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Ecdysozoa. Protostomes are divided into two large groups the Lophotrochozoa and the Ecdysozoa. The molluscs are the last group of the Lophotrochozoa we will deal with this semester. The Ecdysozoa includes two major groups the Nematoda and the Arthropoda. Ecdysozoa.

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  • Protostomes are divided into two large groups the Lophotrochozoa and the Ecdysozoa. The molluscs are the last group of the Lophotrochozoa we will deal with this semester.
  • The Ecdysozoa includes two major groups the Nematoda and the Arthropoda.
  • Members of the Ecdysozoa are characterized by the fact that they shed their cuticle as they grow.
phylum nematoda
Phylum Nematoda
  • The nematodes are quite species diverse (about 10,000 species) and the many parasitic forms have a significant impact on humans.
  • Most nematodes are under 5cm and many are microscopic. However, some parasitic forms can be over a meter in length.
phylum nematoda4
Phylum Nematoda
  • Nematodes use their pseudocoelom as a hydrostatic skeleton.
  • A pseudocoelom is a fluid-filled body cavity in which mesoderm lines only the outer edge of the developing blastocoel. No peritoneal lining develops.
  • The body has a thick cuticle (made primarily of collagen) secreted by the underlying epidermis, which resists the high hydrostatic pressure exerted by the fluid in the pseudocoelom.
phylum nematoda5
Phylum Nematoda
  • Beneath the epidermis is a layer of longitudinal muscles.
  • Muscles in nematodes are not arranged in antagonistic pairs, the antagonistic role is played by the cuticle.
  • Contraction of a longitudinal muscle on one side is transmitted through the hydrostatic skeleton and stretches the cuticle on the opposite side of the body.
  • When the muscle relaxes, the cuticle contracts and the body returns to its resting position.
phylum nematoda6
Phylum Nematoda
  • Nematodes have a complete gut with a mouth, muscular pharynx, intestine, rectum, and anus.
  • Most nematodes are dioecious and males are smaller than females.
  • Fertilization is internal and juveniles go through several developmental stages, each time molting or shedding their cuticle.
free living nematodes
Free-living nematodes
  • Free-living nematodes live in the sea, in fresh water, and in the soil. They occur worldwide in all environments and most live in the interstitial spaces of sediments and soils.
  • Vast numbers of nematodes occur. One square meter of sea bottom mud has been estimated to hold 4.4 million nematodes and 90,000 were counted on a single decomposing apple.
free living nematodes8
Free-living nematodes
  • The slender, tapered body of nematodes equips them to live in interstitial spaces.
  • Most free-living nematodes are less than 2.5mm in length and often are microscopic. The largest soil dwelling nematodes may be 7mm long and the largest marine forms a whopping 5cm.

Free-living nematode

free living nematodes10
Free-living nematodes
  • Most free-living nematodes are carnivorous.
  • However, some feed on algae and fungi and some are detritivores. Others feed on plants, especially the roots.
free living nematodes11
Free-living nematodes
  • Many root feeding nematodes are major agricultural pests. These species pierce root cells and suck out their contents.
  • Nematodes are estimated to destroy 12% of the world’s cash crops annually.
parasitic nematodes
Parasitic nematodes
  • There are a great many species of parasitic nematodes and they attack virtually all groups of animals and plants.
  • Parasitic forms include ascarids, hookworms, Guinea worms, trichina worms, pinworms, and filarial worms.
ascaris lumbricoides large roundworm of humans
Ascaris lumbricoides: large roundworm of humans
  • It’s estimated that worldwide as many as 1.4 billion people are infected with Ascarislumbricoides which lives in the small intestine.
  • Females may be a foot long and produce 200,000 eggs a day.
  • Infection occurs when parasite eggs are eaten with uncooked food or when soiled fingers are put into the mouth.
ascaris lumbricoides large roundworm of humans14
Ascaris lumbricoides: large roundworm of humans
  • The larvae penetrate the intestinal wall and travel through the blood stream to the lungs where they break out of the alveoli (often causing pneumonia).
  • Then they make their way up the trachea where they are swallowed and eventually settle in the small intestine.
ascaris lumbricoides large roundworm of humans15
Ascaris lumbricoides: large roundworm of humans
  • In the intestines the worms cause abdominal symptoms and allergic reactions and may produce an intestinal blockage.


Male (top) and female Ascaris lumbricoides

  • Hookworms are named for the dorsal curve in their anterior end.
  • Hookworms are quite small, the commonest species Necatoramericanus is only 11mm long. However, because they feed on blood a heavy infection can produce severe anemia.
  • Large plates in the hookworm’s mouth are used to cut the intestinal lining of the host.
  • The parasite then pumps blood through its gut, partially digesting it before excreting it.
  • Because hookworms suck more blood than they use, they can cause debilitating anemia. In children a hookworm infection can stunt growth and cause a general lack of energy.


