaetes</a>, but the similarities are too vague for these fossils to be classified with confidence.<a class="footnote-ref" id="fnref:143" href="#fn:143"><sup>143</sup></a> The <a href="/facts/Small_shelly_fossil/5Ek2F3BX">small shelly fossil</a> <i><a href="/facts/Cloudina/ujnTmI8x">Cloudina</a></i>, from <a href="https://geoltime.github.io/?Ma=549–542">549 to 542</a> million years ago, has been classified by some authors as an annelid, but by others as a <a href="/facts/Cnidarian/d0HRc6w0">cnidarian</a> (i.e. in the phylum to which <a href="/facts/Jellyfish/oLmExpvU">jellyfish</a> and <a href="/facts/Sea_anemone/uWTgHLzS">sea anemones</a> belong).<a class="footnote-ref" id="fnref:144" href="#fn:144"><sup>144</sup></a><a class="footnote-ref" id="fnref:145" href="#fn:145"><sup>145</sup></a> Until 2008 the earliest fossils widely accepted as annelids were the polychaetes <i><a href="/facts/Canadia_(annelid)/bszNaQCL">Canadia</a></i> and <i><a href="/facts/Burgessochaeta/l05DNTyS">Burgessochaeta</a></i>, both from Canada's <a href="/facts/Burgess_Shale/4NX5HqIM">Burgess Shale</a>, formed about <a href="https://geoltime.github.io/?Ma=505">505</a> million years ago in the <a href="/facts/Miaolingian/5NcBhyEU">Middle</a> <a href="/facts/Cambrian/jIfzKtod">Cambrian</a>.<a class="footnote-ref" id="fnref:146" href="#fn:146"><sup>146</sup></a> <i><a href="/facts/Myoscolex/WH98prhp">Myoscolex</a></i>, found in Australia and a little older than the Burgess Shale, was possibly an annelid. However, it lacks some typical annelid features and has features which are not usually found in annelids and some of which are associated with other phyla.<a class="footnote-ref" id="fnref:147" href="#fn:147"><sup>147</sup></a>  Then <a href="/facts/Simon_Conway_Morris/c4kHVblJ">Simon Conway Morris</a> and John Peel reported <i><a href="/facts/Phragmochaeta_canicularis/nXXlkzm3">Phragmochaeta</a></i> from <a href="/facts/Sirius_Passet/LQUUUS8T">Sirius Passet</a>, about <a href="https://geoltime.github.io/?Ma=518">518</a> million years old, and concluded that it was the oldest annelid known to date.<a class="footnote-ref" id="fnref:148" href="#fn:148"><sup>148</sup></a> There has been vigorous debate about whether the Burgess Shale fossil <i><a href="/facts/Wiwaxia/gIFtCpjT">Wiwaxia</a></i> was a <a href="/facts/Mollusc/1elgknsQ">mollusc</a> or an annelid.<a class="footnote-ref" id="fnref:149" href="#fn:149"><sup>149</sup></a> Polychaetes diversified in the early <a href="/facts/Ordovician/BjMrLIEA">Ordovician</a>, about <a href="https://geoltime.github.io/?Ma=488–474">488 to 474</a> million years ago.  It is not until the early Ordovician that the first annelid jaws are found, thus the crown-group cannot have appeared before this date and probably appeared somewhat later.<a class="footnote-ref" id="fnref:150" href="#fn:150"><sup>150</sup></a> By the end of the <a href="/facts/Carboniferous/BRKlkYyd">Carboniferous</a>, about <a href="https://geoltime.github.io/?Ma=299">299</a> million years ago, fossils of most of the modern mobile polychaete groups had appeared.<a class="footnote-ref" id="fnref:151" href="#fn:151"><sup>151</sup></a> Many fossil tubes look like those made by modern <a href="/facts/Sessility_(zoology)/f0xV4VV1">sessile</a> polychaetes,<a class="footnote-ref" id="fnref:152" href="#fn:152"><sup>152</sup></a> but the first tubes clearly produced by polychaetes date from the <a href="/facts/Jurassic/o5Cy4KzX">Jurassic</a>, less than <a href="https://geoltime.github.io/?Ma=199">199</a> million years ago.<a class="footnote-ref" id="fnref:153" href="#fn:153"><sup>153</sup></a> In 2012, a 508 million year old species of annelid found near the <a href="/facts/Burgess_shale/4NX5HqIM">Burgess shale</a> beds in <a href="/facts/British_Columbia/Ty3e3g1Q">British Columbia</a>, <i><a href="/facts/Kootenayscolex/eEf2MpBX">Kootenayscolex</a></i>, was found that changed the hypotheses about how the annelid head developed. It appears to have bristles on its head segment akin to those along its body, as if the head simply developed as a specialized version of a previously generic segment.
