Phylum Annelida Notes (copy)

Phylum Annelida (Segmented Worms)

Includes earthworms, polychaete worms, and leeches.
Found in marine, freshwater, and terrestrial environments.
Exhibit diverse ecological roles, from filter feeders to bloodsuckers and active predators.
Characterized by segmentation (metamerism).
Each segment contains elements of circulatory, nervous, and excretory systems.
Body wall consists of circular and longitudinal muscle fibers surrounded by an acellular cuticle secreted by the epithelium.
Most annelids (except leeches) have setae projecting from their cuticle.
Setae may be located on paddle-like appendages called parapodia.
True coelom present, with parietal and visceral layers of mesoderm.
Coelom is partially subdivided by septa (except in leeches).
Hydrostatic pressure maintained across segments provides body rigidity, enabling muscle contractions to bend the body without collapsing.

Traditional division: polychaetes, oligochaetes, and leech-like annelids.
Classification is useful but not entirely accurate.
Polychaetes:
- Almost all are marine.
- Occupy various niches: swimmers (preying or being preyed upon), crawlers (scavenging), sitters (filter feeding), and burrowers (consuming sediments).
Oligochaetes:
- Include most freshwater segmented worms.
- Similar to marine polychaetes but often have setae and no parapods.
- Earthworms belong to this group.
- Burrow through moist soils and extract nutrients from dirt.
Leeches:
- Less segmented than other annelids; basic body plan starts with 34 segments.
- Have flatter bodies, often used for swimming.
- Possess suckers at each end for crawling.
- Only some leeches are obligate parasites.

Segmentation:
- Enhances flexibility and strength during movement.
- Each segment has muscles and setae.
- Setae help anchor and control movement through soil.
- Most segments possess the same organs.
Digestive System:
- Includes pharynx, esophagus, crop, intestine, and gizzard.
- Gizzard uses small stones to grind food.
- Gland cells in the intestine release enzymes.
- Intestinal wall contains blood vessels for nutrient absorption.
Closed Circulatory System:
- Dorsal and ventral blood vessels connected by circular vessels.
- Five circular vessels are enlarged and function as a heart.
- Dorsal vessels carry blood to the body's front part.
- Ventral vessels carry blood to the body's back part.
Respiration:
- Earthworms breathe through their skin.
- Body fluid and mucous are released to keep the skin moist, supporting gas diffusion.
Excretion:
- Metanephridia function similarly to nephrons in mammals.

Earthworms are hermaphrodites but usually require a mate to reproduce.
During mating, two worms align inverted and exchange sperm.
A slime tube (clitellum) forms, filled with fluid.
The worm moves forward out of the tube, which picks up eggs from the female pore.
The tube continues moving and picks up spermatozoa from the male pore.
Fertilized eggs develop within the closed clitellum.

Possess anterior and posterior suckers.
Posterior sucker aids movement and attachment to the host.
Anterior sucker houses three sharp teeth.
Leeches can feed for 30 minutes to 6 hours, ingesting several times their body weight.
Saliva contains about 100 pharmacologically active substances:
- Hirudin
- Hyaluronidase
- Vasodilators
- Anesthetics
- Anti-inflammatory factors
- Collagenase
These substances are injected into the host's body during sucking.

An old and inexpensive technique.
Studied for effects on inflammation related to arthritis, trauma, or surgical manipulation.
Considered a complementary choice rather than an alternative treatment.
Possible application: blood aspiration to prevent hematoma after surgical procedures.