{"id":755,"date":"2018-01-18T19:58:50","date_gmt":"2018-01-18T19:58:50","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/chapter\/phylum-annelida\/"},"modified":"2024-04-26T21:33:15","modified_gmt":"2024-04-26T21:33:15","slug":"phylum-annelida","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/chapter\/phylum-annelida\/","title":{"raw":"Phylum Annelida","rendered":"Phylum Annelida"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Describe the features of animals classified in phylum Annelida<\/li>\r\n<\/ul>\r\n<\/div>\r\nPhylum\u00a0<strong>Annelida<\/strong>\u00a0includes segmented worms. These animals are found in marine, terrestrial, and freshwater habitats, but a presence of water or humidity is a critical factor for their survival, especially in terrestrial habitats. The name of the phylum is derived from the Latin word\u00a0<em>annellus<\/em>, which means a small ring. Animals in this phylum show parasitic and commensal symbioses with other species in their habitat. Approximately 16,500 species have been described in phylum Annelida. The phylum includes earthworms, polychaete worms, and leeches. Annelids show protostomic development in embryonic stages and are often called \u201csegmented worms\u201d due to their key characteristic of\u00a0<strong>metamerism<\/strong>, or true segmentation.\r\n<h2>Morphology<\/h2>\r\n[caption id=\"attachment_2434\" align=\"alignright\" width=\"350\"]<img class=\"wp-image-2434\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18195842\/Figure_28_03_16.jpg\" alt=\"The clitellum is a swollen, smooth section of the earthworm.\" width=\"350\" height=\"263\" \/> Figure\u00a01. The clitellum, seen here as a protruding segment with different coloration than the rest of the body, is a structure that aids in annelid reproduction. (credit: Rob Hille)[\/caption]\r\n\r\nAnnelids display bilateral symmetry and are worm-like in overall morphology. Annelids have a segmented body plan wherein the internal and external morphological features are repeated in each body segment. Metamerism allows animals to become bigger by adding \u201ccompartments\u201d while making their movement more efficient. This metamerism is thought to arise from identical teloblast cells in the embryonic stage, which give rise to identical mesodermal structures. The overall body can be divided into head, body, and pygidium (or tail). The\u00a0<strong>clitellum<\/strong>\u00a0is a reproductive structure that generates mucus that aids in sperm transfer and gives rise to a cocoon within which fertilization occurs; it appears as a fused band in the anterior third of the animal (Figure\u00a01).\r\n<h2>Anatomy<\/h2>\r\nThe epidermis is protected by an acellular, external cuticle, but this is much thinner than the cuticle found in the ecdysozoans and does not require periodic shedding for growth. Circular as well as longitudinal muscles are located interior to the epidermis. Chitinous hairlike extensions, anchored in the epidermis and projecting from the cuticle, called\u00a0<strong>setae<\/strong>\/<strong>chaetae<\/strong>\u00a0are present in every segment. Annelids show the presence of a true coelom, derived from embryonic mesoderm and protostomy. Hence, they are the most advanced worms. A well-developed and complete digestive system is present in earthworms (oligochaetes) with a mouth, muscular pharynx, esophagus, crop, and gizzard being present. The gizzard leads to the intestine and ends in an anal opening. A cross-sectional view of a body segment of an earthworm (a terrestrial type of annelid) is shown in\u00a0Figure 2; each segment is limited by a membranous septum that divides the coelomic cavity into a series of compartments.\r\n\r\n[caption id=\"attachment_2435\" align=\"aligncenter\" width=\"544\"]<img class=\"size-full wp-image-2435\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18195845\/Figure_28_03_17.jpg\" alt=\"The illustration shows a cross-section of an annelid. The body is divided into segmented compartments. A U-shaped intestine runs through the middle of the compartments, and two ventral nerve cords run along the bottom. In each segment, the nerve cords are connected to each other. A dorsal blood vessel sits on top of the intestine, and a ventral blood vessel rests beneath it. Other vessels connect the dorsal and ventral vessels together. The nephridium is connected to the barrier separating the compartments, and consists of a long coil connected to a trumpet-like bell.\" width=\"544\" height=\"409\" \/> Figure 2. This schematic drawing shows the basic anatomy of annelids in a cross-sectional view.