</p><p>The earliest good evidence for <a href="/facts/Oligochaete/08sBMVpF">oligochaetes</a> occurs in the <a href="/facts/Tertiary/Krcz3olj">Tertiary</a> period, which began <a href="https://geoltime.github.io/?Ma=65">65</a> million years ago, and it has been suggested that these animals evolved around the same time as <a href="/facts/Flowering_plant/E5eGYIvY">flowering plants</a> in the early <a href="/facts/Cretaceous/nwKtFtKx">Cretaceous</a>, from <a href="https://geoltime.github.io/?Ma=130–90">130 to 90</a> million years ago.<a class="footnote-ref" id="fnref:154" href="#fn:154"><sup>154</sup></a> A <a href="/facts/Trace_fossil/kaucgzNj">trace fossil</a> consisting of a convoluted <a href="/facts/Burrow/vgY0GpEH">burrow</a> partly filled with small <a href="/facts/Fecal/r9HDKB8G">fecal</a> pellets may be evidence that earthworms were present in the early <a href="/facts/Triassic/r6CNxKXC">Triassic</a> period from <a href="https://geoltime.github.io/?Ma=251–245">251 to 245</a> million years ago.<a class="footnote-ref" id="fnref:155" href="#fn:155"><sup>155</sup></a><a class="footnote-ref" id="fnref:156" href="#fn:156"><sup>156</sup></a> <a href="/facts/Body_fossil/by7NzIA1">Body fossils</a> going back to the mid <a href="/facts/Ordovician/BjMrLIEA">Ordovician</a>, from <a href="https://geoltime.github.io/?Ma=472–461">472 to 461</a> million years ago, have been tentatively classified as oligochaetes, but these identifications are uncertain and some have been disputed.<a class="footnote-ref" id="fnref:157" href="#fn:157"><sup>157</sup></a><a class="footnote-ref" id="fnref:158" href="#fn:158"><sup>158</sup></a>
</p>
<h3>Internal relationships</h3>
<p>Traditionally the annelids have been divided into two major groups, the <a href="/facts/Polychaete/Uo2YIINM">polychaetes</a> and <a href="/facts/Clitellata/2lD2UWIu">clitellates</a>. In turn the clitellates were divided into <a href="/facts/Oligochaete/08sBMVpF">oligochaetes</a>, which include <a href="/facts/Earthworm/V7IUvWgQ">earthworms</a>, and hirudinomorphs, whose best-known members are <a href="/facts/Leech/eYSMx97F">leeches</a>.<a class="footnote-ref" id="fnref:159" href="#fn:159"><sup>159</sup></a> For many years there was no clear arrangement of the approximately 80 polychaete <a href="/facts/Family_(biology)/qTrn6E8E">families</a> into higher-level groups.<a class="footnote-ref" id="fnref:160" href="#fn:160"><sup>160</sup></a> In 1997 Greg Rouse and Kristian Fauchald attempted a "first heuristic step in terms of bringing polychaete systematics to an acceptable level of rigour", based on anatomical structures, and divided polychaetes into:<a class="footnote-ref" id="fnref:161" href="#fn:161"><sup>161</sup></a>
</p>
<table><tbody><tr><td><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Echiura/CYxsCTIc">Echiura</a></p></td></tr><tr><td></td></tr><tr><td><a href="/facts/Articulata_hypothesis/e0Dfvbaf">Articulata</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Euarthropoda/JKJLTFzp">Euarthropoda</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Onychophora/CVn4wdwx">Onychophora</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td><a href="/facts/Annelida/PQvb7NAM">Annelida</a></td><td rowspan="2"><table><tbody><tr><td><a href="/facts/Palpata/rW8z82Sy">Palpata</a></td><td rowspan="2"><table><tbody><tr><td><a href="/facts/Canalipalpata/C3dm7jwX">Canalipalpata</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Sabellida/Fgga0rIQ">Sabellida</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Terebellida/BsNhsyqh">Terebellida</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Spionida/Usm2QSSS">Spionida</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td><a href="/facts/Aciculata/U2MUvNKa">Aciculata</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Phyllodocida/L3jkFY9r">Phyllodocida</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Eunicida/kNKqH9U3">Eunicida</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Scolecida/AAungNN7">Scolecida</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Sipuncula/uJxuIwvB">Sipuncula</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td>Morphological phylogeny of Annelida (1997)<a class="footnote-ref" id="fnref:162" href="#fn:162"><sup>162</sup></a></td></tr></tbody></table>
<ul><li><a href="/facts/Scolecida/AAungNN7">Scolecida</a>, less than 1,000 burrowing species that look rather like earthworms.<a class="footnote-ref" id="fnref:163" href="#fn:163"><sup>163</sup></a></li>
<li><a href="/facts/Palpata/rW8z82Sy">Palpata</a>, the great majority of polychaetes, divided into:
<ul><li><a href="/facts/Canalipalpata/C3dm7jwX">Canalipalpata</a>, which are distinguished by having long grooved palps that they use for feeding, and most of which live in tubes.<a class="footnote-ref" id="fnref:164" href="#fn:164"><sup>164</sup></a></li>
<li><a href="/facts/Aciculata/U2MUvNKa">Aciculata</a>, the most active polychaetes, which have parapodia reinforced by internal spines (aciculae).<a class="footnote-ref" id="fnref:165" href="#fn:165"><sup>165</sup></a></li></ul></li></ul>

<p>Also in 1997 Damhnait McHugh, using <a href="/facts/Molecular_phylogenetics/FysplHP5">molecular phylogenetics</a> to compare similarities and differences in one gene, presented a very different view, in which: the clitellates were an offshoot of one branch of the polychaete family tree; the <a href="/facts/Siboglinidae/2clzTfUY">pogonophorans</a> and <a href="/facts/Echiuran/CYxsCTIc">echiurans</a>, which for a few decades had been regarded as a separate <a href="/facts/Phylum/ELT3Jyax">phyla</a>, were placed on other branches of the polychaete tree.<a class="footnote-ref" id="fnref:166" href="#fn:166"><sup>166</sup></a> Subsequent molecular phylogenetics analyses on a similar scale presented similar conclusions.<a class="footnote-ref" id="fnref:167" href="#fn:167"><sup>167</sup></a>
</p><p>In 2007 Torsten Struck and colleagues compared three genes in 81 <a href="/facts/Taxon/caQC1e87">taxa</a>, of which nine were outgroups,<a class="footnote-ref" id="fnref:168" href="#fn:168"><sup>168</sup></a> in other words not considered closely related to annelids but included to give an indication of where the organisms under study are placed on the larger <a href="/facts/Tree_of_life_(science)/woHaYdLT">tree of life</a>.<a class="footnote-ref" id="fnref:169" href="#fn:169"><sup>169</sup></a> For a cross-check the study used an analysis of 11 genes (including the original 3) in ten taxa. This analysis agreed that clitellates, pogonophorans and echiurans were on various branches of the polychaete family tree. It also concluded that the classification of polychaetes into Scolecida, Canalipalpata and Aciculata was useless, as the members of these alleged groups were scattered all over the family tree derived from comparing the 81 taxa. It also placed <a href="/facts/Sipuncula/uJxuIwvB">sipunculans</a>, generally regarded at the time as a separate phylum, on another branch of the polychaete tree, and concluded that leeches were a sub-group of oligochaetes rather than their <a href="/facts/Cladistics/JasKVABk">sister-group</a> among the clitellates.<a class="footnote-ref" id="fnref:170" href="#fn:170"><sup>170</sup></a> Rouse accepted the analyses based on molecular phylogenetics,<a class="footnote-ref" id="fnref:171" href="#fn:171"><sup>171</sup></a> and their main conclusions are now the scientific consensus, although the details of the annelid family tree remain uncertain.<a class="footnote-ref" id="fnref:172" href="#fn:172"><sup>172</sup></a>
