Phylum Annelida Learning Outcomes

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Phylum AnnelidaLearning Outcomes

• Students should be able to
• Describe the unifying characteristics of members
  of phylum annelida
• Describe how annelids carry out their life
  functions
• Describe the ecological roles of annelids

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Phylum Annelida
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Leeches
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General Information

• Annelids are ringed, segmented worms
• Examples include earthworms and leeches
• Habitat terrestrial, fresh water, marine
• Have bilateral symmetry with 3 body layers
  (ectoderm, mesoderm, endoderm)

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3 Unique Features

• Have a true coelom (body cavity) lined by
  mesoderm
• Have externally visible body segmentation
• Have a closed circulatory system

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Coelom

• Located between the body wall and the intestine
• Lined by mesoderm
• Filled with fluid
• Acts as a buffer between body muscles and the
  intestine (allows worm to move without putting
  pressure on the intestine)

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Body Segmentation

• The body is divided into segments
• Each segment contains a portion of the coelomic
  cavity

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Class Polychaeta

• Polychaete worms
• Marine
• Flattened bodies
• Body segments with paired paddle-like appendages
  (parapodia)

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Class Oligochaeta

• Earthworms
• Have a few hairs
• Small head for burrowing
• May have a saddle-like swelling (clitellum)

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Class Hirudinea

• Leeches
• Often aquatic
• Bodies with suckers
• Many are ectoparasites feeding on blood

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Earthworm Ingestion

• Earthworms have a complete digestive system (one
  way digestive tract)
• Food (organic material in the soil) enters
  through the mouth
• The pharynx draws food into the mouth (suck in
  and swallows food)
• Food moves through the esophagus to the crop
  where it is stored temporarily

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Earthworm Digestion

• Food moves from the crop to the gizzard
• Gizzard contains sand grains which help grind up
  food by muscular contractions
• Food moves into the intestine where enzymes
  chemically digest (break down) food into
  particles small enough to be absorbed into blood
• Intestine has a typhlosole which increases the
  surface area for nutrient absorption

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Earthworm Elimination

• Undigested food waste moves to the end of the
  intestine and exits through the anus

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Earthworm Respiration

• Gas exchange (O2 CO2) happens via diffusion
  through the moist skin
• Mucus glands secrete mucus to keep skin moist
• O2 diffuses across the moist skin and into the
  capillaries and blood
• O2 is carried by hemoglobin (Hb) in the blood

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Earthworm Circulation

• Earthworms have a closed circulatory system
  (blood is always contained in vessels)
• Closed circulatory systems allow for more
  effective delivery of nutrients which allows for
  increased activity and size
• Earthworms have 5 pairs of hearts which pump
  blood through the vessels

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Earthworm Excretion

• Excretory organs called nephridia filter excess
  H2O and nitrogenous wastes from the coelomic
  cavity of one segment
• This waste passes through a long tube surrounded
  by blood vessels (much like our kidneys) and
  eventually exits through an excretory pore in the
  next segment
• Excess water is reabsorbed into the blood vessels

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Earthworm Sexual Reproduction

• Earthworms are hermaphrodites (male female on
  same individual)
• 2 earthworms cross-fertilize (exchange sperm)
• Earthworms have a swelling called a clitellum
  which secretes a mucus cocoon into which
  fertilized eggs are deposited

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Earthworm Asexual Reproduction

• Earthworms are able to regenerate missing body
  parts

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Earthworm Nervous System

• Earthworms have a paired ventral nerve cord which
  swells into ganglia (a mass of nerve cell bodies
  acting as a primitive brain) at the head

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Earthworm Locomotion

• Uses longitudinal and circular muscles combined
  with the fluid pressure in the coelom and the
  setae (bristles) to help burrow and move
• Contraction of the circular muscles of anterior
  (front) cause worm to lengthen
• Paired setae (bristles) anchor worm to soil
• Longitudinal muscles contract causing worm to
  shorten pulling its posterior (tail end forward)

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Evolutionary Significance

• Segmentation! Body segmentation lead to the
  ability to have specialized structures in the
  body plan of organisms.
• Closed Circulatory System heart(s) and blood
  vessels
• True Coelom mesoderm lines the digestive tract

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Ecological Significance

• Aerate the soil
• Food for organisms such as birds
• Predators in the food chain (polychaetes)