Annelids

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Annelids Part 1

• AKA Segmented Roundworms

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Annelids

• The word annelid is derived from the word annulus
  which means ring.
• In this way, annelids get their name from their
  most defining characteristic their external and
  internal segments, or rings.

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Classes of Annelids

• Annelids have three main classes based on habitat
  and differences in body structure.
• They are
• Polychaete (paul-ee-keet)
• Oligochaete (awl-ee-go-keet)
• Hirudinea (hir-oo-din-ee-uh)

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Polychaete Worms

• AKA clamworms
• Size millimeters to meters long
• Change in body structure has many bristles.
• Habitat marine environment
• Types of movement sedentary or mobile

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Mobile Polychaete

• Mobile worms
• Tend to swim around and catch food
• Have parapodia (little flaps of skin that act as
  legs) for movement.
• Parapodia have setae (bristles) and cirri
  (needle-like projections) on them.
• Have eyes, sensory tentacles, and a retractable
  pharynx with jaws.
• Ex. Nereis virens

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Sedentary Polychaete

• Sedentary Worms
• Tend to burrow in sand or mud filter feed.
• They suck up sand or mud and selectively absorb
  nutrients.
• Often lack eyes and have much smaller parapodia.
• Some secrete their own tube to burrow into.
• Ex. Arenicola

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Oligochaete Worms

• Change in body structure few bristles.
• Habitat damp terrestrial environments
• Best known oligochaete is the earthworm

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Earthworm

• The earthworm is mostly nocturnal and tends to
  live in underground tunnels.
• They help to develop and add nutrients to the
  soil.
• They are considered to be more advanced than
  polychaete.

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Hirudinea

• Change in body structure flattened body, with no
  parapodia or bristles.
• Habitat freshwater or terrestrial parasites or
  predators.
• They attach to their prey by suckers.
• Ex. Leeches
• After having its fill of a victims blood they
  can go for weeks or months without eating.

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Annelid Body Structure

• Bilateral symmetry
• Coelomates have a lined coelom (body cavity).
• This is an advancement over nematodes.
• Cuticle covered body to keep the worm from drying
  out, while still allowing movement.

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Digestive System

• More advanced than the nematodes.
• One way system, in addition to the nematode parts
  they have
• Calciferous glands
• Monitor acid-base levels of the coelomic and
  blood fluids.
• Crop
• Used to store food.
• Gizzard
• Food is ground into digestible pieces.
• Mouth ? buccal cavity ? pharynx ? esophagus
  (attached to calciferous glands) ? crop ? gizzard
  ? intestine ? anus

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Nervous System

• Anterior brain with paired ventral nerve cords.
• Has peripheral nerves that supply the muscle
  layers and reach the sense receptors of the outer
  wall.
• Also have ganglia that allow for movement.
• Each segment has a nerve cord that passes through
  a ganglia and attaches to sense receptors.
• This nerve cord consists of giant nerve fibers
  and is responsible for coordinating movement of
  all the segments.

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Muscular System

• Both longitudinal and circular muscles another
  advancement over nematodes.
• Seta retractors are muscles that connect from one
  set of setae to another. This allows for
  coordinated movement.
• Remember, setae are bundles of bristles.

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Circulatory System

• We havent seen one of these since the piggy!
• Two kinds are seen in annelids
• Open no vessels, blood empties into sinuses, or
  cavitites. (in the leech)
• Closed has vessels
• to hold and direct
• blood. (in the clam-
• worm earthworm)

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Reproduction

• Annelids are hermaphroditic.
• They reproduce sexually.

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Excretory System

• The excretory system is called a metanepridium.
• Each segment has a funnel like nephrostome which
  is covered by tiny hair-like structures (cilia)
  leading into the coelom of that segment.
• The segments are connected together by a
  convuluted tubule which leads from the
  nephrostome into the bladder of another segment.
• Wastes are excreted through the nephridiopore.

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A note on leeches

• Leeches are sometimes considered to be parasitic
  organisms.
• However, their relationship with the host is
  relatively brief, so it is more correct to
  classify them as predatory.
• 13 on page 99 in your resource book incorrectly
  states that they are not free-living, but rather
  are considered to be a parasitic organism.

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Annelid Importance

• Annelids are important to humans for the
  following reasons
• Earthworms help to add nutrients to our soil.
• Leeches are used medicinally to help promote
  blood flow to areas.
• They make good bait for fishing!!