Segmented Worms

1
Segmented Worms

• Chapter 17

2
Annelids and Allied Taxa

• Phylum Annelida
• Clade Pleistoannelida
• Errantia
• Sedentaria
• Phylum Sipuncula

3
Annelids and Allied Taxa

• Members of Phylum Sipuncula are benthic marine
  animals with unsegmented bodies.
• Molecular sequence data place echiurans within
  phylum Annelida.
• Sipunculans - Sister taxon to Annelida.

4
Phylum Annelida

• Annelids are protostome coelomates in superphylum
  Lophotrochozoa.
• Spiral, determinate cleavage.
• Nervous system more centralized circulatory
  system more complex than in previous phyla.

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Phylum Annelida

• Annelids are segmented worms.
• They have bodies composed of a series of fused
  rings.
• Earthworms, leeches, clam worms.

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Phylum Annelida

• The evolutionary innovation shown by annelids is
  segmentation (metamerism).
• Segmentation evolved separately in annelids,
  arthropods, and chordates.
• The body is divided into a series of segments,
  each having similar components of all major organ
  systems.
• Built in fail-safe.
• Allows for specialization.

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Phylum Annelida

• Many annelids have chitinous bristles called
  setae.
• Help in locomotion
• Anchor worm in place
• Deter predators

8
Phylum Annelida

• Annelids can be found worldwide in marine,
  freshwater, and terrestrial habitats.

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Phylum Annelida Body Plan

• Prostomium anterior part followed by segmented
  body.
• Pygidium terminal portion.

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Phylum Annelida Body Plan

• Peritonia (layers of mesodermal epithelium) of
  adjacent segments meet to form septa.
• Fluid-filled coelom acts as a hydrostatic
  skeleton.

11
Phylogeny

• Traditionally, annelids are divided among 3
  classes
• Class Polychaeta
• Class Oligochaeta
• Class Hirudinida
• Polychaeta is a paraphyletic class because
  ancestors of the clitellates arose from within
  it.
• Oligochaeta and Hirudinida form a monophyletic
  group called Clitellata.
• Characterized by reproductive structure called a
  clitellum.
• Class Oligochaeta is a paraphyletic group because
  ancestors of leeches arose from within it.

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Clade Pleistoannelida

• New Classification
• Clade Errantia
• Clade Sedentaria

13
Errantia

• Errant polychaetes have some features other
  annelids do not
• A well developed head.
• Paired appendages, parapodia, that function as
  gills and aid in locomotion.
• No clitellum.
• Many setae

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Errantia

• Errant forms include pelagic and benthic types
  and are often predators or scavengers.

15
Reproduction

• Gonads are temporary structures in polychaetes.
• Sexes usually separate.
• Fertilization is external.
• Early larva is a trochophore.

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Circulation and Respiration

• Most have parapodia and gills for gaseous
  exchange.
• Others use the body surface.
• Circulation varies.
• In Nereis a dorsal vessel carries blood forward
  and a ventral vessel carries blood posteriorly.
• Blood flows across between these major vessels in
  networks around the parapodia and intestine.
• In some, septa are incomplete and coelomic fluid
  serves circulatory function.
• Many polychaetes have respiratory pigments -
  Hemoglobin, chlorocruorin or hemerythrin.

17
Excretion

• Excretory organs vary, from protonephridia to
  metanephridia, and mixed forms.
• One pair per metamere.
• Inner end (nephrostome) opens into the coelomic
  cavity.
• Coelomic fluid enters the nephrostome.
• Selective resorption occurs along the nephridial
  duct.

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Nervous System and Sense Organs

• Double ventral nerve cord runs length of the worm
  with ganglia in each metamere.
• Sense organs include
• Eyes, nuchal organs and statocysts.
• Eyes vary from simple eyespots to well-developed
  image-resolving eyes similar to mollusc eyes.
• Nuchal organs are ciliated sensory pits that are
  probably chemoreceptive.
• Some burrowing and tube-building polychaetes use
  statocysts to orient their body.

