🧬 Phylum Nematoda – The Thread-like Roundworms of Aschelminths

Phylum Nematoda (Roundworms)

Literal Meaning:

• The word “Nematoda” is derived from Greek:

  • “Nema” means thread.

  • “Eidos” means form or shape.

• So, Nematoda literally means “thread-like” or “thread-shaped” organisms, referring to their elongated, slender body shape.

PTB

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General Characteristics:

• Body Shape: Elongated, cylindrical (round) body with pointed ends, resembling threads.

• Habitat:

  • Free-living species found in soil and water (e.g., Caenorhabditis elegans).

  • Parasitic species that live inside hosts (e.g., Ascaris, hookworm, pinworm, whipworm).

• Symmetry: Bilateral symmetry.

• Germ Layers: Triploblastic (three germ layers: ectoderm, mesoderm, endoderm).

• Segmentation: Body is unsegmented.

• Body Cavity: Pseudocoelomates – they possess a false body cavity (pseudocoelom) filled with fluid.

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Digestive System:

• Type: Complete and tube-like digestive system.

• Openings: Two – mouth at the anterior end, and anus at the posterior end.

• Note: In parasitic nematodes, the digestive system is often simplified.

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Excretory System:

• Composed of protonephridia.

• Two excretory canals run longitudinally and join at the anterior end, forming a single canal.

• The canal opens outside through a nephridiopore on the ventral surface.

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Nervous System:

• Consists of a nerve ring around the pharynx.

• Four longitudinal nerve cords extend from the nerve ring.

• Sense organs: Sensory papillae (hair-like structures) are present on the lips.

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Circulatory and Respiratory Systems:

• Absent – no specialized blood or respiratory systems.

• Exchange of gases occurs by diffusion.

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Reproduction:

• Unisexual (separate male and female individuals).

  • Male nematodes have testes.

  • Female nematodes have ovaries.

• Sexual dimorphism is often visible (males are usually smaller than females).

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Taxonomic Note:

• Pseudocoelomate animals are divided into seven phyla, grouped under a collective term: Aschelminths.

• Phylum Nematoda is the main representative of this group.

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