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General characteristics of Phylum Nemathelminthes, • They are commonly called thread worm or round worm., • Mostly parasitic, few are free-living., • 20,000 species, out of which 15,000 are parasitic in nature, • They are cylindrical, elongated, slender worm like and tapers at both end., • Triploblastic., • Bilaterally symmetrical., • Organ system level of organization., • Body is unsegmented., • Body cavity is filled with muscle. They are pseudocoelomate i.e. body cavity is, not lined by mesodermal layer.

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• Internal cephalization is present but externally there is little differentiation, between the anterior and posterior regions., • Distinct head is lacking. However, mouth is present in anterior region., • Body is covered with tough and resistant cuticle., • Digestive system is complete and straight with both mouth and anus., • Mouth is terminal and surrounded by lips bearing sense organ., • Respiratory and circulatory organs are absent., • Respiration occurs through general body surface. Respiration is aerobic in freeliving forms and anaerobic in parasitic form., • Excretory system consists of intracellular canal or lateral excretory ducts., • Nervous system is not much developed., • Nervous system consists of circumpharyngeal nerve ring and longitudinal nerve, cords., • Sense organs are poorly developed

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• These are unisexual i.e. sexes are separate with sexual dimorphism., • Fertilization is internal, may be cross or self., • Development may be direct or indirect., • Larval forms are Rhabditiform, Filariform and Micrifilaria., • Various lateral lines and pores are present on the surface of body., • Eutyly: no. of cells are the fixed in every system, • Example: Ascaris lumbricoides (Roundworm)

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Class 1: Rotifera, • Commonly known as wheel animalcules, • Mostly freshwater, rarely marine, • Anterior end of the body is provided with a ciliary disc, called corona. It helps in, locomotion and food capturing, • Tail is segmented, • Dioecious (sexes are separated), • Examples: Rotaria, Hydalina, etc.

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Class 2: Gastrotricha, • Commonly known as hairy bellies or hairy backs, • Generally aquatic; freshwater or marine, • Body unsegmented and corona absent, • Ventral surface of the body bears two longitudinal rows of cilia, which helps in, locomotion, • Dioecious or monoecious, • Examples: Lepidodermella, Chetonotus, etc.

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Class 3: Nematomorpha, • Commonly called hair-worms, • Adult are aquatic and fee-living but juveniles are parasitic, • Body is thread-like with blunt anterior end, • Dioecious, • Examples: Paragordius, Nectonema, etc

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Class 4: Nematoda, • Commonly called round worms, • Aquatic or terrestrial, free-living or parasitic, • Body cylindrical and tapering at both ends, • Dioecious, • Examples: Ascaris sp. (round worm), Ancylostoma sp. (hook worm)

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Class 5: Kinorhyncha, • Commonly called mud dragons, • All marine and free-living, • Body divided into 13 or 14 segments, each with spiny cuticle for locomotion, • Dioecious, • Examples: Kinorhynchus, Pycnophyes, etc.

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Class 6: Priapuloidea, • Commonly called penis worms, • All marine and free-living, • Body is unsegmented with superficially segmented trunk, • Body bears an interior introvent (reversible part), • Dioecious, • Examples: Priapulus, Halicryplius, etc.