</p><p>In addition to re-writing the classification of annelids and three previously independent phyla, the molecular phylogenetics analyses undermine the emphasis that decades of previous writings placed on the importance of <a href="/facts/Segmentation_(biology)/S4Cw0UcN">segmentation</a> in the classification of <a href="/facts/Invertebrate/3CyqCVnN">invertebrates</a>. Polychaetes, which these analyses found to be the parent group, have completely segmented bodies, while polychaetes' echiurans and sipunculan offshoots are not segmented and pogonophores are segmented only in the rear parts of their bodies. It now seems that segmentation can appear and disappear much more easily in the course of evolution than was previously thought.<a class="footnote-ref" id="fnref:173" href="#fn:173"><sup>173</sup></a><a class="footnote-ref" id="fnref:174" href="#fn:174"><sup>174</sup></a> The 2007 study also noted that the ladder-like nervous system, which is associated with segmentation, is less universal than previously thought in both annelids and arthropods.<a class="footnote-ref" id="fnref:175" href="#fn:175"><sup>175</sup></a><a class="footnote-ref" id="fnref:176" href="#fn:176"><sup>176</sup></a>
</p><p>The updated <a href="/facts/Phylogenetic_tree/f5vVEpeY">phylogenetic tree</a> of the annelid phylum is comprised by a <a href="/facts/Evolutionary_grade/ssv1Meoz">grade</a> of basal groups of polychaetes: Palaeoannelida, Chaetopteriformia and the <a href="/facts/Amphinomida/ALaCDeyK">Amphinomida</a>/<a href="/facts/Sipuncula/uJxuIwvB">Sipuncula</a>/<i>Lobatocerebrum</i> clade. This grade is followed by <a href="/facts/Pleistoannelida/qPgs8bRq">Pleistoannelida</a>, the clade containing nearly all of annelid diversity, divided into two highly diverse groups: <a href="/facts/Sedentaria/mA941E6P">Sedentaria</a> and <a href="/facts/Errantia/U2MUvNKa">Errantia</a>. <a href="/facts/Sedentaria/mA941E6P">Sedentaria</a> contains the <a href="/facts/Clitellate/2lD2UWIu">clitellates</a>, <a href="/facts/Siboglinidae/2clzTfUY">pogonophorans</a>, <a href="/facts/Echiurans/CYxsCTIc">echiurans</a> and some archiannelids, as well as several polychaete groups. <a href="/facts/Errantia/U2MUvNKa">Errantia</a> contains the <a href="/facts/Eunicida/kNKqH9U3">eunicid</a> and <a href="/facts/Phyllodocida/L3jkFY9r">phyllodocid</a> polychaetes, and several archiannelids. Some small groups, such as the <a href="/facts/Myzostomida/Jguck1aa">Myzostomida</a>, are more difficult to place due to <a href="/facts/Long_branch_attraction/I3v1CrEN">long branching</a>, but belong to either one of these large groups.<a class="footnote-ref" id="fnref:177" href="#fn:177"><sup>177</sup></a><a class="footnote-ref" id="fnref:178" href="#fn:178"><sup>178</sup></a><a class="footnote-ref" id="fnref:179" href="#fn:179"><sup>179</sup></a><a class="footnote-ref" id="fnref:180" href="#fn:180"><sup>180</sup></a><a class="footnote-ref" id="fnref:181" href="#fn:181"><sup>181</sup></a>
</p>

<table><tbody><tr><td>Annelida</td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td>Palaeoannelida </td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Oweniidae/eHFy72hg">Oweniidae</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Magelonidae/4VLUDLm2">Magelonidae</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td>Chaetopteriformia </td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><i>Apistobranchus</i></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Psammodrilidae/m3p3oSHa">Psammodrilidae</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Chaetopteridae/ytSaS3nk">Chaetopteridae</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Amphinomida/ALaCDeyK">Amphinomida</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><i>Lobatocerebrum</i></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Sipuncula/uJxuIwvB">Sipuncula</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td><a href="/facts/Pleistoannelida/qPgs8bRq">Pleistoannelida</a> </td><td rowspan="2"><table><tbody><tr><td><a href="/facts/Errantia/U2MUvNKa">Errantia</a></td><td rowspan="2"><table><tbody><tr><td><a href="/facts/Aciculata/U2MUvNKa">Aciculata</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Eunicida/kNKqH9U3">Eunicida</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Phyllodocida/L3jkFY9r">Phyllodocida</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td><a href="/facts/Protodriliformia/xyGSlUh7">Protodriliformia</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Polygordiidae/4qbUkFxT">Polygordiidae</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Protodrilida/szKdw65r">Protodrilida</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td><i>?</i></td><td rowspan="2"><p><a href="/facts/Myzostomida/Jguck1aa">Myzostomida</a></p></td></tr><tr><td></td></tr><tr><td><a href="/facts/Sedentaria/mA941E6P">Sedentaria</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Orbiniida/AwvkKnQ9">Orbiniida</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p>Cirratuliformia</p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Siboglinidae/2clzTfUY">Siboglinidae</a> (pogonophorans)</p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Sabellida/Fgga0rIQ">Sabellida</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Spionida/Usm2QSSS">Spionida</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Opheliida/jg6dXH9k">Opheliida</a></p></td></tr><tr><td></td></tr><tr><td><a href="/facts/Capitellida/LxkWU1Bf">Capitellida</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Capitellidae/5ECoPzAq">Capitellidae</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Echiura/CYxsCTIc">Echiura</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Terebelliformia/3vPVAVBj">Terebelliformia</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Maldanomorpha/Cxa4ox8s">Maldanomorpha</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Clitellata/2lD2UWIu">Clitellata</a> (<a href="/facts/Oligochaete/08sBMVpF">oligochaetes</a>, <a href="/facts/Leech/eYSMx97F">leeches</a>...)</p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table>
<h3>External relationships</h3>