[\/caption]\r\n\r\nAnnelids possess a closed circulatory system of dorsal and ventral blood vessels that run parallel to the alimentary canal as well as capillaries that service individual tissues. In addition, these vessels are connected by transverse loops in every segment. These animals lack a well-developed respiratory system, and gas exchange occurs across the moist body surface. Excretion is facilitated by a pair of metanephridia (a type of primitive \u201ckidney\u201d that consists of a convoluted tubule and an open, ciliated funnel) that is present in every segment towards the ventral side. Annelids show well-developed nervous systems with a nerve ring of fused ganglia present around the pharynx. The nerve cord is ventral in position and bears enlarged nodes or ganglia in each segment.\r\n\r\nAnnelids may be either monoecious with permanent gonads (as in earthworms and leeches) or dioecious with temporary or seasonal gonads that develop (as in polychaetes). However, cross-fertilization is preferred in hermaphroditic animals. These animals may also show simultaneous hermaphroditism and participate in simultaneous sperm exchange when they are aligned for copulation.\r\n<div class=\"textbox\">This combination\u00a0<a href=\"http:\/\/shapeoflife.org\/video\/annelid-animation-body-plan\" target=\"_blank\" rel=\"nofollow noopener\">video and animation\u00a0provides a closeup look at annelid anatomy<\/a>.<\/div>\r\n<h2>Classification of Phylum Annelida<\/h2>\r\nPhylum Annelida contains the class Polychaeta (the polychaetes) and the class Oligochaeta (the earthworms, leeches and their relatives).\r\n\r\nEarthworms are the most abundant members of the class Oligochaeta, distinguished by the presence of the clitellum as well as few, reduced chaetae (<em>oligo<\/em>- = \u201cfew\u201d; -<em>chaetae<\/em> = \u201chairs\u201d). The number and size of chaetae are greatly diminished in Oligochaeta compared to the polychaetes (<em>poly<\/em>=many,\u00a0<em>chaetae<\/em>\u00a0= hairs). The many chetae of polychaetes are also arranged within fleshy, flat, paired appendages that protrude from each segment called\u00a0<strong>parapodia<\/strong>, which may be specialized for different functions in the polychates. The subclass Hirudinea includes leeches such as\u00a0<em>Hirudo medicinalis<\/em>\u00a0and\u00a0<em>Hemiclepsis marginata<\/em>. The class Oligochaeta includes the subclass Hirudinia and the subclass Brachiobdella. A significant difference between leeches and other annelids is the development of suckers at the anterior and posterior ends and a lack of chaetae. Additionally, the segmentation of the body wall may not correspond to the internal segmentation of the coelomic cavity. This adaptation possibly helps the leeches to elongate when they ingest copious quantities of blood from host vertebrates. The subclass Brachiobdella includes species like\u00a0<em>Branchiobdella balcanica sketi<\/em>\u00a0and\u00a0<em>Branchiobdella astaci<\/em>, worms that show similarity with leeches as well as oligochaetes.\r\n\r\n[caption id=\"attachment_2436\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-2436\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18195848\/Figure_28_03_18-1024x269.jpg\" alt=\"Part a shows an earthworm, and part b shows a large leech trying to latch onto a person\u2019s hand. Part c shows a worm on that is anchored to the ocean floor. Featherlike appendages extend from the tube-like body.\" width=\"1024\" height=\"269\" \/> Figure 3. The (a) earthworm, (b) leech, and (c) featherduster are all annelids. (credit a: modification of work by S. Shepherd; credit b: modification of work by \u201cSarah G...\u201d\/Flickr; credit c: modification of work by Chris Gotschalk, NOAA)[\/caption]\r\n\r\n<div class=\"textbox learning-objectives\">\r\n<h3>In Summary:\u00a0Phylum Annelida<\/h3>\r\nPhylum Annelida includes vermiform, segmented animals. Segmentation is seen in internal anatomy as well, which is called metamerism. Annelids are protostomes. These animals have well-developed neuronal and digestive systems. Some species bear a specialized band of segments known as a clitellum. Annelids show the presence numerous chitinous projections termed chaetae, and polychaetes possess parapodia. Suckers are seen in order Hirudinea. Reproductive strategies include sexual dimorphism, hermaphroditism, and serial hermaphroditism. Internal segmentation is absent in class Hirudinea.\r\n\r\n<\/div>\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/d97e6a9f-154c-4011-8ce6-8af6f727a84a\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Describe the features of animals classified in phylum Annelida<\/li>\n<\/ul>\n<\/div>\n<p>Phylum\u00a0<strong>Annelida<\/strong>\u00a0includes segmented worms. These animals are found in marine, terrestrial, and freshwater habitats, but a presence of water or humidity is a critical factor for their survival, especially in terrestrial habitats. The name of the phylum is derived from the Latin word\u00a0<em>annellus<\/em>, which means a small ring. Animals in this phylum show parasitic and commensal symbioses with other species in their habitat. Approximately 16,500 species have been described in phylum Annelida. The phylum includes earthworms, polychaete worms, and leeches. Annelids show protostomic development in embryonic stages and are often called \u201csegmented worms\u201d due to their key characteristic of\u00a0<strong>metamerism<\/strong>, or true segmentation.