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Annelida: The Metameric Body
Form
Chapter 13
Phylum Annelida
• Triploblastic, coelomate
animals
• Bilateral symmetry
• 1 mm to 3 m in length
• Ventral nerve cords
• Closed circulatory system
Phylum Annelida
• Segmented body
– metameric
• Setae
– Absent in leeches
• Coelom divided by
septa
• Nephridia for each
segment
(metanephridia or
protonephridia)
Metamerism
Segmental arrangement of
Advantages
body parts in an animal
1. Each segment can be
controlled independently
• Have external and internal
ring-like segments
2. Lessons the impact of
injury
– Each has own excretory,
nervous and circulatory
3. Permits modification of
structures
certain regions for
specialized functions
such as: feeding,
locomotion, and
reproduction
Phylum Annelida
Three classes:
– Polychaeta – mostly marine
– Oligochaeta – earthworms
– Hirudinea – leeches
Class Polychaeta
• Mostly marine
• 5-10 cm long
• Largest class of the
annelid phylum
• Adapted to a variety
of habitats
– Under rocks,
burrowers or in tubes
• Most distinct feature:
parapodia & setae
Fireworm
Polychaete Structure
• Prostomium: Contains numerous
sensory structures, including eyes,
antennae, and palps.
• Peristomium: First body segment;
surrounds the mouth and bears sensory
tentacles or cirri.
Jaw
Tentacle
Palp
Cirrus
Parapodium
Everted pharynx
Eye
Digestive System
Feeding
• If predatory, evert proboscis (pharynx) through
mouth to capture prey and bring back to burrow
• Others are herbivores and scavengers or filter
feeders
Digestion
• Contain long, straight intestine used to defecate
Circulation
• Closed circulatory
system
• Blood may be
colorless, green or
red
• Contains dorsal and
ventral aorta
Each segment contains
a pair of excretory tubes
called metanephridia
or protonephridia
Regeneration, Reproduction and
Development
• All have remarkable powers of regeneration
• They can replace lost parts
• Some have break points that allow the worm to sever
themselves when a predator grabs them
• Some reproduce asexually by budding
• Most use sexual reproduction
• Dioecious
• Fertilization is external in most
Platynereis megalops from Woods Hole, Massachusetts
Figure 12.09
Class Hirudinea
Leeches
• Habitat: Vegetated
spots in lakes and
sluggish parts of rivers
• Dorsoventrally flattened
• No setae
• Anterior & posterior
sucker
Class Hirudinea
• Parasite of vertebrates
and predator of small
invertebrates
• Three “teeth” in mouth
allow it to cut into host
– Anticoagulants keep
blood flowing
– After decent meal, may
not need to feed for 100
days
Class Hirudinea
• Leech locomotion
– Use anterior and
posterior suckers in
sequence to anchor
body while muscles
selectively contract
– Needs hard substrate
for locomotion: cannot
live in disturbed, silty
habitats
Credit: Josee Soucie, Biodidac
Additional Resources
Circulatory Systems
Metamerism
Prostomium
Peristomium
Setae: Bristles that aide in
locomotion and hold it in its
burrow
Parapodia:
lateral extensions