<p>Annelids are members of the <a href="/facts/Protostomes/XLtWYdt8">protostomes</a>, one of the two major <a href="/facts/Superphyla/ELT3Jyax">superphyla</a> of <a href="/facts/Bilaterian/nfUwVYrx">bilaterian</a> animals – the other is the <a href="/facts/Deuterostomes/Uc3V9mGA">deuterostomes</a>, which includes <a href="/facts/Vertebrate/tT54FSTT">vertebrates</a>.<a class="footnote-ref" id="fnref:182" href="#fn:182"><sup>182</sup></a> Within the protostomes, annelids used to be grouped with <a href="/facts/Arthropod/JKJLTFzp">arthropods</a> under the super-group <a href="/facts/Articulata_hypothesis/e0Dfvbaf">Articulata</a> ("jointed animals"), as segmentation is obvious in most members of both phyla. However, the genes that drive segmentation in arthropods do not appear to do the same in annelids. Arthropods and annelids both have close relatives that are unsegmented. It is at least as easy to assume that they evolved segmented bodies <a href="/facts/Convergent_evolution/dd0L8z4R">independently</a> as it is to assume that the ancestral protostome or bilaterian was segmented and that segmentation disappeared in many descendant phyla.<a class="footnote-ref" id="fnref:183" href="#fn:183"><sup>183</sup></a> The current view is that annelids are grouped with <a href="/facts/Mollusc/1elgknsQ">molluscs</a>, <a href="/facts/Brachiopod/XVBnEPj1">brachiopods</a> and several other phyla that have <a href="/facts/Lophophore/ESrVMezI">lophophores</a> (fan-like feeding structures) and/or <a href="/facts/Trochophore/bSNMnxRv">trochophore</a> larvae as members of <a href="/facts/Lophotrochozoa/sz5UzpHC">Lophotrochozoa</a>.<a class="footnote-ref" id="fnref:184" href="#fn:184"><sup>184</sup></a> Meanwhile, arthropods are now regarded as members of the <a href="/facts/Ecdysozoa/L3hxqnxA">Ecdysozoa</a> ("animals that <a href="/facts/Molt/2wdfTSL5">molt</a>"), along with some phyla that are unsegmented.<a class="footnote-ref" id="fnref:185" href="#fn:185"><sup>185</sup></a><a class="footnote-ref" id="fnref:186" href="#fn:186"><sup>186</sup></a>
</p>

<table><tbody><tr><td><a href="/facts/Bilateria/nfUwVYrx">Bilateria</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Acoelomorpha/9sf0zjLv">Acoelomorpha</a> (<a href="/facts/Acoela/WjUIboyJ">Acoela</a> and <a href="/facts/Nemertodermatida/YhPjLfMc">Nemertodermatida</a>)</p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Deuterostomia/Uc3V9mGA">Deuterostomia</a> (<a href="/facts/Ambulacraria/tAgtNCeP">Ambulacrarians</a> and <a href="/facts/Chordate/AldL6ARm">chordates</a>)</p></td></tr><tr><td></td></tr><tr><td><a href="/facts/Protostomia/XLtWYdt8">Protostomia</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Ecdysozoa/L3hxqnxA">Ecdysozoa</a> (<a href="/facts/Arthropod/JKJLTFzp">Arthropods</a>, etc.) </p></td></tr><tr><td></td></tr><tr><td><a href="/facts/Spiralia/I4tKtgbI">Spiralia</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Gnathifera_(clade)/P8zySGC7">Gnathifera</a> </p></td></tr><tr><td></td></tr><tr><td><a href="/facts/Platytrochozoa/xkP233tS">Platytrochozoa</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Rouphozoa/rbVpqMDr">Rouphozoa</a> </p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Mesozoa/jNvUg9Yq">Mesozoa</a> </p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td><a href="/facts/Lophotrochozoa/sz5UzpHC">Lophotrochozoa</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Cycliophora/XSM7QM8F">Cycliophora</a> </p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p>Annelida</p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Mollusca/1elgknsQ">Mollusca</a> </p></td></tr><tr><td></td></tr><tr><td><a href="/facts/Kryptotrochozoa/MGmJkFBP">Kryptotrochozoa</a></td><td rowspan="2"><table><tbody><tr><td><a href="/facts/Lophophorata/772EG9EP">Lophophorata</a></td><td rowspan="2"><table><tbody><tr><td><a href="/facts/Brachiozoa/HHooSupM">Brachiozoa</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Brachiopoda/XVBnEPj1">Brachiopoda</a> </p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Phoronida/cDXn9pBV">Phoronida</a> </p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td><a href="/facts/Bryozoa/obAgpA6M">Bryozoa s.l.</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Entoprocta/IHpCbHfj">Entoprocta</a> </p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Ectoprocta/obAgpA6M">Ectoprocta</a> </p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Nemertea/D1HKC5NT">Nemertea</a> </p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table>
<p>The "Lophotrochozoa" hypothesis is also supported by the fact that many phyla within this group, including annelids, <a href="/facts/Mollusc/1elgknsQ">molluscs</a>, <a href="/facts/Nemertean/D1HKC5NT">nemerteans</a> and <a href="/facts/Flatworm/lvqbrb2V">flatworms</a>, follow a similar pattern in the fertilized egg's development. When their cells divide after the 4-cell stage, descendants of these four cells form a spiral pattern. In these phyla the "fates" of the embryo's cells, in other words the roles their descendants will play in the adult animal, are the same and can be predicted from a very early stage.<a class="footnote-ref" id="fnref:187" href="#fn:187"><sup>187</sup></a> Hence this development pattern is often described as "spiral determinate <a href="/facts/Cleavage_(embryo)/v0osHpJC">cleavage</a>".<a class="footnote-ref" id="fnref:188" href="#fn:188"><sup>188</sup></a>
</p>

<p>Fossil discoveries lead to the hypothesis that Annelida and the <a href="/facts/Lophophorates/ESrVMezI">lophophorates</a> are more closely related to each other than any other <a href="/facts/Phylum/ELT3Jyax">phyla</a>. Because of the body plan of <a href="/facts/Lophotrochozoan/sz5UzpHC">lophotrochozoan</a> fossils, a <a href="/facts/Phylogenetic_analysis/bw4rsovt">phylogenetic analysis</a> found the <a href="/facts/Lophophorate/772EG9EP">lophophorates</a> as the <a href="/facts/Sister_group/pNNuVex4">sister group</a> of annelids. Both groups share in common: the presence of <a href="/facts/Chaeta/hcc9Kb7v">chaetae</a> secreted by <a href="/facts/Microvillus/tlI7DwCh">microvilli</a>; paired, <a href="/facts/Metameric/8FGR2KpP">metameric</a> <a href="/facts/Coelom/PD25dd6m">coelomic</a> compartments; and a similar <a href="/facts/Metanephridium/YdMaNPHn">metanephridial</a> structure.<a class="footnote-ref" id="fnref:189" href="#fn:189"><sup>189</sup></a>
</p>

<table><tbody><tr><td><a href="/facts/Lophotrochozoa/sz5UzpHC">Lophotrochozoa</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Nemertea/D1HKC5NT">Nemertea</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p>†<i><a href="/facts/Wiwaxia/gIFtCpjT">Wiwaxia</a></i></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p>†<i><a href="/facts/Kimberella/lH4oAEz6">Kimberella</a></i></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Mollusca/1elgknsQ">Mollusca</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p>†<i><a href="/facts/Phragmochaeta/nXXlkzm3">Phragmochaeta</a></i></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p>Annelida</p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td><a href="/facts/Lophophorata/772EG9EP">Lophophorata</a></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p>†<i><a href="/facts/Wufengella/oX17GfR1">Wufengella</a></i></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><table><tbody><tr><td></td><td rowspan="2"><p><a href="/facts/Phoronida/cDXn9pBV">Phoronida</a></p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Bryozoa/obAgpA6M">Bryozoa</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p>†"<a href="/facts/Tommotiids/TCAEbMHr">Tommotiids</a>"</p></td></tr><tr><td></td></tr><tr><td></td><td rowspan="2"><p><a href="/facts/Brachiopoda/XVBnEPj1">Brachiopoda</a></p></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table></td></tr><tr><td></td></tr></tbody></table>
<h2 id="notes">Notes</h2>

<h2 id="external-links">External links</h2>

The Wikibook <i>Dichotomous Key</i> has a page on the topic of: <i>Annelida</i>

<ul><li> Media related to Annelida at Wikimedia Commons</li>
