环节动物门（學名：Annelida）是动物界的一个门，该门动物为两侧对称、同律分节的裂生体腔动物，有的具疣足和刚毛，多闭管式循环系统、链式神经系统。常见环节动物有：蚯蚓、蚂蟥（又称水蛭）、沙蚕等。


身体构造 环节动物的消化系统由口，咽，食道，腺囊，肌肉胃，中肠和肛门组成，循环系统由心脏，腹背两套血管和滋养器官的小型侧边血管组成。
每一个体节里有两个后肾管，它们负责收集前一体节的体液。体液通过肾孔流到下一体节，途中经过盘曲的肾管，直到膀胱。然后通过肾孔排出体外。终尿里面是含氮的物质。而无机盐则会被肾管上皮重吸收回到血液。为了平衡通过皮肤吸收的水分，尿液会稀释得很厉害。每天可排出約体重的60%稀薄尿液。后肾管被腹膜“绑”着固定于体腔壁上。
环节动物的繁殖能力非常强：体节的分离和出芽生殖的无性生殖。但有性生殖也很重要。多毛纲的动物大多是雌雄异体。而原环虫纲则是雌雄同体。


下属分类 见：环节动物分类表
本门包括以下纲：

环带纲 Clitellata 
多毛纲 Polychaeta 

纲和目的地位未定的科：
 Amphinomea 
 Antillesomatidae 
 Cloudinidae 
 Hercolepadidae 
 Hrabeiellidae 
 Lepidocoelidae 
 Lobatocerebridae 
 Rhynchobdellae 
 Siphonosomatidae 