Section through hookworm

attached to dog intestine

  • Hookworms do not permanently attach in one spot, but move around the gut and reattach when they are ready to feed.
  • Hookworms have evolved sophisticated anti-clotting factors that keep platelets from clumping and forming a clot while the hookworm is feeding.
  • When the hookworm releases, a clot forms and the tissue can recover.
  • By using this approach instead of producing a crude blood thinner to ensure blood flow, hookworms prevent hemophilia developing in their hosts, which would be fatal for the hookworm.
  • The life cycle of hookworms is similar to that of ascarids.
  • Infection occurs after a larva hatches from an egg and penetrates the skin of a person. It then makes its way to the lungs where eventually it is coughed up and swallowed and travels to the intestines..
guinea worms
Guinea worms
  • Guinea worm infections (also referred to as Dracunculiasis) are now confined to sub-Saharan Africa. Adults are threadlike nematode worms that can grow to 1 meter in length.
  • The adult lives in humans and the intermediate host is tiny crustaceans.
  • Humans become infected when they drink water containing the crustaceans.
guinea worms26
Guinea worms
  • The immature worm penetrates the gut wall and wanders through the body, maturing and growing.
  • After about a year the female makes her way to the surface of the skin (usually in the legs) causing very painful blistering.
guinea worms27
Guinea worms
  • To ease the pain, sufferers immerse their feet in water. This bursts the blisters and the female worm then protrudes from the sore and lays her eggs, thus continuing the life cycle.
guinea worms28
Guinea worms
  • There is no cure for Guinea worms and the only way to remove one is to slowly over the course of weeks wind the worm out on a stick.
  • If the worm breaks,

a serious bacterial

infection results.


Interestingly, the traditional symbols for medicine

and healing the staff of Asclepias (showing a

snake entwined around a staff) and the caduceus

(which shows two snakes entwined about

a winged staff) very likely are derived from the

Guinea worm removal technique.

guinea worms31
Guinea worms
  • Guinea worm infection is avoidable with relatively simple precautions such as preventing people walking in drinking water sources and boiling or filtering water before drinking it.
guinea worms32
Guinea worms
  • Since the mid 1980’s a campaign to eradicate Guinea worms coordinated by the U.N. and the Carter Center has had tremendous success.
  • In 1986, an estimated 3.5 million people were infected, but by 2000 the number of cases had been reduced to about 75,000.
guinea worms33
Guinea worms
  • Guinea worms have been eliminated from Pakistan, India, and Iran and infections greatly reduced over much of sub-Saharan Africa.
  • The major barrier to elimination at this point is the ongoing conflict in southern Sudan where the majority of cases now occur.
trichina worms
Trichina worms
  • Trichinellaspiralis is a tiny nematode that causes the potentially fatal disease trichinosis.
  • Humans typically become infected by eating undercooked pork. Trichinella lives in cysts formed in individual muscle cells of the host.

trichina worms35
Trichina worms
  • Trichinella when it hatches from an ingested cyst in its host’s gut drills through the wall of the gut where females produce living young.
  • These juveniles travels in the circulatory system to a muscle.
  • The juvenile penetrates an individual muscle cell and breaks the cell down so it can be remade.
trichina worms36
Trichina worms
  • Trichinella, just as a virus does, manipulates the host cell’s DNA. It causes the cell to recruit a blood supply to supply food to the cell and also produce collagen to form a cyst around the cell.
  • The Trichinella juvenile awaits ingestion by another host. When ingested it emerges from its cysts enters the mucosal lining of gut, develops into an adult and continues the life cycle.

Trichinella life cycle in humans

trichina worms38
Trichina worms
  • Adults usually do not persist long in the gut before being expelled by the host’s immune system.
  • Trichinella occurs commonly in wild animals such as foxes, wolves and bears. Smaller mammals such as skunks, raccoons and rats, which commonly associate with people, are the main sources of domestic pig infections.

trichina worms40
Trichina worms
  • Pigs may become infected by eating fecal matter or the bodies of animals infected with the parasite. Humans are an inadvertent host of Trichinella.
  • In humans, infection with a few Trichinella parasites may cause no symptoms, but heavy infections can cause intense muscle pain and in some cases death.
filarial worms
Filarial worms
  • Filarial worms are thread-like nematodes of which there are at least 8 species for which humans, especially in tropical regions, are the definitive host.
  • Approximately 250 million people worldwide are infected with these worms which are spread by mosquitoes.
  • Different species inhabit different locations in the body. Some live in the lymphatic system, others subcutaneously and others in the abdominal cavity.
filarial worms43
Filarial worms
  • Females can be 10cm long and they release live young microfilariae into the blood and lymphatic system.
  • The microfilariae are picked up by mosquitoes where they develop, become infective and can infect another person.
filarial worms44
Filarial worms
  • In some people exposed to persistent infections with filarial parasites that live in the lymphatic system, elephantiasis may develop.
  • This is caused by blockage of lymphatic ducts and inflammation. There may be excessive growth of connective tissue and enormous swelling of infected parts including legs, arms and scrotum.

Elephantiasis of leg

caused by filarial worms


filarial worms46
Filarial worms
  • The most common filarial worm in the U.S. Dirofilaria immitis is the cause of heartworm in dogs.
  • Adult worms may be as long as 40cm and they live in the dog's heart and lungs.
  • Because they damage the heart, infection is often fatal, and killing adult worms is difficult and dangerous to the dog.
  • Prevention of infection by regular dosing of a dog with drugs that kill circulating larvae is a better strategy.

Diriofilaria immitis

Dog heartworm


river blindness
River blindness
  • River blindness is also caused by filarial worms that live subcutaneously.
  • In this case the insect that transmits the disease is a blackfly.
  • 18- 30 million people are infected worldwide (mainly central Africa and parts of South America) and more than 300,000 have been made blind.
river blindness50
River blindness
  • When a black fly becomes infected, the worm larvae spread to its salivary glands. When it bites someone the larvae pass into the skin.
  • The larvae develop into adults and form nodules under the skin. The adults breed and produce thousands of larvae, which spread all over the body - including the eyes.
river blindness51
River blindness
  • The worst problem is caused when the parasites die. The immune system produces a severe inflammation, which if it occurs in the eye can cause blindness.
  • People infected at birth with river blindness commonly become blind by their 40s.