<li> Data related to Annelida at Wikispecies</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>McIntosh, William Carmichael (1878). "Annelida" . In Baynes, T. S. (ed.). Encyclopædia Britannica. Vol. 2 (9th ed.). New York: Charles Scribner's Sons. pp. 65–72. <a href="/wiki/William_Carmichael_McIntosh" target="_blank">/wiki/William_Carmichael_McIntosh</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>The term originated from Jean-Baptiste Lamarck's annélides.[3][4] <a href="/wiki/Jean-Baptiste_Lamarck" target="_blank">/wiki/Jean-Baptiste_Lamarck</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>Renne, Paul R.; Deino, Alan L.; Hilgen, Frederik J.; Kuiper, Klaudia F.; Mark, Darren F.; Mitchell, William S.; Morgan, Leah E.; Mundil, Roland; Smit, Jan (7 February 2013). "Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary" (PDF). Science. 339 (6120): 684–687. Bibcode:2013Sci...339..684R. doi:10.1126/science.1230492. PMID 23393261. S2CID 6112274. <a href="http://www.cugb.edu.cn/uploadCms/file/20600/20131028144132060.pdf" target="_blank">http://www.cugb.edu.cn/uploadCms/file/20600/20131028144132060.pdf</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>Rouse, G. W. (2002). "Annelida (Segmented Worms)". Encyclopedia of Life Sciences. John Wiley & Sons. doi:10.1038/npg.els.0001599. ISBN 978-0470016176. <a href="978-0470016176" target="_blank">978-0470016176</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>Blakemore, R.J. (2012). Cosmopolitan Earthworms. VermEcology, Yokohama. <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>Blakemore, R.J. (2012). Cosmopolitan Earthworms. VermEcology, Yokohama. <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
<li id="fn:7"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li>
<li id="fn:8"><p>Lavelle, P. (July 1996). "Diversity of Soil Fauna and Ecosystem Function" (PDF). Biology International. 33. Retrieved 2009-04-20. <a href="http://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_6/b_fdi_45-46/010008126.pdf" target="_blank">http://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_6/b_fdi_45-46/010008126.pdf</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li>
<li id="fn:9"><p>Struck, T.H.; Schult, N.; Kusen, T.; Hickman, E.; Bleidorn, C.; McHugh, D.; Halanych, K.M. (5 April 2007). "Annelid phylogeny and the status of Sipuncula and Echiura". BMC Evolutionary Biology. 7 (1): 57. Bibcode:2007BMCEE...7...57S. doi:10.1186/1471-2148-7-57. PMC 1855331. PMID 17411434. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1855331" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1855331</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li>
<li id="fn:10"><p>Hutchings, P. (2007). "Book Review: Reproductive Biology and Phylogeny of Annelida". Integrative and Comparative Biology. 47 (5): 788–789. doi:10.1093/icb/icm008. <a href="https://doi.org/10.1093%2Ficb%2Ficm008" target="_blank">https://doi.org/10.1093%2Ficb%2Ficm008</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li>
<li id="fn:11"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li>
<li id="fn:12"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 176–179. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li>
<li id="fn:13"><p>Rouse, G. W. (2002). "Annelida (Segmented Worms)". Encyclopedia of Life Sciences. John Wiley & Sons. doi:10.1038/npg.els.0001599. ISBN 978-0470016176. <a href="978-0470016176" target="_blank">978-0470016176</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li>
<li id="fn:14"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li>
<li id="fn:15"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D.T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 179–183. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li>
<li id="fn:16"><p>Blakemore, R.J. (2012). Cosmopolitan Earthworms. VermEcology, Yokohama. <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li>
<li id="fn:17"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 176–179. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li>
<li id="fn:18"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. p. 459. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li>
<li id="fn:19"><p>Blakemore, R.J. (2012). Cosmopolitan Earthworms. VermEcology, Yokohama. <a href="#fnref:19" class="footnote-back-ref">↩</a></p></li>
<li id="fn:20"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></p></li>
<li id="fn:21"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></p></li>
<li id="fn:22"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D.T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 179–183. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></p></li>
<li id="fn:23"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></p></li>
<li id="fn:24"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D.T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 179–183. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></p></li>
<li id="fn:25"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R.D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 471–482. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:25" class="footnote-back-ref">↩</a></p></li>
<li id="fn:26"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 176–179. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></p></li>
<li id="fn:27"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D.T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 179–183. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></p></li>
<li id="fn:28"><p>Halanych, K. M.; Dahlgren, T. G.; McHugh, D. (2002). "Unsegmented Annelids? Possible Origins of Four Lophotrochozoan Worm Taxa". Integrative and Comparative Biology. 42 (3): 678–684. doi:10.1093/icb/42.3.678. PMID 21708764. S2CID 14782179. <a href="https://doi.org/10.1093%2Ficb%2F42.3.678" target="_blank">https://doi.org/10.1093%2Ficb%2F42.3.678</a> <a href="#fnref:28" class="footnote-back-ref">↩</a></p></li>
<li id="fn:29"><p>Rouse, G. W. (2002). "Annelida (Segmented Worms)". Encyclopedia of Life Sciences. John Wiley & Sons. doi:10.1038/npg.els.0001599. ISBN 978-0470016176. <a href="978-0470016176" target="_blank">978-0470016176</a> <a href="#fnref:29" class="footnote-back-ref">↩</a></p></li>
<li id="fn:30"><p>Halanych, K. M.; Dahlgren, T. G.; McHugh, D. (2002). "Unsegmented Annelids? Possible Origins of Four Lophotrochozoan Worm Taxa". Integrative and Comparative Biology. 42 (3): 678–684. doi:10.1093/icb/42.3.678. PMID 21708764. S2CID 14782179. <a href="https://doi.org/10.1093%2Ficb%2F42.3.678" target="_blank">https://doi.org/10.1093%2Ficb%2F42.3.678</a> <a href="#fnref:30" class="footnote-back-ref">↩</a></p></li>
<li id="fn:31"><p>McHugh, D. (July 1997). "Molecular evidence that echiurans and pogonophorans are derived annelids". Proceedings of the National Academy of Sciences of the United States of America. 94 (15): 8006–8009. Bibcode:1997PNAS...94.8006M. doi:10.1073/pnas.94.15.8006. PMC 21546. PMID 9223304. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC21546" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC21546</a> <a href="#fnref:31" class="footnote-back-ref">↩</a></p></li>
