Phylum Annelida: summary of characteristics

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Phylum Annelida summary of characteristics

• Name from Latin annulus meaning a ring.
• Vermiform. Possess tissues and organs.
• Muscular gut with mouth and anus.
• Body divided into segments.
• Outer epithelium with clumps of bristles (except
  in forms with suckers). May be covered with a
  cuticle.
• Body wall muscular with both circular and
  longitudinal muscles.
• Closed circulatory system.
• Nervous system with supraoesophageal ganglion,
  circum-oesophageal ring and ventral nerve cord.
• Nephridia responsible for most excretion

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Phylum Annelida

• The annelids (L. annelus a little ring) are the
  segmented worms.
• Annelids are coelomate, protostomes and the body
  is metameric being composed of serially repeated
  segments or metameres.
• Each segment is separate from the next segments
  being divided by partitions or septa.

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Segmentation

• Within each segment are components of most organ
  systems such as the circulatory, nervous and
  excretory systems.
• Thus, there is a degree of redundancy in annelids
  so that if a segment is damaged it need not be
  fatal.

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Segmentation

• The evolution of segmentation is the great
  evolutionary innovation of the annelids.
• Segmentation allows annelids to make more precise
  body movements than organisms that have a
  hydrostatic skeleton, but lack segmentation e.g.
  the pseudocoelomate nematodes.

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Segmentation

• Because the coelom is divided by septa the force
  of muscle contraction in a segment is not
  transmitted throughout the body, but instead is
  confined to the single segment.
• Thus, one segment may elongate while the adjacent
  one contracts and this allows the animal to make
  fine, controlled movements.

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Movement

• With the exception of the leeches, the coelom is
  filled with fluid and acts as a hydrostatic
  skeleton.
• Annelids possess circular and longitudinal
  muscles and this enables individual segment to be
  elongated or contracted.
• Crawling is achieved by alternating waves of
  contraction by circular and longitudinal muscles
  passing down the body (peristalsis).

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Movement

• Because they have fine control of movement
  annelids have evolved a relatively sophisticated
  nervous system.
• Most annelids are burrowing forms and as an
  adaptation to this lifestyle bear short chitinous
  bristles called setae on each segment. The setae
  enable the annelid to gain traction against the
  side of the burrow.

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Movement

• In other annelids longer hair-like setae assist
  the animal in swimming.
• For the annelids that live in burrows or in tubes
  the setae help to prevent the animal from being
  pulled out.

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Annelids

• Annelids occur worldwide being found in the sea,
  freshwater, and in the soil.
• They feed on organic matter in the mud or soil,
  by filtering suspended particles from the water,
  act as predators, or suck blood.

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Annelids

• The typical annelid body has a two part head made
  up of a prostomium and a peristomium, a series of
  segments, and a terminal pygidium which contains
  the anus.
• Neither the head nor the pygidium are considered
  true segments. In growth, new segments form
  anterior to the pygidium. If an annelid is cut
  in two the posterior segments can be regrown.

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Annelid Classification

• There are approximately 12,000-15,000 species of
  annelids divided into 4 classes
• Polychaeta polychaete worms
• Oligochaeta earthworms
• Hirundinea leeches
• Siboglinidae pogonophorans

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Class Polychaeta

• The polychaetes are the largest of the annelid
  classes and include more than 10,000 described
  species, most of which are marine.
  Morphologically very diverse.
• The name poly chaete refers to the numerous
  chaetae or bristles they possess.

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Polychaetes

• Polychaetes have a well differentiated head that
  has sense organs including eyes and cirri (short
  tentacles), jaws (in predatory forms), or a fan
  for filter feeding.
• Most segments bear parapodia, which are lobed
  structures used in swimming, crawling, or for
  anchorage in tubes. Parapodia also serve as
  gills.

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Class Polychaeta

• Polychaetes follow one of two basic lifestyles
  being either sedentary/sediment burrowing
  (sedentary) or active hunting (errant)
  species.
• Sedentary polychaetes usually exhibit variation
  in the structure of segments. All are
  filter-feeders or deposit feeders.
• Sedentary polychaetes burrow in mud and soil or
  build their own tubes from which they filter
  feed.
• Tubes may be made from calcium carbonate, a
  secreted paper-like material, or sand grains.

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Fanworms

• Most of the sedentary polychaetes, which inhabit
  burrows or build tubes, are filter feeders and
  consume plankton or detritus.
• Forms such as fanworms extend long, modified
  feathery crowns of stiff prostomial tentacles to
  feed. Ciliary action draws in food, which is
  trapped in mucus and delivered down grooves to
  the mouth.

