Biology 320 Invertebrate Zoology Fall 2005

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Biology 320Invertebrate ZoologyFall 2005

• Chapter 13 Phylum Annelida
• Part Two

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Taxon Clitellata

• Two classes
• Class Oligochaeta
• Class Hirudinomorpha
• Posses a clitellum
• Several (6 or 7 in Lumbricus) anterior segments
  with a thick, glandular epidermis
• Conspicuous during reproduction
• Close proximity to gonopores
• Produces mucus for copulation, albumen for eggs,
  and cocoons
• Lack parapodia, prostomial and pygidial
  appendages
• Copulating hermaphrodites with direct development

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Class Oligochaeta

• Name means few chaetae
• 3500 spp.
• Earthworms are most familiar, but also small FW
  and marine varieties
• 200 marine spp., mostly interstitial
• Giant Australian earthworm (Megascolides
  australis)
• Up to 3 m long

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Body Form

• Only exceptions to generalized annelid are listed
• Four bundles of chaetae per segment
• Two ventral
• Two lateral
• Chaetae are structurally simple
• Genital chaetae are more complex
• One to 25 chaetae per bundle
• Two per bundle in most earthworms
• Protractor and retractor muscles attached to each
  chaeta
• Small in terrestrial varieties and longer in
  aquatics

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Coelom

• Earthworms posses coelomopores
• Connect coelomic cavities with exterior
• Posses a sphincter
• Pores are located in intersegmental furrows
• Exude coelomic fluid dorsally
• Keeps animal moist
• Deters predators
• Squirter worm (Didymogaster) can squirt fluid
  30 cm high

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Locomotion

• Crawl or burrow using peristalsis
• Mucus and egested soil coat burrow walls
• Lumbricus terrestris covers burrow opening
• Larger worms can burrow deeper
• Chaetae are extended or retracted as needed to
  apply optimal traction
• Animal moves forward in steps
• 2 3 cm per step
• Approximately 30 cm per minute in some
• Can even crawl backwards

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Nutrition

• Scavengers of dead organic matter (such as
  leaves) and / or deposit feeders
• Swallow soil
• Takes 1 2.5 hr to process
• Castings are excreted
• Great for soil (aerates, mixes, and transfers
  nutrients)
• Aquatic varieties may be carnivores of amebas,
  ciliates, rotifers, etc.
• Some may parasitize FW snails

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Nervous System

• One large ventral nerve cord instead of two
  lateral cords
• Five giant axons
• Stimulation of either end of worm elicits an
  escape response (wriggling, or withdrawal into
  burrow)
• Subpharyngeal ganglion is motor control center
• Most lack eyes but have simple ocelli
• Some have rings of chemoreceptors (called
  tubercles) that project from cuticle

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Digestive System

• Pharyngeal bulb
• Eversible in aquatic spp.
• Muscular pump in terrestrial spp.
• Esophageal glands secrete mucus and enzymes
• Compartmentalized esophagus
• Crop for food storage
• Gizzard for grinding
• Lined with chitin-like cuticle

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Calciferous Glands

• Located in wall of esophagus
• Produce calcite crystals that are secreted into
  esophageal lumen and pass in feces
• Two functional hypothesis
• 1) Removal of excess CO2
• Soil CO2 is high (relative to atmospheric
  levels), due to bacterial respiration
• Therefore there is a negative concentration
  gradient for diffusion of worms CO2
• CO2 combines with calcium ions to form calcite
• 2) Removal of excess calcium

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Digestive System Cont

• Intestine makes up posterior 3/4 of digestive
  system
• Anterior half secretes digestive enzymes
• Cellulase and chitinase (breaks down fungal cell
  walls) are released by mutualistic bacteria
• Posterior half is absorptive
• Typhlosole is a large dorsal fold in the
  intestine that increases its surface area
• Chlorogogen cells surround intestine
• Waste - laden cells are released into coelom
• Exit body via nephridiopores or coelomopores