<li id="fn:32"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D.T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 179–183. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:32" class="footnote-back-ref">↩</a></p></li>
<li id="fn:33"><p>Halanych, K. M.; Dahlgren, T. G.; McHugh, D. (2002). "Unsegmented Annelids? Possible Origins of Four Lophotrochozoan Worm Taxa". Integrative and Comparative Biology. 42 (3): 678–684. doi:10.1093/icb/42.3.678. PMID 21708764. S2CID 14782179. <a href="https://doi.org/10.1093%2Ficb%2F42.3.678" target="_blank">https://doi.org/10.1093%2Ficb%2F42.3.678</a> <a href="#fnref:33" class="footnote-back-ref">↩</a></p></li>
<li id="fn:34"><p>Hausdorf, B.; et al. (2007). "Spiralian Phylogenomics Supports the Resurrection of Bryozoa Comprising Ectoprocta and Entoprocta". Molecular Biology and Evolution. 24 (12): 2723–2729. doi:10.1093/molbev/msm214. PMID 17921486. <a href="https://doi.org/10.1093%2Fmolbev%2Fmsm214" target="_blank">https://doi.org/10.1093%2Fmolbev%2Fmsm214</a> <a href="#fnref:34" class="footnote-back-ref">↩</a></p></li>
<li id="fn:35"><p>Shen, X.; Ma, X.; Ren, J.; Zhao, F. (2009). "A close phylogenetic relationship between Sipuncula and Annelida evidenced from the complete mitochondrial genome sequence of Phascolosoma esculenta". BMC Genomics. 10: 136. doi:10.1186/1471-2164-10-136. PMC 2667193. PMID 19327168. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2667193" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2667193</a> <a href="#fnref:35" class="footnote-back-ref">↩</a></p></li>
<li id="fn:36"><p>Wanninger, Andreas; Kristof, Alen; Brinkmann, Nora (Jan–Feb 2009). "Sipunculans and segmentation". Communicative and Integrative Biology. 2 (1): 56–59. doi:10.4161/cib.2.1.7505. PMC 2649304. PMID 19513266. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2649304" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2649304</a> <a href="#fnref:36" class="footnote-back-ref">↩</a></p></li>
<li id="fn:37"><p>Annelida <a href="https://books.google.com/books?id=gyRXEAAAQBAJ&dq=Orthonectida+taxa+members+Annelida&pg=PA331" target="_blank">https://books.google.com/books?id=gyRXEAAAQBAJ&dq=Orthonectida+taxa+members+Annelida&pg=PA331</a> <a href="#fnref:37" class="footnote-back-ref">↩</a></p></li>
<li id="fn:38"><p>Mitochondrial Genome Evolution in Annelida—A Systematic Study on Conservative and Variable Gene Orders and the Factors Influencing its Evolution <a href="https://academic.oup.com/sysbio/article/72/4/925/7135606?login=false" target="_blank">https://academic.oup.com/sysbio/article/72/4/925/7135606?login=false</a> <a href="#fnref:38" class="footnote-back-ref">↩</a></p></li>
<li id="fn:39"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:39" class="footnote-back-ref">↩</a></p></li>
<li id="fn:40"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 176–179. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:40" class="footnote-back-ref">↩</a></p></li>
<li id="fn:41"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:41" class="footnote-back-ref">↩</a></p></li>
<li id="fn:42"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:42" class="footnote-back-ref">↩</a></p></li>
<li id="fn:43"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:43" class="footnote-back-ref">↩</a></p></li>
<li id="fn:44"><p>Cutler, B. (August 1980). "Arthropod cuticle features and arthropod monophyly". Cellular and Molecular Life Sciences. 36 (8): 953. doi:10.1007/BF01953812. S2CID 84995596. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:44" class="footnote-back-ref">↩</a></p></li>
<li id="fn:45"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Introduction to Arthropoda". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 523–524. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:45" class="footnote-back-ref">↩</a></p></li>
<li id="fn:46"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:46" class="footnote-back-ref">↩</a></p></li>
<li id="fn:47"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:47" class="footnote-back-ref">↩</a></p></li>
<li id="fn:48"><p>Struck, T.H.; Schult, N.; Kusen, T.; Hickman, E.; Bleidorn, C.; McHugh, D.; Halanych, K.M. (5 April 2007). "Annelid phylogeny and the status of Sipuncula and Echiura". BMC Evolutionary Biology. 7 (1): 57. Bibcode:2007BMCEE...7...57S. doi:10.1186/1471-2148-7-57. PMC 1855331. PMID 17411434. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1855331" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1855331</a> <a href="#fnref:48" class="footnote-back-ref">↩</a></p></li>
<li id="fn:49"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Echiura and Sipuncula". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 490–495. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:49" class="footnote-back-ref">↩</a></p></li>
<li id="fn:50"><p>Anderson, D. T. (1998). "The Annelida and their close relatives". In Anderson, D.T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:50" class="footnote-back-ref">↩</a></p></li>
<li id="fn:51"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Nemertea". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 271–282. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:51" class="footnote-back-ref">↩</a></p></li>
<li id="fn:52"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Arthropoda". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 518–521. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:52" class="footnote-back-ref">↩</a></p></li>
<li id="fn:53"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Onychophora and Tardigrada". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 505–510. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:53" class="footnote-back-ref">↩</a></p></li>
<li id="fn:54"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:54" class="footnote-back-ref">↩</a></p></li>
<li id="fn:55"><p>Paxton, H. (June 2005). "Molting polychaete jaws—ecdysozoans are not the only molting animals". Evolution & Development. 7 (4): 337–340. doi:10.1111/j.1525-142X.2005.05039.x. PMID 15982370. S2CID 22020406. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:55" class="footnote-back-ref">↩</a></p></li>
<li id="fn:56"><p>Nielsen, C. (September 2003). "Proposing a solution to the Articulata–Ecdysozoa controversy" (PDF). Zoologica Scripta. 32 (5): 475–482. doi:10.1046/j.1463-6409.2003.00122.x. S2CID 1416582. Archived (PDF) from the original on 20 March 2009. Retrieved 2009-03-11. <a href="http://www.museunacional.ufrj.br/mndi/Aracnologia/pdfliteratura/Nielsen%202003%20articulata%20vs%20ecdiso.pdf" target="_blank">http://www.museunacional.ufrj.br/mndi/Aracnologia/pdfliteratura/Nielsen%202003%20articulata%20vs%20ecdiso.pdf</a> <a href="#fnref:56" class="footnote-back-ref">↩</a></p></li>
<li id="fn:57"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Introduction to Arthropoda". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 523–524. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:57" class="footnote-back-ref">↩</a></p></li>
<li id="fn:58"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:58" class="footnote-back-ref">↩</a></p></li>
<li id="fn:59"><p>The Invertebrate Tree of Life <a href="https://books.google.com/books?id=YHetDwAAQBAJ&dq=Sipuncula+Echiura+Lobatocerebrum+Diurodrilus+Polygordius&pg=PA399" target="_blank">https://books.google.com/books?id=YHetDwAAQBAJ&dq=Sipuncula+Echiura+Lobatocerebrum+Diurodrilus+Polygordius&pg=PA399</a> <a href="#fnref:59" class="footnote-back-ref">↩</a></p></li>
