Phylum Aschelminthes (Nematoda/Round worm)
MAIN CHARACTERS
HABIT AND HABITAT
§ Nematoda have a very wide distribution and they seem to have
mastered almost every habitat.
§ Free living nematodes are found in the sea, fresh water or in
the soil in all kinds of environment.
§ There are also many Parasitic nematodes found in all groups of
Plants and animals.
§ The Saprophagous species live in decomposing plant and animal
bodies and in rotting fruits.
NATURE
They have a
bilaterally symmetrical, cylindrical body, glistening smooth surface. They are
triploblastic.
EXTERNAL FEATURES
§ They show no trace of segmentation.
§ Most of the free living nematodes are less then a millimeter
length.
§ Some of the parasitic species attain a length of several meters
e.g. Guinea worm (Dracunculus medinensis).
§ They are usually long, round, tapered at both ends showing very
little morphological diversity from species to species.
§ The mouth of nematodes is modified for various modes of feeding
such as cutting, tearing, piercing and sucking fluids from the host.
§ Body is covered by cuticle, which moults only during the period
of growth.
INTERNAL FEATURES
§ The organs are packed in parenchyma when young, but later on it
disappears in adult. So that organs lie in a fluid filled cavity. This cavity
is termed as PSEUDOCOEL and it has not peritoneum.
§ Muscles are only longitudinal.
§ Excretory system has no flame cells.
§ Alimentary canal is straight with ectodermal fore and hind gut
and an endodermal mid gut.
REPRODUCTION
§ Sexes are generally separate.
§ Gonades are tubular and continues with their ducts.
§ Female organs are usually paired and open by vulva.
§ Male organs are single and open into a cloaca.
§ The life cycle of Parasitic species involves one, two or more
hosts
EXAMPLES
Ascaris (Round
worms), Hookworms and Thread worms etc.
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