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NEMATODA GENERAL CHARACTERS AND CLASSIFICATION:Nematodes are commonly referred to as roundworms, threadworms or pinworms.Nematodes is one of the largest groups of metazoan.Nematodes are pseudocoelomates. Bilaterally symmetrical cylindrical body with triploblastic nature.GENERAL CHARACTERS OF NEMATODA:Some are parasites and others are free living in soil or water.Example: Rhabditis Body is long cylindrical without segmentation.Nematodes are characterized by a species specific number of cells or nuclei. Body of nematodes is slender cylindrical unsegmented with resistant cuticle.Body wall contains longitudinal muscles. Circular muscles are absent.Body cavity is pseudocoelom.Epidermis is syncytial. Locomotory cilia are absent. Some have ciliated gastrodermalcells.Alimentary canal is simple straight tube. Mouth at anterior end and anus at the posteriorend.Digestion is both extracellular and intracellular. Intestine is non muscular.Blood vascular system is absent. Pseudocoelomicfluid helps in the transport of nutrients.Excretory system contains 'H' shaped canals or excretory gland. Respiration is by anaerobic method.Nervous system consists of a circumenteric nerve ring with ganglia.Sense organs are amphids (mechanical and chemoreceptors present at the anterior region of the body) and phasmidsglandulo sensory structures in the posterior region of the body.Sexes are separate. Sexual dimorphism is present. Males are smaller with curved posterior end. They possess a cloaca with one or two copulatory spicules. Females are longer with straight posterior end. Genital pore is separate from anus.Fertilization is internal.Development involves four moults of the cuticle.Nematodes exhibiteutely. It is a phenomenon in which cell division cease near the end of embryonic development, so that number of cells of the adult is constant.Developmentincludes rabdtiformlarva.CLASSIFICATION OF NEMATODA:Nematoda is divided into two classes.Class 1: Aphasmidia.Class 2: Phasmidia.Class 1: APHASMIDIA. These are mostly free living.Amphids are of various shape and highly modified.Phasmids are absent.Excretory glands are present and excretory canals are absent.Examples: Greeffiella Free living.Trichinella Trichina worm parasite.Class 2: PHASMIDIA.Some are freeliving and others are parasitic.Amphids are porelike.Phasmids are present.Excretory glands orexcretory canals or both are present.Examples: Ascaris common round warm,Enterobius pinworm, Wuchereria Filarial worm,Ancylostoma hook worm.See the table showing common names and scientific names of important phasmids and aphasmids.