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CHAPTER 17
Annelids 17-1
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Characteris*cs of the Phylum Annelida
Diversity •  Exhibit segmenta1on or metamerism •  Bodies composed of repeated units •  Each unit contains components of most MAJOR organ systems •  Increased burrowing efficiency by permiAng independent movement of segments •  More sophis1cated nervous system •  Provided a safety factor: •  If one segment (metamere/
somite) failed, others could s1ll func1on 17-2
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Characteris1cs •  About 15,000 species •  2/3 are the more obscure marine worms. •  Segmenta1on •  Body segments marked by circular grooves called annuli •  Setae •  Tiny chi1nous bristles •  Absent in leeches •  Short setae anchor segments in earthworms •  Prevent it from slipping backward •  Long setae help aqua1c worms swim •  Polychaetes •  Primarily marine and usually benthic (lowest level of water) •  Oligochaetes and leeches •  Freshwaters, or terrestrial soils •  Many leeches are predators 17-3
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Body Plan •  Head is composed of 2 parts: •  Prostomium •  Perstonium •  Terminal por1on us the pygidium which bears the anus •  Head and anus are not considered metameres •  New metameres form in front of the anus •  Surface is covered with an epidermis and a thin outer layer of non-­‐chi1nous cu1cle •  Strong circular and longitudinal muscles underlie the body wall 17-4
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• Coelom •  Peritoneum (mesodermal epithelium) lines body wall and forms dorsal and ventral mesenteries •  Peritonea of adjacent segments meet to form the septa •  Gut and longitudinal blood vessels extend through the septa 17-5
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• Hydrosta1c Skeleton •  Except in leeches, coelom is filled with fluid and serves as a hydrosta:c skeleton •  Contrac1on of longitudinal muscles causes body to shorten and expand •  Contrac1on of circular muscles causes body to narrow and lengthen •  By separa1ng this force into sec1ons, widening and elonga1on move the whole animal •  Alternate waves of contrac1on, or peristalsis, allow efficient burrowing •  Swimming annelids use (wavelike) movements 17-6