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Representative Errant Polychaetes

• Clam Worms Nereis
• Errant polychaetes
• Live in mucus-lined burrows near low tide level.
• Come out of hiding places at night to search for
  food.
• Prostomium bears a pair of palps sensitive to
  touch and taste, a pair of short sensory
  tentacles, and two small dorsal eyes sensitive to
  light.

• Peristomium has a ventral mouth, a pair of jaws,
  and four pairs of sensory tentacles.

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Representative Errant Polychaetes

• Scale worms
• Flattened bodies are covered with broad scales.
• Some are large, all are carnivores and some are
  commensals in burrows of other organisms.

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Representative Errant Polychaetes

• Fireworms
• Have hollow, brittle setae that contain poisonous
  secretions.
• Feed on cnidarians.

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Sedentaria

• Sedentaria now includes the sedentary polychaetes
  (tubeworms, siboglinids), echiurans, oligochaetes
  and leeches.

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Sedentaria

• Sedentary forms often have elaborate devices for
  feeding and respiration.
• Filter or deposit feeders.

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Representative Sedentary Polychaetes

• Tubeworms
• Tube-dwellers
• May line their burrows with mucus
• Use cilia or mucus to obtain food

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Representative Sedentary Polychaetes

• Fanworms or Featherduster worms
• Unfurl tentacular crowns to feed.
• Food moved from radioles to mouth by ciliary
  action.

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Representative Sedentary Polychaetes

• Parchment Worms
• Lives in a U-shaped tube.
• Modified segments pump water through tube.

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Clade Siboglinidae (Pogonophorans)

• Formerly members of phylum Pogonophora
  (beardworms).
• Discovered in 1900.
• 150 species described.
• Most are small, less than 1 mm in diameter.
• Giant beardworms that live in deepwater
  hydrothermal vents are 3 m long and 5 cm in
  diameter.

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Family Siboglinidae (Pogonophorans)

• Most live in mud on ocean floor at depths of 100
  - 10,000m.
• Sessile animals that secrete and live in long
  chitinous tubes.
• Tubes have general upright orientation in bottom
  sediments.
• Tubes are generally three or four times the
  length of the animal.

http//www.youtube.com/watch?v2FFnrW_SUdM
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Family Siboglinidae (Pogonophorans)

• Long cylindrical body covered with cuticle.
• Divided into a short anterior forepart, a long
  slender trunk, and a small, segmented
  opisthosoma.
• Tentacles are hollow extensions of the coelom and
  bear minute pinnules.

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Family Siboglinidae (Pogonophorans)

• No mouth or digestive tract.
• Nutrients such as glucose and amino acids
  absorbed from seawater through pinnules and
  microvilli of tentacles.

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Family Siboglinidae (Pogonophorans)

• Most energy derived from a mutualistic
  relationship with chemoautrophic bacteria that
  oxidizes hydrogen sulfide.
• Trophosome, derived embryonically from midgut,
  houses the bacteria.

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Family Siboglinidae (Pogonophorans)

• Sexes are separate.
• Research suggests that cleavage is unequal and
  atypical.
• Appears to be spiral.
• Coelom formed by schizocoely.
• Embryo
• Worm-shaped and ciliated.
• Poor swimmer.
• Probably carried by water currents until it
  settles.

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Osedax bone eating worms
http//www.youtube.com/watch?vURi8KccVkks
34
Family Echiuridae

• Approximately 140 species of marine worms that
  burrow into mud or sand.
• Live in empty snail shells or sand-dollar tests,
  or rocky crevices.
• Found in all oceans.
• Length varies from a few millimeters to 40 or 50
  cm.

35
Family Echiuridae Form and Function

• Sausage-shaped.
• Inextensible proboscis anterior to the mouth.
• Often called spoon worms.
• Simple nervous system with a ventral nerve
  running length of the body.
• Ciliated groove on the proboscis allows them to
  gather detritus over the mud while lying buried.

• Muscular body wall is covered by a cuticle and
  epidermis which may be smooth or covered by
  papillae.