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Burrowing polychaetes

• A number of families of sedentary polychaetes
  burrow in soft sediments either swallowing
  sediment or scraping it of bacteria, algae, fungi
  and other live material.
• Many functionally resemble oligochaetes and have
  reduced parapodia, lack prominent sense organs
  and have well developed circular muscles and
  septa.
• Some have soft prehensile tentacles they use
  collect food particles.

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Polychaete.jpg
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Burrowing Polychaetes

• Burowing polychaetes such as lugworms are very
  common on estuaries.
• They make burrows in the sand and consume large
  quantities of sand. After theyve extracted the
  digestible material the remaining material is
  defecated and forms a characteristic pile outside
  the burrow.

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Lugworm (two images above) from http//marinebio.o
rg/species.asp?id57
Above right Lugworm casts. http//upload.wikimedia
.org/wikipedia/commons/7/7b/Lugworm_cast.jpg
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Burrowing Polychaetes

• Lugworms are an important source of food for
  wading birds.

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Predatory polychaetes

• Predatory forms of polychaetes such as Nereis
  have a muscular pharynx equipped with jaws that
  can be quickly everted to grab prey.

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Bobbit worm a predatory polychaete.
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Predatory polychaetes

• Predatory polychaetes typically can crawl rapidly
  using their parapodia.
• They are active hunters that can sqeeze through
  small spaces (e.g. in coral, crevices, etc.) is
  search of prey.
• They consume any other invertebrates that they
  can catch and dismember.

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Class Oligochaeta

• There are over 3000 species of oligochaetes, the
  most familiar of which are the earthworms.
• Lumbricus terrestris, the common earthworm, grows
  from 4-12 inches, but tropical forms may reach 12
  feet in length.

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Class Oligochaeta

• Earthworms burrow in rich, damp soil and leave
  their burrows at night to eat vegetation and to
  breed.
• Earthworms play a significant role in soil
  fertility by aerating the soil with their
  burrows, adding vegetable material, and mixing
  subsoil and topsoil.

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http//www.cheshirewildlifetrust.co.uk/IMAGES/watc
h_earthworm.jpg
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Class Oligochaeta

• Darwin studied earthworms and published a book on
  their effects on soil.
• He estimated that an earthworm eats its own
  weight in soil daily and that in an acre of land
  10-18 tons of dry soil passed through their guts
  annually.
• Earthworms consume dead organic material and
  partially digest it, the waste passing out of
  them containing nutrients valuable to plants and
  supplemented with nitrogenous wastes from the
  worm.

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Class Oligochaeta

• In addition to the earthworms there are many
  freshwater species, most of which burrow in silt
  and mud or creep along the bottom, although some
  live among submerged vegetation.
• Freshwater forms usually are smaller than
  terrestrial and have more conspicuous setae.
• Most respire through their skins, but some have
  gills. Most are algae or detritus feeders.

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Freshwater oligochaetes
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Class Oligochaeta

• Oloigochaetes, like all annelids, have a double
  circulatory system as both the coelomic fluid and
  circulatory system are used to carry food, wastes
  and gases.
• The blood system is closed, with the dorsal blood
  vessel being the main pumping organ.

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Class Oligochaeta

• The excretory organs are called nephridia and
  there is a pair in each segment, each of which
  occupies parts of two successive segments.
• A ciliated funnel (the nephrostome) opens just
  anterior of an intersegmental septum and from
  this a tubule leads into the posterior segment
  and forms a series of loops that are closely
  surrounded by blood vessels.

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Excretory organs

• The tubule eventually opens to the outside via an
  aperture called a nephridiopore.
• The system works by cilia drawing coelomic fluid
  into the nephrostome and selective reabsorbtion
  of salts and water occurs in the loops leaving
  only a dilute urine to be excreted to the outside.

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Reproduction in earthworms

• Earthworms are hermaphroditic and mate by
  aligning their ventral surfaces together.
• Each worms clitellum (thickened section of some
  midbody segments) secretes mucus, which holds the
  two worms together.
• Sperm is exchanged and stored in a seminal
  receptacle.

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Reproduction in earthworms

• After sperm has been exchanged the worms separate
  and each secretes a cocoon around its clitelleum.
  The cocoon slides along the body and picks up
  eggs and sperm.
• Fertilization occurs within the cocoon as does
  later embryonic development.
• As the cocoon slides off the worm its ends seal.
  Young worms emerge several weeks later.

