Animals - Phylum Nematoda

PHYLUM NEMATODA (ROUNDWORMS)

Nematoda = Thread

  • General Characteristics:

    • Unsegmented worms with a digestive tract having two openings (mouth and anus).

    • Bilateral symmetry.

    • Very numerous (>80,000 species) found in almost every environment.

    • Includes both free-living and parasitic species.

    • Body covered with a tough cuticle.

    • Complete digestive system.

    • Only lengthwise muscles, producing whip-like movements.

Life Functions

Feeding:

  • Muscular pharynx sucks food through the mouth into the intestine.

  • Intestine is thin (1 cell layer thick) for efficient nutrient diffusion.

  • Digestion and absorption occur in the intestine.

  • Parasitic species lack gland cells and digestive enzymes.

  • Undigested food and waste exit through the anus.

Respiration:

  • O2 & CO2 enter/exit by diffusion across the body wall.

Internal Transport:

  • No internal transport system; nutrients, gases, and waste are transported by diffusion.

Excretion:

  • Nitrogenous waste (NH3NH_3, Urea).

  • Cellular wastes diffuse into the intestine and exit with undigested food.

  • Some species have excretory canals to remove excess H2OH_2O or waste.

Response:

  • Nervous system: ring of nervous tissue around the pharynx connected to dorsal and ventral nerve cords.

  • Transmit sensory information and control muscles.

  • Chemical receptors to sense and locate prey or hosts.

Movement:

  • Long bands of longitudinal muscles.

  • Contraction and relaxation controlled by the nervous system.

  • Whip-like, thrashing movement.

Reproduction:

  • Reproduce sexually (no asexual reproduction).

  • Most species have separate sexes (few are hermaphroditic).

  • Internal fertilization occurs inside the female.

Ascaris

  • Adult worms live in human intestines where sexual reproduction occurs.

  • Fertilized eggs leave the host's body in feces.

  • Eggs hatch in a new host who ingested infected water.

  • Larval worms burrow into the intestinal wall and enter the host’s blood.

  • Larvae travel to the lungs, then to the throat, and are swallowed back into the intestines as adults.

  • Secretes a substance to counteract the host's digestive enzymes.

  • Large numbers can block the host's intestines.

Ecological Roles of Roundworms

  • Food for larger organisms.

  • Help aerate soil.

  • Parasitic species cause diseases.

    • Necator americanus (Hookworm): feeds on blood, causing energy loss and poor development; larvae enter by crossing the skin of feet.

    • Trichinella spiralis (Trichina Worm): obtained by eating raw/undercooked pork; larvae migrate through the bloodstream, causing muscle pain, fever, or death.