Phylum Aschelminthes:
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Animals belonging to this phylum are commonly called roundworms or thread worms or bagworms or nematodes.
It includes about 15,000 species. Out of which about 15 species are parasites of man.
Important characteristic features:
• Aquatic or terrestrial, either free living or parasitic in plants and animals.
• Organ system level, bilaterally symmetrical, triploblastic, pseudocoelomate animals.
• Body wall: It is tubular tapering at both ends and is covered with a thin cuticle.
• Epidermis is multinucleated.
• Digestive system: They have a tubular and complete alimentary canal within their body consisting of mouth, muscular pharynx, intestine and anus. The mouth may possess teeth to cut and pierce the tissues. The muscular pharynx is well developed which helps in sucking food from the host.
• Circulatory and respiratory system are absent. Nervous system is present.
• They show flexible body movements.
• Reproduction: Dioecious, sexual dimorphism present. The male is smaller than the female. Males have copulatory spicules which help in the process of copulation.
• Fertilization: it is internal. The zygote has a thick wall to survive adverse conditions. Development is direct or indirect.
The common examples of this phylum are
Parasites in human beings- Round worms, Hook worms, Filarial worms, Pinworms, Guinea worms.
Other members of this phylum-Eye worm (Loaloa), Whip worm (Trichuristrichura), Trichina worm (Trichinella)
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