<li id="fn:60"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:60" class="footnote-back-ref">↩</a></p></li>
<li id="fn:61"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:61" class="footnote-back-ref">↩</a></p></li>
<li id="fn:62"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R.D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 471–482. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:62" class="footnote-back-ref">↩</a></p></li>
<li id="fn:63"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 176–179. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:63" class="footnote-back-ref">↩</a></p></li>
<li id="fn:64"><p>Rouse, G. W. (2002). "Annelida (Segmented Worms)". Encyclopedia of Life Sciences. John Wiley & Sons. doi:10.1038/npg.els.0001599. ISBN 978-0470016176. <a href="978-0470016176" target="_blank">978-0470016176</a> <a href="#fnref:64" class="footnote-back-ref">↩</a></p></li>
<li id="fn:65"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:65" class="footnote-back-ref">↩</a></p></li>
<li id="fn:66"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:66" class="footnote-back-ref">↩</a></p></li>
<li id="fn:67"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:67" class="footnote-back-ref">↩</a></p></li>
<li id="fn:68"><p>Minelli, Alessandro (2009), "Chapter 6. A gallery of the major bilaterian clades", Perspectives in Animal Phylogeny and Evolution, Oxford University Press, ISBN 978-0-19-856620-5, p. 82: This is the case for circular muscles, which have been reported as absent in many families (Opheliidae, Protodrilidae, Spionidae, Oweniidae, Aphroditidae, Acoetidae, Polynoidae, Sigalionidae, Phyllodocidae, Nephtyidae, Pisionidae, and Nerillidae; Tzetlin et al. 2002). Tzetlin and Filippova (2005) suggest that absence of circular muscles is possibly plesiomorphic in the Annelida. <a href="978-0-19-856620-5" target="_blank">978-0-19-856620-5</a> <a href="#fnref:68" class="footnote-back-ref">↩</a></p></li>
<li id="fn:69"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:69" class="footnote-back-ref">↩</a></p></li>
<li id="fn:70"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:70" class="footnote-back-ref">↩</a></p></li>
<li id="fn:71"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:71" class="footnote-back-ref">↩</a></p></li>
<li id="fn:72"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:72" class="footnote-back-ref">↩</a></p></li>
<li id="fn:73"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:73" class="footnote-back-ref">↩</a></p></li>
<li id="fn:74"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:74" class="footnote-back-ref">↩</a></p></li>
<li id="fn:75"><p>Jenner, R. A. (2006). "Challenging received wisdoms: Some contributions of the new microscopy to the new animal phylogeny". Integrative and Comparative Biology. 46 (2): 93–103. doi:10.1093/icb/icj014. PMID 21672726. <a href="https://doi.org/10.1093%2Ficb%2Ficj014" target="_blank">https://doi.org/10.1093%2Ficb%2Ficj014</a> <a href="#fnref:75" class="footnote-back-ref">↩</a></p></li>
<li id="fn:76"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:76" class="footnote-back-ref">↩</a></p></li>
<li id="fn:77"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:77" class="footnote-back-ref">↩</a></p></li>
<li id="fn:78"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 176–179. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:78" class="footnote-back-ref">↩</a></p></li>
<li id="fn:79"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 425–429. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:79" class="footnote-back-ref">↩</a></p></li>
<li id="fn:80"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:80" class="footnote-back-ref">↩</a></p></li>
<li id="fn:81"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:81" class="footnote-back-ref">↩</a></p></li>
<li id="fn:82"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:82" class="footnote-back-ref">↩</a></p></li>
<li id="fn:83"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:83" class="footnote-back-ref">↩</a></p></li>
<li id="fn:84"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 425–429. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:84" class="footnote-back-ref">↩</a></p></li>
<li id="fn:85"><p>Bok, Michael J.; Macali, Armando; Garm, Anders (2024-04-08). "High-resolution vision in pelagic polychaetes". Current Biology. 34 (7): R269 – R270. doi:10.1016/j.cub.2024.02.055. ISSN 0960-9822. PMID 38593767. <a href="https://doi.org/10.1016%2Fj.cub.2024.02.055" target="_blank">https://doi.org/10.1016%2Fj.cub.2024.02.055</a> <a href="#fnref:85" class="footnote-back-ref">↩</a></p></li>
<li id="fn:86"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:86" class="footnote-back-ref">↩</a></p></li>
<li id="fn:87"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 425–429. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:87" class="footnote-back-ref">↩</a></p></li>
<li id="fn:88"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 425–429. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:88" class="footnote-back-ref">↩</a></p></li>
<li id="fn:89"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:89" class="footnote-back-ref">↩</a></p></li>
<li id="fn:90"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:90" class="footnote-back-ref">↩</a></p></li>
<li id="fn:91"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:91" class="footnote-back-ref">↩</a></p></li>
<li id="fn:92"><p>Ruppert, E.E.; Fox, R.S. & Barnes, R.D. (2004). "Introduction to Metazoa". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 103–104. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:92" class="footnote-back-ref">↩</a></p></li>
<li id="fn:93"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:93" class="footnote-back-ref">↩</a></p></li>
<li id="fn:94"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:94" class="footnote-back-ref">↩</a></p></li>
<li id="fn:95"><p>Ruppert, E.E.; Fox, R.S. & Barnes, R.D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 423–425. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:95" class="footnote-back-ref">↩</a></p></li>
<li id="fn:96"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:96" class="footnote-back-ref">↩</a></p></li>
<li id="fn:97"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:97" class="footnote-back-ref">↩</a></p></li>
<li id="fn:98"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:98" class="footnote-back-ref">↩</a></p></li>
<li id="fn:99"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:99" class="footnote-back-ref">↩</a></p></li>
<li id="fn:100"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R.D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 471–482. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:100" class="footnote-back-ref">↩</a></p></li>
<li id="fn:101"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R.D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 471–482. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:101" class="footnote-back-ref">↩</a></p></li>
