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 Setae = strong hair

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

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

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

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.

Fig. 17.3

Jaw Tentacle Everted pharynx Palp Eye Cirrus Parapodium

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