Invertebrate Zoology

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Invertebrate Zoology

• Lecture 15 Phylum Annelida, Part 2

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Lecture outline

• Phylum Annelida, Part 2
• Circulation/gas exchange
• Osmoregulation/excretion
• Nervous system/Sensory structures
• Class Polychaeta
• Class Clitellata next time

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Circulation/gas exchange Overview

• Why do members of Phylum Annelida need a
  circulatory system?

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Circulation/gas exchange Class Polychaeta

• Closed circulatory system
• Key gas exchange sites
• Parapodia
• Additional structures (some)
• Anterior gills (Terebellidae)
• Tentacular crown (Sabellidae)
• Body surface
• If no other structures

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Circulation/gas exchange Class Polychaeta

• Two major vessels
• Dorsal blood ? anteriorly
• Ventral blood ? posteriorly
• Extensive capillary beds in
• Parapodia
• Other gas exchange structures
• Anterior and posterior
• Intestine
• Blood moves between main vessels via
• Capillary beds
• Segmental vessels (ring vessel)

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Circulation/gas exchange Class Polychaeta
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Circulation/gas exchange Class Polychaeta

• No hearts
• Movement of blood via vessel contraction body
  wall contraction
• Respiratory pigments
• Primarily dissolved within plasma
• Primarily hemoglobin
• Two forms of hemoglobin for polychaetes in
  potentially low-oxygen environments
• How do these two forms differ? Adaptive value?

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Circulation/gas exchange Class Polychaeta

• Terrebellidae circulatory system

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Circulation/gas exchange Class Clitellata,
Subclass Oligochaeta

• Closed circulatory system
• Organization similar to Polychaeta
• Key differences include
• Subneural vessel (so 3 main vessels)
• More highly-developed pumping structures
• Large, muscular dorsal vessel
• 2-5 pairs of enlarged circumesophageal vessels
• One-way valves prevent backflow
• Key respiratory surface is the epidermis
• Must stay moist. Why? How?

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Circulation/gas exchange Class Clitellata,
Subclass Oligochaeta
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Circulation/gas exchange Class Clitellata,
Subclass Hirudinoidea

• Combination of the ancestral annelid
  circulatory system reduced coelomic spaces.
• Sometimes reduced coelomic passages only
• Blood movement via vessel body wall contraction

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Osmoregulation/Excretion Class Polychaeta

• Protonephridia or metanephridia (many)
• Often associated with coelomoducts
• What is it?
• Function?
• Nephridia and coelomoduct linked in different
  ways
• Osmoregulatory excretory
• Fluid entry ? selective reabsorption along tube

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Osmoregulation/Excretion Class Clitellata,
Subclass Oligochaeta

• Metanephridia
• Fluid entry
• Via coelom
• Nephrostome
• Via circulatory system
• Capillaries
• Specialization of regions
• Such as..?

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Osmoregulation/Excretion Class Clitellata,
Subclass Hirudinoidea

• Metanephridia
• Nephrostome connected to a capsule with amoeboid
  cells
• Function?
• Intracellular duct system
• Temporary
• Formed by coalescing intracellular vacuoles.

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Nervous System Class Polychaeta

• Dorsal brain within prostomium
• What happens if brain is removed?
• Anterior sensory structures connect with brain
• Often a variety of tentacles, etc
• Circumenteric connectives to ventral nerve cord

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Nervous System Class Polychaeta

• Paired, fused ventral nerve cords
• One ganglion per segment
• Lateral nerves ? pedal ganglia
• Giant axons
• Why giant? Function?

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Nervous System sensory organs Class Polychaeta

• A variety of tactile and chemoreceptors
• Anterior nuchal organs
• Ciliated pits
• Chemosensory
• Dorsal cirri of parapodia
• Tactile, chemosensory
• Also for gas exchange

Photos www.tolweb.org
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Nervous System sensory organs Class Polychaeta

• Photoreceptors
• Simple pigment cup (Chaetopteridae)
• Pigment cup with lens (Nereidae)
• Complex eye with cornea, lens and retina