Phylum Annelida

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Phylum Annelida

• Segmented Worms

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Annelids
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What is a segmented worm?

• Body is divided into segments that are separated
  by septa (septum, sing.)
• Septa are internal walls between each segment
• Bristles called setae are attached to septa in
  most annelids
• Allows great flexibility mobility (adaptation
  for movement)

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Annelid Characteristics

• True coelom (coelomates)
• More specialized systems
• Organs are paired in segments
• Bilateral symmetry
• About 15,000 species
• Range in length from less than 1mm to 3-m-long
• Live in sea, most freshwater habitats damp soil

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Annelid Features

• Earths most advanced worms
• All are eucoelomate, segmented protostomes
• Segmentation more correctly known as metamerism,
  meaning that parts like hearts, nephridia, and
  muscle groups repeat in series down the animals
  length
• Most have hairs or setae
• Head specialized to include differentiated organs
  like tentacles, palps and eyespots

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Anatomy

• Integumentary
• Thin but effective protein cuticle secreted by
  epidermis, similar to but softer than that of
  arthropods
• Skeletal
• hydrostatic skeleton with increased efficiency
  due to segmented body with sealed internal
  chambers

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Anatomy

• Respiration
• Often via gills (filamentous organs specialized
  for gas exchange underwater)
• Exchange via moist (mucus) skin
• Excretion
• Digestive waste passes out the anus
• Nitrogenous waste eliminated by nephridia,
  excretory organs that filter fluid in the coelom.

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Anatomy

• Endocrine
• no specific organs, from nerves only
• Nervous
• Most advanced so far. Large ventral nerve cord
  with fast-conduction neurons. Paired cerebral
  ganglia form a brain near the pharynx.

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Reproduction

• Most reproduce sexually
• Some use external fertilization and have separate
  sexes
• Others are hermaphrodites (earthworms leeches)
  although rarely fertilize themselves
• Exchange sperm store is special sacs
• Clitellum (band of thickened, specialized
  segments) secretes mucus ring were fertilization
  takes place
• Ring slips off worms body forms protective
  cocoon that hatches young worms

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Feeding Methods

• Eat decayed plants and leaves via sticky mucus
  covering pharynx
• Sometimes eat other invertebrates
• Some filter feed fan water through tubelike
  burrow which captures food in a mucus bag
• Pharynx is pushed out the mouth, grabs the food,
  and brings it back in
• Soaks food with saliva
• Sit-and-wait predators, hide in burrows from prey

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Response

• Well-developed nervous system in most
• Brain many nerve cords
• Various adaptations in species
• Sensory tentacles
• Chemical receptors
• Statocysts to detect gravity
• Two or more pairs of eyes

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Movement

• Two major groups of muscles function
  alternatively as a part of a hydrostatic skeleton
• Longitudinal muscles contract to make worm
  shorter fatter
• Circular muscles wrap around each body segment
  contract to make worm longer thinner
• Marine annelids have paddlelike appendages called
  parapodia on each segment for swimming crawling

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The Closed Circulatory System

• Most annelids have closed system where blood is
  contained within a network of blood vessels
• Blood vessels are the length of the worms body
• Blood in dorsal (top) vessel runs toward head
• Blood in ventral vessel runs toward tail
• In each segment, pair of smaller vessels called
  ring vessels connect ventral dorsal vessels and
  supply internal organs with blood
• 5 pairs of organs that act like hearts on dorsal
  side
• Moves blood quicker than an open circulatory
  system

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Open and Closed Circulatory Systems
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Phylum Annelida Classes

• Oligochaeta (oligo few chaeta bristles)
• Polychaeta (poly many chaeta bristles)
• Hirudinea

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Class Oligochaeta

• (Oligochaetes)
• Earthworms
• Tubifex worms
• Sludge worms

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Class Oligochaeta

• 3000 species, most live in freshwater or in soil
• Name of class means few hairs many have only
  2-4 per segment
• Each seta (made of chitin) has muscles which
  control it
• Setae are used (in earthworms) along with
  peristaltic muscle contractions to move worm
  through soil
• most highly organized animals to have
  regeneration
• Reduced head no parapodia

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Lumbricus
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Lumbricus, the earthworm

• Earthworms do eat soil, but target high
  concentrations of decayed vegetation and other
  refuse
• The muscular and chemosensory prostomium aids in
  tunneling and in determining direction of travel
• Soil is ingested after being moistened by
  secretions of the pharynx, then held in crop
  before processing in the gizzard and intestine.
• The intestines typhlosole helps increase surface
  area
• Remember, two nephridia (excretory) per segment
• Circulatory system has muscular dorsal vessel and
  aortic arches. The rest of the system is venous

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Earthworm External Anatomy
Metamerism serial repetition of body
parts Setae Used to grip surface during
motion Clitellum produces protective cocoon
around eggs
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Earthworm Internal Anatomy
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Focus on Earthworms

• Live in burrows
• Live in nearly all environments
• Tunnel for a living
• Eat decayed plants and animal remains
• Droppings make soil fertile

• Length - 1mm up to 3m
• Body is made of two tubes, one inside the other

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Class Polychaeta

• (Polychaetes)
• Sandworms, Bloodworms
• Feather worms, Clam worms,
• Tube worms

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Class Polychaeta

• Largest group of annelids
• General Characteristics
• Well-developed head
• Tube-dwelling free-living species
• Have numerous setae at ends of parapodia
• Parapodia fleshy segmented appendages for
  locomotion breathing

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Class Polychaeta

• Most numerous class (in species) of annelids
  10,000 species
• Bodies covered in many bristles, which are
  modified for feeding, swimming, or defense
• Complex cephalization in many, with reversible
  pharynx and jaws, also prostomium with eyes and
  tentacles.
• Group roughly divided into sessile members and
  motile members
• Most sessile worms filter feeding, while errant
  worms are efficient predators and scavengers.

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Nereis, a polychaete annelidthe clam worm

• Marine predatory worm which lives in crevices in
  reefs, and burrows of other animals. Fierce
  predator, grows to 40 cm
• Most segments contain rudimentary appendages with
  many setae
• Pharynx is reversible, and includes chitinous
  jaws
• Prostomium with palps (food manipulation),
  tentacles (sensory and tactile) and eyespots.
• Some polychaetes have vertebrate-like eyes

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Polychaeta Neries
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Polychaeta Christmas Tree Worm
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Class Hirudinea

• Leeches

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Class Hirudinea

• 500 species, mostly freshwater and terrestrial
• Often dorso-ventrally flattened, and have a
  reduced coelom reduced segmentation
• Often highly patterned for camouflage
• Leeches are predatory, temporarily parasitic or
  fully parasitic
• Most lack setae and have sucker discs instead
  one on front and back
• The gut is large and branched for storage of
  large quantities of blood

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Class Hirudinea

• A clitellum is present - apparently only during
  reproduction
• True bloodsuckers have cutting plates for cutting
  through tissue of the host organism
• Most species are free-living carnivores (eat
  snails insects)
• Usually attaches by posterior sucker until
  suitable spot is found for attachment of anterior
  sucker

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Hirudinea leech
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Class Hirudinea

• Hirudo medicinalis medicinal leech
• Leeches secrete saliva containing a strong
  anesthetic anticoagulant painless bite
  blood doesnt clot
• Leech anticoagulant now synthetically made
• Leeches are still occasionally used
• Remove blood from bruised tissues
• Help relieve swelling in fingers or toes sewn
  back on after accidents to allow small blood
  vessels time to grow back
• Remove excess blood