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Circulation and Gas Exchange

• Often have hearts (described earlier)
• Five pairs in Lumbricus (segments 7-11)
• Gas exchange is carried out across body wall
• Large species have capillary loops in epidermis
  and hemoglobin dissolved in plasma
• Moist surface facilitates diffusion
• Oxygen levels in soil drop after heavy rains,
  forcing earthworms to the surface

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Excretion

• Ureotelic but excrete some ammonia
• Depends on environmental conditions
• Osmoregulation
• Urine is hyposmotic
• Often several types of nephridia
• Allows them to tolerate dry soils
• Those without special nephridia must burrow
  deeper during dry periods

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Encystment

• Can secrete tough mucus covering
• Summer cysts to avoid desiccation
• Winter cysts during periods of low temperature
• Undergo diapause
• Some migrate deeper into soil during dry or cold
  periods
• Up to three meters down in some
• 70 of body water can be lost

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Reproduction

• Clonal reproduction always occurs via transverse
  fission
• Sexual
• Monoecious (hermaphroditic)
• Reproductive organs located in a few anterior
  segments
• Paired ovaries release eggs which develop in
  ovisacs
• Paired testes release sperm which develop in
  seminal vesicles
• Genital segments each posses a pair of gonoducts
  (sperm ducts or oviducts), which open to the
  ventral surface
• Female genital segments have ventral openings
  that lead to seminal receptacles (store sperm
  prior to fertilization)

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• Lumbricus breeds continually, but some have one
  yearly reproductive season
• Often reabsorb reproductive system and grow back
  later
• Copulation with mutual sperm transfer
• Ventral contact between oppositely oriented worms
• Genital chaetae and mucus produced by clitellum
  hold worms together
• Sperm swim in a ventral sperm groove from male
  gonopores to openings of partners seminal
  receptacles
• Indirect sperm transfer
• Entire process can take 2 3 hours
• Some worms have copulatory organ for direct sperm
  transfer

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Cocoons

• Secreted a few days after copulation
• Clitellum secretes mucus tube
• Clitellum secretes chitinous material which
  becomes wall of cocoon
• Albumen from clitellum deposited into cocoon
• Tube slides forward as worm moves backwards
• Eggs and partners sperm (from seminal
  receptacles) deposited into cocoon
• External fertilization
• Cross fertilization
• Cocoon slides of head, and ends pinch off mucus
  tube disintegrates

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• Terrestrial species deposit cocoons in soil
• Aquatic species deposit cocoons in debris or mud,
  or attach to vegetation
• Ovoid and yellow in color
• Contain 1 to 20 eggs
• 7.5 cm X 2 cm in Megascolides

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Development

• Direct development
• Therefore create eggs containing yolk or albumen
• Eight days to several months before juveniles
  emerge from cocoons
• Live several years
• Six years in captivity
• Reach sexual maturity at approximately 200 days

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Class Hirudinomorpha

• Leeches and closely related worms
• 500 spp
• Marine, mostly freshwater, a few terrestrial
  species (restricted to moist environments)
• Lack chaetae
• Have a fixed number of segments (typically 33)
• All have a posterior sucker for adhering to prey
  or substratum
• Most have an anterior sucker as well

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• Blood-sucking ectoparasites or carnivores
• 1 cm to 30 cm long (Haementeria, giant Amazonian
  leech)
• Black, brown, olive, or red in color
• May have striped / spotted patterns
• Typically inhabit stagnant or slow moving
  freshwater
• Estivate in mud during periods of drought
• Can lose 90 of body water
• Often extremely abundant

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Body Form

• Dorsoventrally flattened
• Tapered at anterior end
• Suckers
• Anterior (if present) is smaller and surrounds
  mouth
• Posterior is disc-shaped anus located directly
  anterior
• Annulations that dont accurately depict segments
• Number of annulations per segment varies
• Clitellum spans segments 9-11, but is only
  conspicuous during reproduction

