PHYLUM-ASCHELMINTHES
Phylum—Aschelminthes or Nemathelminthes or Nematoda
(Nema — thread; helminth — worms) includes roundworms
They are commonly called as nematodes.
General Features
Some important general features of phylum—
Aschelminthes are discussed below
(i) Habitat and Habit They are mostly free-living and
may occur in water or within the soil. There are
several parasitic species which live within the body Of
animals or plants, e.g., Guineaworms, whipworms
eyeworms, etc.
(ii) Symmetry They show bilateral symmetry and have
organ system level of organisation.
(iii) Germ Layers They are triploblastic animals and have
tube within tube body plan.
(iv) Body Wall The body wall contains an outer cuticle,
syncytial epidermis and a muscle layer. Circular
muscles are absent.
(v) Body Cavity Aschelminthes are pseudocoelomates as
the cavity present between the body wall and gut is not
lined by mesodermal epithelium.
(vi) Digestive System Alimentary canal is complete with a
well-developed muscular pharynx.
(vii) Excretory System It consists of a pair of gland cells or
intracellular canals or both.
(vii) Nervous System It contains a nerve ring around the
pharynx with dorsal and ventral longitudinal nerve
cords running throughout the length of the body.
ix) Reproduction Nematodes reproduce only by sexual
method. Sexes are separate and exhibit sexual
dimorphism. The males are generally smaller than the
females to aid copulation.
(x) Fertilisation It is internal. The fertilised eggs develop
directly or indirectly through larvae that undergo
moulting and finally grow into adults.
Diseases Caused by Aschelminthes
Aschelminthes may cause the following diseases in humans
(i) Ascaris lumbricoides or giant intestinal roundworm is an
endoparasite of the small intestine of humans. It causes
ascariasis.
(ii) Wuchereria (filaria) or filarial worm is an endoparasite
in the lymphatic vessels and lymph nodes of humans. It
causes elephantiasis in the legs, arms, scrotum, etc.
(iii) Ancyclostoma duodenale or hookworm is an
endoparasite in the small intestine of humans. It
causes ancylostomiosis disease.