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Class Hirudinea

• There are more than 500 species of leeches, most
  of which are freshwater inhabitants.
• Leeches have anterior and posterior suckers which
  they use in locomotion. With the exception of
  one group, leeches lack septae and their coelom
  is largely filled with connective tissue and
  muscle.

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Class Hirudinea

• Many leeches are carnivorous, but leeches are
  best known as blood-sucking ectoparasites.
• The leech penetrates its host using its jaws or
  proboscis and sucks blood with its powerful
  pharynx.
• To ensure blood continues to flow the leech
  secretes a powerful anticoagulant (hirudin) in
  its saliva.

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Worlds largest leech Haementeria ghilianii
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Class Hirudinea

• For hundreds of years leeches were used for
  blood letting, in the belief that too much blood
  caused a variety of medical conditions
• After being discarded as a medical tool leeches
  are again being used by surgeons.

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Class Hirudinea

• In reattachments of severed digits and in the
  case of skin grafts, because the blood vessels
  are damaged, pooling of blood often threatens to
  kill the attached tissue.
• Leeches, however, can remove the pooling blood
  safely allowing time for veins to develop.

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Medicinal leech feeding
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Class Siboglinidae (pogonophorans)

• The pogonophorans (or beardworms) were formerly
  considered to be a phylum, but now are considered
  to be derived from the polychaetes.
• These were first discovered during deep sea
  dredging in 1900 off Indonesia, but since then
  about 80 species have been identified in seas
  worldwide.

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Class Siboglinidae (pogonophorans)

• While similar to tube dwelling polychaetes, the
  first pogonophorans were considered to be a
  separate group because they lack a complete gut
  and appeared not to be segmented.

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Class Siboglinidae (pogonophorans)

• The lack of segmentation proved to be illusory.
  Pogonophorans live buried in the mud and their
  lower ends were broken off when collected during
  dredging.
• In 1964 complete pogonophorans were dredged up
  and it was discovered that the posterior end of
  pogonophorans (called the opisthosoma) is
  segmented and bears setae.

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Class Siboglinidae (pogonophorans)

• Most siboglinids live in the mud and silt of the
  seafloor usually at depths gt 200m.
• The body is divided into a short forepart, which
  bears tentacles, a long, slender trunk, and the
  small segmented opisthosoma. The body is covered
  with a cuticle and has setae on the trunk and
  opisthosoma.

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Siboglinum fiordicum
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Opisthosoma
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Class Siboglinidae (pogonophorans)

• Because siboglinids have no mouth or complete gut
  its unclear how they obtain nutrition.
• They absorb some nutrients in the water through
  their tentacles, but most energy apparently is
  derived from a mutualistic association with
  chemoautotrophic bacteria.
• The bacteria oxidize hydrogen sulfide to produce
  energy and live in an expanded section of the
  midgut called a trophosome. There is no foregut
  or hindgut.

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Phylum Echiura

• The Echiura (from Greek -- echis a viper and
  ura a tail) are worms that are closely related
  to the Annelids.
• Like annelids thay have a trochophore larva, but
  differ from the annelids in being unsegmented.
• They are widely distributed in shallow marine
  benthic habitats.
• Possess a characteristic extensible proboscis
  (used in feeding on detritus) and a set of small
  hooks or spines on the tail.
• Echiurans live in permanent burrows in soft
  sediments. Most are unselective detritus feeders

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Echiura http//www.usp.br/cbm//images/rsgallery/d
isplay/echiura02.JPG.jpg
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Phylum Echiura

• Urechis caupo the innkeeper worm is a common
  inhabitant of mudflats along the coast of
  California.
• It builds and lives permanently in a U-shaped
  burrow and it uses a mucus net secreted by its
  proboscis to trap plankton in water it draws
  through its burrow.
• Urechis is called the "innkeeper worm" because
  many marine organisms, such as small crustaceans,
  polychaete worms and fish, live commensally
  inside its burrow.

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Urechis caupo the Innkeeper worm http//www.ryanph
otographic.com/images/JPEGS/Echiuran.jpg
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Phylum Sipuncula

• The Phylum Sipuncula (from Latin meaning little
  pipe) consists of approximately 250 species of
  benthic, marine worms, most from 15-30 cm in
  length.
• Sometimes referred to as the peanut worms most
  burrow in sand or silt or occupy crevices or
  empty mollusc shells or worm tubes.

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Phylum Sipuncula

• The body is unsegmented and divided into an
  anterior introvert and a posterior trunk.
• Like the Echiurans the sipunculids are generally
  non-selective deposit feeders and they use the
  tentacles surrounding the tip of the introvert to
  collect food.
• They produce a trochophore larva similar in
  structure to that of the annelids.

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