Phylum : Nematoda

#biologynotes #grade11

It is made up of 2 Greek words, nema = thread & edios = form

Characters:

Body form:
They have elongated, cylindrical and unsegmented body like threads, so often called thread worms.
Habitat:
Most of them are parasites while few are free living found in soil or water.
Cellularity:
They are multicellular organisms with organ-system grade of body organization.
Coelom:
They are pseudocoelomates.
Nutrition:
They exhibit parasitic or holozoic modes of nutrition.
Digestive system:
Digestion is extracellular.
Alimentary canal:
Alimentary canal is complete and straight with mouth, pharynx, intestine and anus.
Excretion:
Excretory system is absent. Gaseous exchange occurs through the general body surface.
Circulatory system:
Blood vascular system is absent. Transport of materials occurs with the help of hydrolymph or pseudocoelomic fluid.
Reproduction:
They are usually dioceious i.e. sexes are separate with sexual dimorphism. No asexual reproduction occurs.
Fertilization:
It is internal and cross.
Nervous system:
Nervous system consists of a nerve ring and the anterior part of the alimentary canal with longitudinal nerve cords.
Germ layers:
They are triploblastic.
Segmentation:
Body is unsegmented but maybe wrinkles.
Cephalsation:
Body lacks the distinct head. However, mouth is present in the anterior system.
Sensory Organs:
Amphids and phasmids are the sensory organs present in them.
Development:
Development is direct without any larval form.

On the basis of presence or absence of Phasmids (caudal sensory organ), phylum nematode is futher classified as :

1. Class 1. Aphasmidia:

2. Class 2. Phasmidia:

Most of them are parasitic forms.
Phasmids are present.
Amphids are pore like.
Excretory organs are developed with paired lateral canals. E.g. Ascarics lumbricoidesetc.

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