Phylum Nematoda

PHYLUM NEMATODA The roudworms Phylum Nematoda Structure     Roundworms Nematodes are bilaterally symmetrical Surrounded by a strong, flexible non-cellular inelastic layer called cuticle. It functions to: Resist penetration – in free-living.  Resist enzymatic digestion – in parasitic.  Maintains internal hydrostatic pressure.  Molts 4 times during maturation They have very high internal pressure This causes body to be round in cross section Growth in juveniles is accompanied by molting,     Phylum Nematoda Structure  Many nematodes are free living    decomposers predators Some are parasitic species    filarial worms hookworms Ascaroids Phylum Nematoda Structure      Nematodes undergo Eutely..having the same pattern of cell division and same number of cells. They are tapered at both ends They have longitudinal muscles only They move by antagonistic contraction of longitudinal muscles. They have a pseudocoelom which is mostly filled with digestive system and oviducts or testes. Phylum Nematoda Structure      They are dioecious (Gonochoristic). Fertilization takes place when males use special copulatory spines to open the females' reproductive tracts and inject sperm into them. Some have hypodermic impregnation Males use barb to punch hole in female and release amoebic sperm Sperm are unique in that they lack flagella and move by pseudopodia, like amoebas. Nematode Nervous System   They have a central nervous system consisting of a circular brain (a circumpharyngeal nerve ring). Have two nerve cords in their bodies.     Ventral nerve cord – runs along the “belly.” Dorsal nerve cord – runs along the “back.” The nervous system allows the nematode to detect its environment and react to it. Aquatic nematodes have a pair of ocelli (eyes). Nematode Nervous System   These nerve cords transmit sensory information and coordinate movement. Nematodes have a variety of sensory receptors,   Tactile (touch) receptors at the front and back ends of body Chemosensory (chemical-sensitive)    Amphids (head region) Phasmids (tail region) Photoreceptors (ocelli or simple eyes) Phylum Nematoda Classification  2 classes based on presence or absence of phasmids   Class Aphasmida (Adenophorea) Class Phasmida (Sercernetea) Class Aphasmida (Adenophorea)      Members possess amphids behind lips. Absence of phasmid in caudal area. Presence of cephalic setae and papillae. Presence of paired testes. Most of species in this class are free-some are parasitic. Class Secernetea (Phasmida)     This class comprises of nematodes that usually possess pore-like amphids in the lateral lips. In the caudal region there is the presence of a pair of phasmid. Only anterior testis is present. Many parasitic forms are member of this class and the free-living species largely inhabit soil. Brief Parasitic Nematode   Pinworms  Live in mammals, common in humans  Adults reside in the large intestine.  Enter humans by being eaten. Hookworms  Live in vertebrates  Adults live in the small intestine of humans.  Enter host through the skin, usually between the toes. Brief Parasitic Nematode  Trichina  Live in humans & other omnivores (like pigs)  Adults live in small intestine of it’s host; larvae encyst in the stomach and skeletal tissue  Enter host by being eaten.  Filaroids  Live in mammals and birds  Adults live in lymph nodes  Transmitted by a vector (mosquito)