纲、目和科的地位未定的属：
 Acritobdella Ringuelet, 1978
 Adaetobdella Ringuelet, 1978
 Adenobdella Leidy, 1885
 Albione Savigny, 1822
 Ambalodus Branson & Mehl, 1933
 Ancyrobdella Oka, 1917
 Anoculobdella Weber, 1915
 Anoigmaichnus Vinn, Wilson, Motus & Toom, 2014
 Argyrobdella Cordero, 1937
 Asabellarifex Klahn, 1932
 Astacobdella Moquin-Tandon, 1846
 Aulacodus Pander, 1856
 Aulacostomum Grube, 1850
 Aulozoon Gehling & Runnegar, 2022
 Bakedebdella Sciacchitano, 1939
 Barbarodina Stauffer, 1935
 Belodus Pander, 1856
 Blennobdella Blanchard, 1849
 Bransonella Harlton, 1933
 Cardiodus Branson & Mehl, 1933
 Caribeobdella Ringuelet, 1976
 Centrognathus Branson & Mehl, 1935
 Cephalopus Gronov, 1762
 Chenidrilus Song, 1994
 Clepsinides Augener, 1926
 Cololites Agassiz, 1836
 Colombobdella Mane-Garzon, 1973
 Conphytobdella Richardson, 1975
 Crossopodia M'Coy, 1851
 Cymatobranchus Selensky
 Dacnobdella Ringuelet, 1978
 Dactylobdella van Beneden & Hesse, 1863
 Dagarobdella Finogenova & Snimschikova, 1991
 Dartevellida Sciacchitano, 1939
 Democedes Kinberg, 1866
 Dermobdella Philippi, 1867
 Dichognathus Branson & Mehl, 1933
 Diplotuba Badve & Ghare, 1984
 Dualigenys Courtinat, 1998
 Dubichaetaspis Eriksson, 1998
 Dundjibdella Sciacchitano, 1939
 Epsteinia Utevsky, 1994
 Erpobdelliformes Sawyer, 1986
 Eubranchella Baird, 1869
 Exochleius Rafinesque, 1819
 Ganymedes Leigh-Sharpe, 1915
 Glossobdella de Blainville, 1827
 Glossophora Gray, 1831
 Glossopora Johnson, 1817
 Gnatho Goldfuss, 1820
 Gnathodella Matern, 1933
 Goel Oken, 1815
 Granelia Harant & Vernieres, 1936
 Gyrognathus Stauffer, 1935
 Haemobdella Brandt, 1911
 Haemocharis Savigny, 1822
 Haemonis Bernouilli, 1849
 Hararbdella Sciacchitano, 1941
 Haughtonia Kinahan, 1859
 Helluo Oken, 1815
 Helminthodes Marsh, 1864
 Herpetonites Otto, 1855
 Heterognathus Stauffer, 1935
 Hippobdella de Blainville, 1827
 Hirudella Munster, 1842
 Histioderma Kinahan, 1858
 Huananoserpula Xu, Lin & Wang, 1992
 Hybobdella Weyenbergh, 1879
 Hylacobdella Sciacchitano, 1935
 Hypsobdella Weber, 1915
 Icthyodus Harris & Hollingsworth, 1933
 Idiobdella Harding, 1913
 Idrisobdella Leigh-Sharpe, 1933
 Igriodus Branson & Mehl, 1933
 Imbricina Rovereto, 1904
 Ischyracanthus Marsh, 1865
 Jatrobdella de Blainville, 1827
 Jennaria Rieger, 1991
 Lapillitubus Belaustegui & Belaustegui, 2017
 Lapispira Lange, 1932
 Lecthaylus Weller, 1925
 Lepodus Branson & Mehl, 1933
 Leptostoma Whitman, 1886
 Lobina Moquin-Tandon, 1846
 Lonchodus Pander, 1856
 Lonchosaccus Ruedemann, 1925
 Maghrebiella Baha & Berra, 2001
 Maiabdella Ringuelet, 1980
 Malayabdella Richardson, 1975
 Matadibdella Sciacchitano, 1939
 Meanderovaleichnus Console-Gonella & Acenolaza, 2014
 Micrapium Thorell, 1870
 Microstoma Whitman, 1885
 Miskoia Walcott, 1911
 Moluccabdella Richardson, 1975
 Mongbwalubdella Sciacchitano, 1939
 Monopus Gosse, 1855
 Mooreotorix Lukin, 1976
 Moquinia Blanchard, 1888
 Multidentodus Harlton, 1933
 Myrianites MacLeay, 1839
 Nemapodia Emmons, 1846
 Nosamia Rafinesque, 1814
 Nothognathella Branson & Mehl, 1933
 Oikobesalon Thomas & Smith, 1998
 Oliveiriana Maury, 1927
 Ophibdella Beneden & Hesse, 1863
 Ornataporta Gardner, 1916
 Ortonia Nicholson, 1872
 Pachysomia Kilwinning-Smith, 1907
 Palaeobdella Blainville, 1829
 Palaeohirudo Kozur, 1970
 Palaeonereis Eichwald, 1856
 Palaeorbis Beneden & Coemans, 1867
 Palmatodella Bassler, 1925
 Paltodus Pander, 1856
 Panderodella Ulrich & Bassler, 1926
 Parapontobdella Harant, 1929
 Paratorix Lukin & Epshtein, 1960
 Pentabdella Llewellyn, 1966
 Pharyngobdelliformes Caballero, 1952
 Phormio Goldfuss, 1820
 Pilothylakos Manum, Bose & Sawyer, 1991
 Pinacobdella Diesing, 1850
 Plagiodina Cooper, 1933
 Plagiogmus Roedel, 1929
 Plectospathodus Branson & Mehl, 1933
 Podobdella Diesing, 1850
 Podoclepsis Dequal, 1916
 Polodorites Douville, 1907
 Polycaulodus Branson & Mehl, 1933
 Polygnathodella Harlton, 1933
 Polyupsilon Howell, 1957
 Prioniodella Bassler, 1925
 Prionocanthus Pander, 1856
 Pseudobdella Blainville, 1827
 Pyrgopolopsis Rovereto, 1903
 Sabellarites Dawson, 1890
 Sabellarites Richter, 1920
 Sanguisuga Bruguiere, 1789
 Schlegelia Weyenbergh, 1880
 Sciobdella Richardson, 1971
 Scolopodella Stauffer & Plummer, 1932
 Scorpaenobdella Saint-Loup, 1886
 Scotlandia Cossmann, 1909
 Semiscoloides Sawyer, 1986
 Serpulina Zborzevski, 1834
 Serpuloides Murchison, 1839
 Skolekichnus Ellenberger, 1974
 Sphagodus Eichwald, 1854
 Spinacoleus Schallreuter, 1985
 Stephanodella Matern, 1933
 Suabdella 
 Subcordylodus Stauffer, 1935
 Subprioniodus Smith, 1907
 Sulcichnus Martinell & Domenech, 2009
 Synprioniodina Bassler, 1925
 Telumodina Cooper, 1931
 Tentaculina Zborzewski, 1834
 Tetralysis Eudes-DesLongchamps, 1878
 Thartharella Elliot, 1980
 Tortoniodus Stauffer, 1935
 Trachelobdella Moore, 1957
 Trachyderma Phillips, 1848
 Tribothrynobdella Ringuelet, 1976
 Trigonobdella Sciacchitano, 1939
 Tristabdella Richardson, 1975
 Trucherognathus Branson & Mehl, 1933
 Trulliodbella Brinkmann, 1948
 Typhlobdella Diesing, 1850
 Urotheca Matthew, 1899
 Valentia Smith, 1907
 Vermicularis Borson
 Vermiculites Bronn, 1848
 Virgulaxonia Yin, 1937
 Wenghuiia Wang & Wang, 2008
 Worthenella Walcott, 1911


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