Describe on Phylum Aschelminthes - QS Study
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The body of the aschelminthes is circular in cross-section, hence, the name roundworms (Figure). They may be free-living, aquatic and terrestrial or parasitic in plants and animals.

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Fig: Aschelminthes (roundworms)

Roundworms have organ-system level of body organization. They are bilaterally symmetrical, triploblastic and pseudocoelomate animals. Alimentary canal is complete with a well-developed muscular pharynx. An excretory tube removes body wastes from the body cavity through the excretory pore. Sexes are separate (dioecious), i.e., males and females are distinct. Often females are longer than males. Fertilization is internal and development may be direct (the young ones resemble the adult) or indirect.

Examples: Ascaris (Round Worm), Wuchereria (Filaria worm), Ancylostoma (Hookworm).