Biology XI Content
Phylum : Nematoda
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 :
- Class 1. Aphasmidia:
- Most of them are free living forms.
- Phasmids are present.
- Amphids are of various types.
- Excretory organs are either reduced or absent
E.g. Enoplus, Mermis etc.
- 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.