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.

Annelida Subgroups

• 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.

Lumbricus terrestris (Earthworm) - Evolutionary Changes

• Coelom: Between two mesodermal layers.
• Segmentation (Metamerism): Division of the body into repeating segments.
• Setae: First structures to support body movement.
• Big Cerebral Ganglia: Primitive brain structure.
• Blood Vessels: Some with pumping function.
• Metanephridia: New type of excretory organs, similar to nephrons.

Earthworm Anatomy and Physiology

• 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.

Earthworm Reproduction

• 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.

Hirudo medicinalis (Leech)

• 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.

Hirudotherapy (Leech Therapy)

• 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.