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Family Echiuridae

• Large coelom.
• Digestive tract long and coiled.
• Most have a closed circulatory system with
  colorless blood.
• Hemoglobin found in certain cells and in coelomic
  corpuscles.
• Respiration probably occurs in hindgut which is
  continually filled and emptied by cloacal
  irrigation.

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Family Echiuridae - Reproduction

• Sexes are separate.
• Gonads produced by special regions in peritoneum
  in each sex.
• Fertilization usually external.
• Early cleavage and trochophore stages similar to
  annelids.

38
Clade Clitellata

• Class Oligochaeta and Class Hirudinida
• Form reproductive structure called a clitellum.
• Ring of secretory cells found in a band around
  the body.
• Permanent in oligochaetes but visible only during
  reproductive season in leeches.
• Members are derived annelids that lack parapodia.
• Hermaphroditic (monoecious) animals that exhibit
  direct development.
• Young develop inside a cocoon secreted by the
  clitellum, and emerge as small worms.

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Oligochaeta

• Class Oligochaeta includes earthworms and many
  freshwater worms.
• They possess setae, but not as much as
  polychaetes.

40
Oligochaeta

• Earthworms are the most familiar oligochaetes,
  found in moist, rich soil.
• They can burrow deep underground and remain
  dormant in a slime chamber during dry weather.
• Setae help prevent slipping while burrowing.

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Oligochaeta

• Darwin wrote about earthworms in The Formation of
  Vegetable Mould Through the Action of Worms.
• He noted the beneficial activities of worms,
  aeration, moving nutrients up from subsoil,
  adding nitrogenous products, breakdown of organic
  matter in dead leaves etc.
• An earthworm can ingest its own weight in soil
  every 24 hours.

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Oligochaeta - Reproduction

• Earthworms are hermaphroditic male and female
  organs in the same animal.
• When mating, two worms are held together by mucus
  secreted by the clitellum.

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Oligochaeta - Reproduction

• After mating, a cocoon forms around the
  clitellum, as it passes forward it gathers both
  gametes, and fertilization occurs inside.

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Oligochaeta - Reproduction

• Development occurs inside the cocoon and young
  worms hatch out.
• Development is direct, no larval stage.

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Oligochaeta - Feeding

• Food is stored in a thin-walled crop.
• Muscular gizzard grinds food into small pieces.
• Digestion and absorption occur in intestine.

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Oligochaeta - Excretion

• Each segment, except the 1st three and terminal
  one, have a pair of metanephridia.
• A ciliated funnel, the nephrostome, draws in
  wastes and leads through the septum.
• These coil until the nephridial duct ends at a
  bladder that empties outside at nephridiopore.
• Wastes from both the coelom and the blood
  capillary beds are discharged.
• Aquatic oligochaetes excrete toxic ammonia.

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Oligochaeta - Circulation and Respiration

• Coelomic fluid and blood transport food, wastes,
  and respiratory gases.
• Blood circulates in a closed system with five
  main trunks running lengthwise in the body.
• Dorsal vessel contains valves and functions as a
  true heart.
• Pumps blood anteriorly into 5 pairs of aortic
  arches.
• Aortic arches ensure steady pressure in ventral
  vessel.

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Oligochaeta - Nervous System and Sense Organs

• Central nervous system and peripheral nerves.
• Pair of cerebral ganglia connect around the
  pharynx to the ganglia of the ventral nerve cord.
• Neurosecretory cells in brain and ganglia secrete
  neurohormones.
• Regulate reproduction, secondary sex
  characteristics, and regeneration.

• Lack eyes but have many photoreceptors in the
  epidermis.
• Free nerve endings in tegument are probably
  tactile structures.

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Oligochaeta - General Behavior

• Avoid bright light (negative phototaxis).
• Chemical stimuli are important in locating food.
• Limited learning ability - primarily
  trial-and-error learning.

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Hirudinida

• Class Hirudinida includes the leeches.
• Primarily freshwater, a few marine terrestrial.
• More common in tropical climates.

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Hirudinida

• Many leeches live as carnivores on small
  invertebrates.
• Some are temporary parasites.
• Some are permanent parasites they never leave
  their host.

