Phylum Nematoda Roundworms Phylum Nematoda Found in nearly

Phylum Nematoda Roundworms

Phylum Nematoda • Found in nearly every conceivable kind of ecological niche – They live in the sea, in freshwater, and in soil from polar regions to tropics, mountaintops to the depths of seas • About 12, 000 species have been named – Libbie Hyman estimated that, if all species were known, the number would be closer to 500, 000

Phylum Nematoda • Good topsoil may contain billions of nematodes per acre • Nematodes parasitize virtually every type of animal and many plants

Form and Function • Most are under 5 cm long, many are microscopic – Some parasitic nematodes are over a meter • They use the pseudocoel (false body cavity) as a hydrostatic skeleton • Outer layer is a relatively thick, noncellular “cuticle” (multilayered exoskeleton), secreted by the underlying epidermis (hypodermis)

Form and Function • Layers of the cuticle are primarily “Collagen” • Beneath the hypodermis is a layer of “longitudinal muscles” – The muscles are arranged in four bands, separated by four epidermis cords that project inward • The pseudocoel is fluid-filled and holds internal organs

Form and Function • The alimentary canal consists of: – A mouth – A muscular pharynx – A long nonmuscular intestine • Food moves through intestine by body movements and food moving into the intestine from pharynx – A short rectum – A terminal anus • (Defecation)

Form and Function • Many adults have an anaerobic energy metabolism – They derive energy through glycolysis • A ring of nerve tissue and ganglia around the pharynx gives rise to small nerves to the anterior end and to two “nerve cords” – One dorsal and one ventral

Form and Function • Most are dioecious – Male and female • Males are smaller than females – Males posterior end usually has a pair of copulatory spicules • Fertilization is internal • Shelled zygotes or embryos are stored in the uterus until deposition

Form and Function • There are four juvenile stages – Each is separated by a molt or shedding of the cuticle • Some require an intermediate host

Ascaris lumbricoides: The Large Roundworm of Humans • Most common worm parasite • Recent survey showed that up to 25% prevalence in some Southeastern areas • Estimated 1. 27 billion people worldwide (20% of the world’s population) • Females can be up to 30 cm in length and can produce 200, 000 eggs per day – Eggs are extremely resistant to direct sunlight and high temperatures – Can survive for months or years in soil

Ascaris lumbricoides: The Large Roundworm of Humans • Shelled juveniles: – Eaten with uncooked vegetables or when children put dirty fingers or toys in mouth – Juveniles travel to host’s intestines – Then penetrate the intestinal wall and travel through the heart in blood to lungs – They break through into the alveoli • May cause serious pneumonia – They make their way up to the throat where they are swallowed and reach intestines again

Hookworms • So named because their anterior end curves dorsally • Large plates in their mouth cut into the intestinal wall where they suck blood and pump it through their intestine • They suck more blood then they need – Heavy infections cause anemia – (in children, can cause a stop in mental and physical growth) • Can borrow through skin into blood (juveniles)

Trichina Worm • Tiny nematodes responsible for the potentially lethal disease “trichinosis” – symptoms such as nausea, heartburn, dyspepsia, and diarrhea • They are found in almost any tissue or body space • Juveniles are able to redirect host cells to become “nurse cells” – Cells that nourish the worm • About 25 cases per year in US

Pinworms • Cause relatively little disease, but are the most common worm parasite in US – Estimated at 30% in children and 16% in adults • Adults live in large intestine and cecum • Females about 12 mm long and migrate to the anal region at night to lay their eggs • Eggs develop rapidly (6 hours) at body temperature

Filarial Worms • The worm lives in the lymphatic system • Females as long as 100 mm • Disease symptoms are associated with inflammation and obstruction of the lymphatic system • Females release live, tiny microfilariae into blood and lymph – Ingested by mosquitoes and then they grow inside – Return by mosquito bite • Most common is the dog heartworm (up to 45%)