<\/p>\n<h2>Morphology<\/h2>\n<div id=\"attachment_2434\" style=\"width: 360px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2434\" class=\"wp-image-2434\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18195842\/Figure_28_03_16.jpg\" alt=\"The clitellum is a swollen, smooth section of the earthworm.\" width=\"350\" height=\"263\" \/><\/p>\n<p id=\"caption-attachment-2434\" class=\"wp-caption-text\">Figure\u00a01. The clitellum, seen here as a protruding segment with different coloration than the rest of the body, is a structure that aids in annelid reproduction. (credit: Rob Hille)<\/p>\n<\/div>\n<p>Annelids display bilateral symmetry and are worm-like in overall morphology. Annelids have a segmented body plan wherein the internal and external morphological features are repeated in each body segment. Metamerism allows animals to become bigger by adding \u201ccompartments\u201d while making their movement more efficient. This metamerism is thought to arise from identical teloblast cells in the embryonic stage, which give rise to identical mesodermal structures. The overall body can be divided into head, body, and pygidium (or tail). The\u00a0<strong>clitellum<\/strong>\u00a0is a reproductive structure that generates mucus that aids in sperm transfer and gives rise to a cocoon within which fertilization occurs; it appears as a fused band in the anterior third of the animal (Figure\u00a01).<\/p>\n<h2>Anatomy<\/h2>\n<p>The epidermis is protected by an acellular, external cuticle, but this is much thinner than the cuticle found in the ecdysozoans and does not require periodic shedding for growth. Circular as well as longitudinal muscles are located interior to the epidermis. Chitinous hairlike extensions, anchored in the epidermis and projecting from the cuticle, called\u00a0<strong>setae<\/strong>\/<strong>chaetae<\/strong>\u00a0are present in every segment. Annelids show the presence of a true coelom, derived from embryonic mesoderm and protostomy. Hence, they are the most advanced worms. A well-developed and complete digestive system is present in earthworms (oligochaetes) with a mouth, muscular pharynx, esophagus, crop, and gizzard being present. The gizzard leads to the intestine and ends in an anal opening. A cross-sectional view of a body segment of an earthworm (a terrestrial type of annelid) is shown in\u00a0Figure 2; each segment is limited by a membranous septum that divides the coelomic cavity into a series of compartments.<\/p>\n<div id=\"attachment_2435\" style=\"width: 554px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2435\" class=\"size-full wp-image-2435\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18195845\/Figure_28_03_17.jpg\" alt=\"The illustration shows a cross-section of an annelid. The body is divided into segmented compartments. A U-shaped intestine runs through the middle of the compartments, and two ventral nerve cords run along the bottom. In each segment, the nerve cords are connected to each other. A dorsal blood vessel sits on top of the intestine, and a ventral blood vessel rests beneath it. Other vessels connect the dorsal and ventral vessels together. The nephridium is connected to the barrier separating the compartments, and consists of a long coil connected to a trumpet-like bell.\" width=\"544\" height=\"409\" \/><\/p>\n<p id=\"caption-attachment-2435\" class=\"wp-caption-text\">Figure 2. This schematic drawing shows the basic anatomy of annelids in a cross-sectional view.<\/p>\n<\/div>\n<p>Annelids possess a closed circulatory system of dorsal and ventral blood vessels that run parallel to the alimentary canal as well as capillaries that service individual tissues. In addition, these vessels are connected by transverse loops in every segment. These animals lack a well-developed respiratory system, and gas exchange occurs across the moist body surface. Excretion is facilitated by a pair of metanephridia (a type of primitive \u201ckidney\u201d that consists of a convoluted tubule and an open, ciliated funnel) that is present in every segment towards the ventral side. Annelids show well-developed nervous systems with a nerve ring of fused ganglia present around the pharynx. The nerve cord is ventral in position and bears enlarged nodes or ganglia in each segment.<\/p>\n<p>Annelids may be either monoecious with permanent gonads (as in earthworms and leeches) or dioecious with temporary or seasonal gonads that develop (as in polychaetes). However, cross-fertilization is preferred in hermaphroditic animals. These animals may also show simultaneous hermaphroditism and participate in simultaneous sperm exchange when they are aligned for copulation.