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Hirudinida

• Leeches are hermaphroditic and have a clitellum
  (only appears during breeding season), like
  oligochaetes.
• Leeches do not have setae.
• Theyve developed suckers for attachment and a
  specialized gut for storing large amounts of
  blood.

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Hirudinida - Respiration and Excretion

• Some fish leeches have gills.
• All other leeches exchange gases across
  epidermis.
• 10 to 17 pairs of nephridia.
• Coelomocytes and other special cells may assist
  in excretion.

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Hirudinida - Nervous and Sensory Systems

• Two brains
• Anterior fused ganglia form a ring around the
  pharynx.
• Seven pairs of posterior fused ganglia.
• 21 pairs of segmental ganglia in between along a
  double nerve cord.
• Epidermis contains free sensory nerve endings and
  photoreceptor cells.
• Pigment-cup ocelli are present.

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Hirudinida - Circulation

• Coelom reduced by invasion of connective tissue.
• Forms system of coelomic sinuses and channels.
• Some have a typical oligochaete circulatory
  system.
• Coelomic system is auxiliary.
• Some lack blood vessels and coelomic sinuses
  serve as only vascular system.

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Class Hirudinida

• Leeches are highly sensitive to stimuli
  associated with the presence of prey.
• Those that feed on mammals are attracted by
  warmth.

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Phylum Sipuncula

• Approximately 250 species of benthic marine
  worms.
• Sedentary, living in burrows of mud or sand,
  snail shells, coral crevices, or among
  vegetation.
• More than ½ restricted to tropical zones.
• Some are tiny, slender worms, but most range from
  3 to 10 cm in length.

• Some are known as peanut worms because when
  disturbed, they contract to a peanut shape.

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Phylum Sipuncula - Form and Function

• No segmentation or setae.
• Slender, retractable introvert or proboscis at
  anterior end.
• Walls of the trunk are muscular.

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Phylum Sipuncula - Nutrition

• Some appear to be detritivores and others
  suspension feeders.
• Some nutrition may come from dissolved organic
  matter in the surrounding water.
• From burrow or hiding place, they extend
  tentacles to explore and feed.
• Collected organic matter moved from mucus on
  tentacles to mouth by ciliary action.
• Large fluid-filled coelom.
• Digestive tract is U-shaped.

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Phylum Sipuncula - Respiration

• Lack a circulatory and respiratory system.
• Gas exchange appears to occur across the
  introvert and tentacles.

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Phylum Sipuncula - Nervous and Sensory Systems

• Bilobed cerebral ganglion behind tentacles.
• Ventral cord extends the length of body.

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Phylum Sipuncula - Reproduction

• Sexes are separate.
• Sex organs develop seasonally within the
  connective tissue covering the origins of the
  retractor muscles.
• Sex cells are released through the nephridia.
• Asexual reproduction occurs by transverse fission.

63
Phylogeny

• Similarities in the early development of
  molluscs, annelids, and some primitive arthropods
  indicate that these three groups are probably
  closely related.
• Trochophore larva
• Spiral cleavage
• Schizocoelous coelom formation

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Evolutionary Significance of Metamerism

• No satisfactory explanation for origins of
  metamerism and coelom has gained acceptance.
• Coelom may have been advantageous as a
  hydrostatic skeleton.
• Coelomic fluid would have acted as a circulatory
  fluid and reduce need for flame cells everywhere.
• Coelom could store gametes for timed release.
• Would require nervous and endocrine control.

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Evolutionary Significance of Metamerism

• Unlikely that segmentation is homologous among
  annelids, arthropods, and chordates.
• Current evidence supports the hypothesis that
  segmentation arose independently multiple times.

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Evolutionary Significance of Metamerism

• Selective advantage of a segmented body for
  annelids appears to lie in the efficiency of
  burrowing.
• However, does not explain segmentation in
  arthropods given the rigidity of the exoskeleton.

67
Phylogeny

• Molluscs and annelids share many developmental
  features so are presumed to be closely related.
• However, shared features are likely to be a
  retained ancestral feature for lophotrochozoan
  protostomes.
• Molecular analyses place sipunculids closely
  related to the annelids.