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Body Wall

• Connective tissue is much thicker
• Makes up a larger proportion of the animal
• Slightly different musculature
• Dorsoventral muscles
• For flattening
• Helical muscles
• For twisting

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Coelom

• Larger connective tissue compartment means a
  reduction of the coelom
• Lack septa, so therefore lack bilateral coelomic
  cavities
• Also lack mesenteries
• Continuous coelom which serves as a hemal system
• Two large lateral coelomic vessels
• Dorsal and ventral coelomic vessels
• Lined with mesothelium (chlorogogen cells)
• Muscle contractions propel fluid
• Body surface conducts gas exchange

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Locomotion

• Not equipped for burrowing due to
• Reduction of coelom
• Loss of septa and chaetae
• Many inchworm by anchoring with anterior and
  posterior suckers
• Many swim
• First contract dorsoventral muscles to increase
  surface area
• Then undulate

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Nervous System

• Similar to other annelids
• Fusions of ganglia in the sucker regions
• Posses ocelli
• Posses sensory papillae
• Projecting discs consisting of many sensory cells
• Typically found dorsally, in rows, on one
  annulation of a particular segment
• Sense organs mainly used for prey detection, and
  respond to
• Moving shadows and water-pressure vibrations in
  fish leeches
• Chemicals from body fluids such as oils, sweat,
  and blood
• Waves
• Temperature differences

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Excretory System

• 10 17 pairs of metanephridia
• One pair per segment in middle 1/3 of animal
• Nephrostomes project into coelomic vessels
• Nephridial tubules are embedded in connective
  tissue
• Posses a bladder that fills before urine is
  released through nephridiopore
• Important for osmoregulation

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Digestive System

• Pharynx types
• Protrusible pharynx that is forced into prey /
  host tissue
• Non-protrusible sucking pharynx
• May or may not have jaws
• Enzymes usually facilitate penetration in those
  lacking jaws
• Salivary glands empty secretions into pharynx
• Hirudin anticoagulant
• Anesthetic
• Vasodilators
• Often 1 11 pairs of lateral intestinal ceca

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Nutrition

• Three fourths are blood suckers
• Usually not host specific
• Usually parasitize a group of organisms (i.e.
  crayfish, fish, reptiles, birds, mammals, etc.)
• Some are carnivores of small invertebrates

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• Water from plasma is excreted via nephridiopores
• Digestion is very slow
• Gut produces hardly any enzymes
• Symbiotic bacteria may produce digestive enzymes
  and vitamins
• May take 200 days to digest a blood meal
• Rarely feed
• One blood meal may increase animals weight by 10X
• May only need to feed twice a year in order to
  grow
• Some can fast for 1.5 years

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Medical Applications

• Historically used in Europe for bloodletting
• Supposedly, George Washington died two days after
  a bloodletting for a soar throat
• Hirudo medicinalis is still used today for
  restoring circulation and reducing swelling
  after
• Skin grafts
• Digit / appendage reattachment
• Natural antibiotic properties
• Symbiotic bacteria produce antibiotics to reduce
  competition with other bacteria

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Reproduction

• No sexual reproduction, including regeneration
• Hermaphroditic, but not simultaneous
• Protandric
• Male portion of reproductive system develops
  first
• Spend first part of existence as males, then
  later reproduce as females
• Copulation and fertilization is always internal
• Most have copulatory organ for direct sperm
  transfer
• Others hypodermically impregnate using a
  spermatophore
• Pressure and cytolytic chemicals rupture body
  wall of mate
• Sperm are released and migrate toward eggs

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• Eggs are laid after copulation
• Time varies
• Clitellum secretes a cocoon and albumen
• The cocoons of fish leeches are attached to fish
  hosts
• Some brood eggs by attaching cocoon to substratum
  and ventilate eggs by fanning flattened ventral
  surface
• Some attach cocoon to ventral surface