Class XI, BIOLOGY, "Phylum Aschelminthes (Nematoda/Round worm)"

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.