Phylum : Nematoda
1. Body form: They have elongated, cylindrical and unsegmented body like threads, so often called thread worms. 2. Habitat: Most of them are parasites while few are free living found in soil or water. 3. Cellularity: They are multicellular organisms with organ-system grade of body organization. 4. Coelom: They are pseudocoelomates. 5. Nutrition: They exhibit parasitic or holozoic modes of nutrition. 6. Digestive system: Digestion is extracellular. 7. Alimentary canal: Alimentary canal is complete and straight with mouth, pharynx, intestine and anus. 8. Excretion: Excretory system is absent. Gaseous exchange occurs through the general body surface. 10. Circulatory system: Blood vascular system is absent. Transport of materials occurs with the help of hydrolymph or pseudocoelomic fluid. 11. Reproduction: They are usually dioceious i.e. sexes are separate with sexual dimorphism. No asexual reproduction occurs. 12. Fertilization: It is internal and cross. 13. Nervous system: Nervous system consists of a nerve ring and the anterior part of the alimentary canal with longitudinal nerve cords. 14. Germ layers: They are triploblastic. 15. Segmentation: Body is unsegmented but maybe wrinkles. 16. Cephalsation: Body lacks the distinct head. However, mouth is present in the anterior system. 17. Sensory Organs: Amphids and phasmids are the sensory organs present in them. 18. Development: Development is direct without any larval form.
Further classifies 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.