<\/p>\n<div class=\"textbox\">This combination\u00a0<a href=\"http:\/\/shapeoflife.org\/video\/annelid-animation-body-plan\" target=\"_blank\" rel=\"nofollow noopener\">video and animation\u00a0provides a closeup look at annelid anatomy<\/a>.<\/div>\n<h2>Classification of Phylum Annelida<\/h2>\n<p>Phylum Annelida contains the class Polychaeta (the polychaetes) and the class Oligochaeta (the earthworms, leeches and their relatives).<\/p>\n<p>Earthworms are the most abundant members of the class Oligochaeta, distinguished by the presence of the clitellum as well as few, reduced chaetae (<em>oligo<\/em>&#8211; = \u201cfew\u201d; &#8211;<em>chaetae<\/em> = \u201chairs\u201d). The number and size of chaetae are greatly diminished in Oligochaeta compared to the polychaetes (<em>poly<\/em>=many,\u00a0<em>chaetae<\/em>\u00a0= hairs). The many chetae of polychaetes are also arranged within fleshy, flat, paired appendages that protrude from each segment called\u00a0<strong>parapodia<\/strong>, which may be specialized for different functions in the polychates. The subclass Hirudinea includes leeches such as\u00a0<em>Hirudo medicinalis<\/em>\u00a0and\u00a0<em>Hemiclepsis marginata<\/em>. The class Oligochaeta includes the subclass Hirudinia and the subclass Brachiobdella. A significant difference between leeches and other annelids is the development of suckers at the anterior and posterior ends and a lack of chaetae. Additionally, the segmentation of the body wall may not correspond to the internal segmentation of the coelomic cavity. This adaptation possibly helps the leeches to elongate when they ingest copious quantities of blood from host vertebrates. The subclass Brachiobdella includes species like\u00a0<em>Branchiobdella balcanica sketi<\/em>\u00a0and\u00a0<em>Branchiobdella astaci<\/em>, worms that show similarity with leeches as well as oligochaetes.<\/p>\n<div id=\"attachment_2436\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2436\" class=\"size-large wp-image-2436\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18195848\/Figure_28_03_18-1024x269.jpg\" alt=\"Part a shows an earthworm, and part b shows a large leech trying to latch onto a person\u2019s hand. Part c shows a worm on that is anchored to the ocean floor. Featherlike appendages extend from the tube-like body.\" width=\"1024\" height=\"269\" \/><\/p>\n<p id=\"caption-attachment-2436\" class=\"wp-caption-text\">Figure 3. The (a) earthworm, (b) leech, and (c) featherduster are all annelids. (credit a: modification of work by S. Shepherd; credit b: modification of work by \u201cSarah G&#8230;\u201d\/Flickr; credit c: modification of work by Chris Gotschalk, NOAA)<\/p>\n<\/div>\n<div class=\"textbox learning-objectives\">\n<h3>In Summary:\u00a0Phylum Annelida<\/h3>\n<p>Phylum Annelida includes vermiform, segmented animals. Segmentation is seen in internal anatomy as well, which is called metamerism. Annelids are protostomes. These animals have well-developed neuronal and digestive systems. Some species bear a specialized band of segments known as a clitellum. Annelids show the presence numerous chitinous projections termed chaetae, and polychaetes possess parapodia. Suckers are seen in order Hirudinea. Reproductive strategies include sexual dimorphism, hermaphroditism, and serial hermaphroditism. Internal segmentation is absent in class Hirudinea.<\/p>\n<\/div>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_d97e6a9f-154c-4011-8ce6-8af6f727a84a\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/d97e6a9f-154c-4011-8ce6-8af6f727a84a?iframe_resize_id=assessment_practice_id_d97e6a9f-154c-4011-8ce6-8af6f727a84a\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-755\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":14,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\"}]","CANDELA_OUTCOMES_GUID":"6941b9f5-1a34-458d-8ca2-fa69bd88f7be, 6647e41b-2f6b-40c0-b992-5df461e99f81","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-755","chapter","type-chapter","status-publish","hentry"],"part":710,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters\/755","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":7,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters\/755\/revisions"}],"predecessor-version":[{"id":3038,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters\/755\/revisions\/3038"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/parts\/710"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters\/755\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/media?parent=755"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapter-type?post=755"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/contributor?post=755"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/license?post=755"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}