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Phylum Nematoda Biology 11 Invertebrates Round Worms Microscopic to almost a meter long 20,000 species; exist in every ecological niche; marine, freshwater, terrestrial, parasitic Bilateral symmetry 3 cell layers (endoderm, ectoderm, mesoderm) Pseudocoelom Digestive System One of the first animals to have a complete, one way digestive system with two openings (mouth and anus) Mouth, pharynx, long intestine, anus Movement Muscles run in strips down the length of the body wall Pseudocoelom acts as a hydrostatic skeleton Enables movement due to the pressure of the fluid filled cavity acting against the muscles Have a rigid cuticle Able to shed cuticle as organism grows Known as molting (molt 4 times before adult) Reproduction Sexually only Either male or female Some hermaphroditic Fertilization occurs in female Development inside or outside the nematode Females release approximately 240,000 eggs per day Nervous System Simple nervous system Nerve ganglion in the head region (NOT a brain) Ventral and dorsal nerve cord extend the length of the body Nerves transmit sensory information and control movement Sense organs that detect chemicals given off by prey or hosts Respiration and Circulation No internal transport system Breathe and excrete through their body walls Depend on diffusion to carry nutrients and wastes through their body Habitat Parasitic or free-living Free-living species found in all climates (soil, salt flats, aquatic sediments, polar regions, tropics, fresh water, oceans, hot springs) Parasitic species live within a host Found within plants and animals Ascaris Adults live in the intestines Eggs leave in feces and eaten by another host Eggs hatch in intestines of new host Larvae enter blood stream Enter into lungs Coughed up into the mouth Ingested into intestines where they mature into adults Ecological Role Important decomposers in the soil Break down and recycle decaying matter Parasitic roundworms affect humans: Directly: human parasites Indirectly: destruction of agricultural crops Ascaris, Hookworm(live in intestines, bore through the skin of feet), Trichinosis(live in muscles, from eating undercooked pork), Elephantiasis, Pinworm (itchy anus)