FSc Notes Class XI Biology Phylum Aschelminthes (Nematoda/Round worm) 1st Year Biology Notes Online Taleem Ilmi Hub Class 11

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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