Phylum Annelida

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Phylum Annelida
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Adapted from Lesser Known ProtostomePhyla. SICB
2001. J.R. Garey.
Possess trochophore larvae
Mollusca and Annelida are closely allied phyla
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Phylum Annelida 12,500 spp 3 classes Class
Polychaeta (8000 spp) Class Clitellata
subclass Oligochaeta (3500 spp) subclass
Hirudinea (630 spp)
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Annelid characteristics

• Trochophore larvae
• Closed circulatory system
• True coelom
• Annuli refer to segmented, metameric body plan
• Each segment has metanephridia, branches of
  nerves and blood vessels, chitinous setae /
  chaetae
• Segments partially divided by septa
• ? efficient hydrostatic skeleton
• ? Pre-adapted for regional specialization and
  tagmatization

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Metamerism
Septa provide regional control over hydrostatic
skeleton
Circular musc.
Longitudinal musc.
setae
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From this pic, know metameric repetition of body
segments (septa, metanephridia, nerves, blood
vessels), modest cephalization, limited regional
specialization
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Polychaeta

• Mostly marine
• Errant and sedentary spp
• Possess parapodia

Capillaries for gas exchange
Swimming Crawling Burrowing
setae
acicula chitinous rods for support
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Cephalization
Jaws Ocelli (photoreceptors) Tentacles Mechano
receptors Chemoreceptors
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Tube worms The sedentary polychaetesParapodia
reduced (no acicula, vascularization)Feathery
appendages at anterior end for feeding, gas
exchange
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Giant tube worms

• Discovered in 1977
• Ocean floor 8000 ft deep
• 4 ft tall
• No mouth. Get food from endosymbiotic bacteria
  that feed on sulphur released at deep sea vents

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Polychaete reproduction

• Separate sexes
• 1. Conventional sexual reproduction
• Sperm egg released into water ? trochophore
  larva
• 2. Epitoky
• Epitokous individual forms an epitoke by
• a. Transformation of posterior portion
• b. Asexual budding
• Mature indiv atoke
• Gamete-bearing part epitoke capable of swimming
  about in search of mates (timed w lunar cycle)

atoke
epitoke
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Class Clitellata

• Possess cylindrical ring used in reproduction
• Formation of a cocoon for the embryos
• Permanent gonads
• Two subclasses
• 1. Oligochaeta
• 2. Hirudinea

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Oligochaeta

• Mostly fossorial, some aquatic
• little cephalization
• No specialized organs for gas exchange
• Feed on detritus, organic debris
• Hermaphroditic
• Sperm exchanged between two mating individuals

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• Sperm exchange between mating earthworms
• Sperm from testis stored in, and later released
  from, seminal vesicles
• Sperm from other worm stored in seminal
  receptacles
• After separation, sperm from other worm used to
  fertilize eggs from the ovary

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• Eggs and sperm from other worm released into
  forming cocoon
• Many terrestrial forms can be parthenogenic

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Hirudinea

• Occur in freshwater or moist terrestrial habitats
• Highly streamlined
• No special organs for gas exchange, sensory
  except ocelli
• Have 2 suckers for feeding, locomotion
• 3-toothed jaws
• Ectoparasitic, but many
• are predaceous or scavenge

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Leech reproduction

• Reproduction similar to oligochaetes (use of
  clitellum to form cocoons)

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Class(?) Echiura spoon worms

• Have trochophore larva but lack segments or any
  indication of metamerism as adults
• Briefly segmented period as embryos
• Molecular data links Echiura to Annelida
• Reproduction similar to polychaeta

• 140 spp
• Marine
• Burrow in sand, mud