Nematoda (1)

Nematoda Overview

  • Phylum Nematoda includes roundworms.

  • Diverse and abundant, with over 30,000 species found in marine, freshwater, and terrestrial environments.

  • Body size ranges from less than 1 mm to 1 m; mainly microscopic.

  • Includes both free-living and parasitic forms; parasitic species can infect most animals and many plants.

  • Distinct body plan common to all taxa.

Ecdysozoa

  • Ecdysozoa: Group includes Nematoda and Arthropoda among others.

  • Characterized by lack of trochophore larva, found in Protostomia and Lophotrochozoa.

  • Features:

    • Cuticle: Non-living external layer secreted by the epidermis, rigid, requiring molting (ecdysis) for growth.

    • Specific shared genes control development and molting.

Body Structure of Nematodes

  • Nematodes are classified based on body structure:

    • Acoelomates: Lack a coelom.

    • Pseudocoelomates: Have a body cavity called a pseudocoelom.

    • Coelomates: True coelom present.

  • General characteristics include:

    • Bilaterally symmetrical and triploblastic.

    • Pseudocoelom providing hydrostatic skeleton.

    • Poorly differentiated head, no metamerism.

    • Absence of circulatory and respiratory systems; gas exchange occurs by diffusion.

Unique Traits of Nematodes

  • Synapomorphy: Presence of three rings of chemosensory sensillae around the mouth.

  • Cuticle Composition: Primarily composed of collagen; protects against harsh environments.

  • Limited lateral expansion due to thick cuticle; allows for increased hydrostatic pressure in the pseudocoelom.

  • Movement is characterized by thrashing due to lack of circular muscles, relying only on longitudinal muscles.

Reproductive Characteristics

  • Most nematodes are dioecious (separate sexes) and exhibit internal fertilization.

  • Growth occurs through eutely in many small species (increased cell size instead of cell number).

Life Cycle of Parasitic Nematodes

Pinworm (Enterobius vermicularis)

  • Most common nematode parasite in the USA and Canada; affects 30% of children and 16% of adults.

  • Lifecycle includes:

    • Eggs ingested, juveniles hatch in the duodenum.

    • Adults migrate to the large intestine; gravid female deposits eggs around the anus, causing itching.

    • Eggs can be transferred to clothing and hands, leading to reinfection.

Human Intestinal Roundworm (Ascaris lumbricoides)

  • Among the most common parasites of vertebrates, infecting about 1.2 billion people (16% of the global population).

  • Lifecycle involves:

    • Eggs consumed (contaminated sources).

    • Larvae hatch in the gut and migrate through the gut wall into the bloodstream, moving to the lungs.

    • Adults mature in the gut and can cause intestinal obstruction if heavy infection occurs.

Summary of Key Characteristics

  • Nematodes are abundant with diverse habitats.

  • Unique body structures and characteristics, including a thick cuticle and pseudocoelom, allow adaptation to various environments.

  • They exhibit complex life cycles, especially in parasitic forms, impacting human health and ecosystems.