1. Annelida: The Metameric Body Form
Chapter 13

4. Phylum Annelida Triploblastic, coelomate animals Bilateral symmetry
1 mm to 3 m in length
Ventral nerve cords
Closed circulatory system
Setae = strong hair

5. Phylum Annelida Segmented body metameric Setae Coelom divided by septa
Absent in leeches
Coelom divided by septa
Nephridia for each segment (metanephridia or protonephridia)

6. Metamerism Segmental arrangement of body parts in an animal
Have external and internal ring-like segments
Each has own excretory, nervous and circulatory structures
Advantages
Each segment can be controlled independently
Lessons the impact of injury
Permits modification of certain regions for specialized functions such as: feeding, locomotion, and reproduction

7. Phylum Annelida Three classes: Polychaeta – mostly marine
Oligochaeta – earthworms
Hirudinea – leeches
Setae = strong hair

8. 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

12. Fireworm

13. 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.

14. Fig. 17.3

15. Jaw
Tentacle
Everted pharynx
Palp
Eye
Cirrus
Parapodium

16. 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

17. Circulation Closed circulatory system
Blood may be colorless, green or red
Contains dorsal and ventral aorta

18. Each segment contains a pair of excretory tubes called metanephridia or protonephridia

19. 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

20. Figure 12.09

21. Class Hirudinea Leeches
Habitat: Vegetated spots in lakes and sluggish parts of rivers
Dorsoventrally flattened
No setae
Anterior & posterior sucker

22. 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

23. 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

25. Additional Resources

27. Circulatory Systems

28. Metamerism
Prostomium
Peristomium

29. Setae: Bristles that aide in locomotion and hold it in its burrow
Parapodia: lateral extensions