<li id="fn:102"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:102" class="footnote-back-ref">↩</a></p></li>
<li id="fn:103"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:103" class="footnote-back-ref">↩</a></p></li>
<li id="fn:104"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:104" class="footnote-back-ref">↩</a></p></li>
<li id="fn:105"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 183–196. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:105" class="footnote-back-ref">↩</a></p></li>
<li id="fn:106"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:106" class="footnote-back-ref">↩</a></p></li>
<li id="fn:107"><p>Ruppert, E.E.; Fox, R.S. & Barnes, R.D. (2004). "Introduction to Bilateria". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 196–224. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:107" class="footnote-back-ref">↩</a></p></li>
<li id="fn:108"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:108" class="footnote-back-ref">↩</a></p></li>
<li id="fn:109"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:109" class="footnote-back-ref">↩</a></p></li>
<li id="fn:110"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 434–441. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:110" class="footnote-back-ref">↩</a></p></li>
<li id="fn:111"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 176–179. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:111" class="footnote-back-ref">↩</a></p></li>
<li id="fn:112"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 466–469. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:112" class="footnote-back-ref">↩</a></p></li>
<li id="fn:113"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 176–179. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:113" class="footnote-back-ref">↩</a></p></li>
<li id="fn:114"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 477–478. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:114" class="footnote-back-ref">↩</a></p></li>
<li id="fn:115"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 176–179. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:115" class="footnote-back-ref">↩</a></p></li>
<li id="fn:116"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 434–441. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:116" class="footnote-back-ref">↩</a></p></li>
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<li id="fn:118"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 477–478. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:118" class="footnote-back-ref">↩</a></p></li>
<li id="fn:119"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:119" class="footnote-back-ref">↩</a></p></li>
<li id="fn:120"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 196–202. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:120" class="footnote-back-ref">↩</a></p></li>
<li id="fn:121"><p>Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7th ed.). Brooks / Cole. pp. 414–420. ISBN 978-0-03-025982-1. <a href="978-0-03-025982-1" target="_blank">978-0-03-025982-1</a> <a href="#fnref:121" class="footnote-back-ref">↩</a></p></li>
<li id="fn:122"><p>Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 196–202. ISBN 978-0-19-551368-4. <a href="978-0-19-551368-4" target="_blank">978-0-19-551368-4</a> <a href="#fnref:122" class="footnote-back-ref">↩</a></p></li>
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<li id="fn:175"><p>Struck, T.H.; Schult, N.; Kusen, T.; Hickman, E.; Bleidorn, C.; McHugh, D.; Halanych, K.M. (5 April 2007). "Annelid phylogeny and the status of Sipuncula and Echiura". BMC Evolutionary Biology. 7 (1): 57. Bibcode:2007BMCEE...7...57S. doi:10.1186/1471-2148-7-57. PMC 1855331. PMID 17411434. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1855331" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1855331</a> <a href="#fnref:175" class="footnote-back-ref">↩</a></p></li>
<li id="fn:176"><p>Since this section was written, a new paper has revised the 2007 results: Struck, T. H.; Paul, C.; Hill, N.; Hartmann, S.; Hösel, C.; Kube, M.; Lieb, B.; Meyer, A.; Tiedemann, R.; Purschke, G. N.; Bleidorn, C. (2011). "Phylogenomic analyses unravel annelid evolution". Nature. 471 (7336): 95–98. Bibcode:2011Natur.471...95S. doi:10.1038/nature09864. PMID 21368831. S2CID 4428998. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:176" class="footnote-back-ref">↩</a></p></li>
<li id="fn:177"><p>Struck, T.; Golombek, Anja; et al. (2015). "The Evolution of Annelids Reveals Two Adaptive Routes to the Interstitial Realm". Current Biology. 25 (15): 1993–1999. Bibcode:2015CBio...25.1993S. doi:10.1016/j.cub.2015.06.007. PMID 26212885. S2CID 12919216. <a href="https://doi.org/10.1016%2Fj.cub.2015.06.007" target="_blank">https://doi.org/10.1016%2Fj.cub.2015.06.007</a> <a href="#fnref:177" class="footnote-back-ref">↩</a></p></li>
<li id="fn:178"><p>Weigert, Anne; Bleidorn, Christoph (2015). "Current status of annelid phylogeny". Organisms Diversity & Evolution. 16 (2): 345–362. doi:10.1007/s13127-016-0265-7. S2CID 5353873. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:178" class="footnote-back-ref">↩</a></p></li>
<li id="fn:179"><p>Marotta, Roberto; Ferraguti, Marco; Erséus, Christer; Gustavsson, Lena M. (2008). "Combined-data phylogenetics and character evolution of Clitellata (Annelida) using 18S rDNA and morphology". Zoological Journal of the Linnean Society. 154: 1–26. doi:10.1111/j.1096-3642.2008.00408.x. <a href="https://doi.org/10.1111%2Fj.1096-3642.2008.00408.x" target="_blank">https://doi.org/10.1111%2Fj.1096-3642.2008.00408.x</a> <a href="#fnref:179" class="footnote-back-ref">↩</a></p></li>
<li id="fn:180"><p>Christoffersen, Martin Lindsey (1 January 2012). "Phylogeny of basal descendants of cocoon-forming annelids (Clitellata)". Turkish Journal of Zoology. 36 (1): 95–119. doi:10.3906/zoo-1002-27. S2CID 83066199. <a href="https://journals.tubitak.gov.tr/zoology/vol36/iss1/9/" target="_blank">https://journals.tubitak.gov.tr/zoology/vol36/iss1/9/</a> <a href="#fnref:180" class="footnote-back-ref">↩</a></p></li>
<li id="fn:181"><p>Struck TH (2019). "Phylogeny". In Purschke G, Böggemann M, Westheide W (eds.). Handbook of Zoology: Annelida. Vol. 1: Annelida Basal Groups and Pleistoannelida, Sedentaria I. De Gruyter. pp. 37–68. doi:10.1515/9783110291582-002. ISBN 9783110291469. S2CID 242569001. <a href="9783110291469" target="_blank">9783110291469</a> <a href="#fnref:181" class="footnote-back-ref">↩</a></p></li>
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<li id="fn:189"><p>Guo, Jin; Parry, Luke A.; Vinther, Jakob; Edgecombe, Gregory D.; Wei, Fan; Zhao, Jun; Zhao, Yang; Béthoux, Olivier; Lei, Xiangtong; Chen, Ailin; Hou, Xianguang; Chen, Taimin; Cong, Peiyun (2022). "A Cambrian tommotiid preserving soft tissues reveals the metameric ancestry of lophophorates". Current Biology. 32 (21): S0960–9822(22)01455–5. Bibcode:2022CBio...32E4769G. doi:10.1016/j.cub.2022.09.011. ISSN 1879-0445. PMID 36170853. <a href="https://doi.org/10.1016%2Fj.cub.2022.09.011" target="_blank">https://doi.org/10.1016%2Fj.cub.2022.09.011</a> <a href="#fnref:189" class="footnote-back-ref">↩</a></p></li>
</ol>

Annelid open-in-new

Annelid