Pseudocoelomate Animals There are 9 pseudocoelomate phyla. These are a diverse lot most of which are small, and some microscopic, although a few are relatively

Pseudocoelomate AnimalsPseudocoelomate Animals

There are 9 pseudocoelomate phyla.There are 9 pseudocoelomate phyla.

These are a diverse lot most of which are small, These are a diverse lot most of which are small, and some microscopic, although a few are and some microscopic, although a few are relatively large.relatively large.

Most are free living although some are Most are free living although some are exclusively (Acanthocephala) parasitic or include exclusively (Acanthocephala) parasitic or include many parasitic species (Nematoda).many parasitic species (Nematoda).


All have a pseudocoelom, a cavity All have a pseudocoelom, a cavity surrounding the gut. surrounding the gut.

Unlike a true coelom, a pseudocoelom is Unlike a true coelom, a pseudocoelom is not a cavity surrounded by mesoderm. not a cavity surrounded by mesoderm. Instead it is a persistent blastocoel and Instead it is a persistent blastocoel and lacks a peritoneum.lacks a peritoneum.


All pseudocoelomates have a body wall of All pseudocoelomates have a body wall of muscles and epidermis that surrounds the muscles and epidermis that surrounds the pseudocoel.pseudocoel.

All pseudocoelomates except for the All pseudocoelomates except for the Acanthocephala have a complete gut.Acanthocephala have a complete gut.

Pseudocoelomate PhylaPseudocoelomate Phyla

The pseudoceolomate phyla are listed below. In The pseudoceolomate phyla are listed below. In the interests of time we will discuss only the first the interests of time we will discuss only the first three:three: Rotifera: wheel animalsRotifera: wheel animals Acanthocephala: spiny-headed wormsAcanthocephala: spiny-headed worms Nematoda: roundwormsNematoda: roundworms GastrotrichaGastrotricha EntoproctaEntoprocta NematomrophaNematomropha KinorhynchaKinorhyncha PriapulidaPriapulida LoriciferaLoricifera

Phylum RotiferaPhylum Rotifera

Rotifers are named for their characteristic Rotifers are named for their characteristic ciliated crown or ciliated crown or corona,corona, which when it beats which when it beats looks like a rotating wheel.looks like a rotating wheel.

Rotifers are tiny animals (most are 100-300Rotifers are tiny animals (most are 100-300µµm m long and the largest only 3mm long) the majority long and the largest only 3mm long) the majority of which live in freshwater and are benthic of which live in freshwater and are benthic inhabitants (live on the bottom). inhabitants (live on the bottom).

About 2000 species have been described.About 2000 species have been described.

9.1

Figure 15.18


The beating of the cilia in the corona draws in The beating of the cilia in the corona draws in plankton-containing water for food.plankton-containing water for food.

The mouth opens to a modified muscular The mouth opens to a modified muscular pharynx known as a pharynx known as a mastaxmastax, which is a , which is a structure unique to rotifers.structure unique to rotifers.

The mastax has a set of complex jaws, which The mastax has a set of complex jaws, which are used to grasp and chew food.are used to grasp and chew food.


One group of rotifers, the Bdelloid rotifers, One group of rotifers, the Bdelloid rotifers, are very unusual in that there are no are very unusual in that there are no males, hermaphrodites, or evidence of males, hermaphrodites, or evidence of meiosis.meiosis.

Molecular evidence suggests that there Molecular evidence suggests that there has been only asexual reproduction in this has been only asexual reproduction in this group for several million years.group for several million years.


Because of the problem of accumulation of Because of the problem of accumulation of deleterious mutations in lineages of exclusively deleterious mutations in lineages of exclusively asexually reproducing animals (a process known asexually reproducing animals (a process known as Muller’s ratchet) it is unclear how the as Muller’s ratchet) it is unclear how the bdelloids have been able to dispense with bdelloids have been able to dispense with sexual reproduction entirely.sexual reproduction entirely.

Other rotifers reproduce using a combination fo Other rotifers reproduce using a combination fo sexual and asexual reproduction.sexual and asexual reproduction.

Phylum AcanthocephalaPhylum Acanthocephala

Acanthocephalans are commonly known Acanthocephalans are commonly known as spiny-headed worms because of the as spiny-headed worms because of the spiny eversible proboscis they use to spiny eversible proboscis they use to attach to the gut of their host.attach to the gut of their host.

All 1100 species of Acanthocephalan are All 1100 species of Acanthocephalan are endoparasitic and most parasitize fish, endoparasitic and most parasitize fish, birds and mammals.birds and mammals.

9.3

Figure 15.20


The body wall is covered with numerous The body wall is covered with numerous minute depressions which enormously minute depressions which enormously increase the surface are of the tegument increase the surface are of the tegument and facilitates (as in cestodes) the and facilitates (as in cestodes) the absorption of food from the host’s gut. absorption of food from the host’s gut.

As is true in cestodes, Acanthocephalans As is true in cestodes, Acanthocephalans lack a gut.lack a gut.


Acanthocpehalans have a lifecycle in Acanthocpehalans have a lifecycle in which a vertebrate is the definitive host which a vertebrate is the definitive host and an invertebrate the intermediate host.and an invertebrate the intermediate host.

Acanthocephalans, as other parasites do, Acanthocephalans, as other parasites do, modify the behavior of the intermediate modify the behavior of the intermediate host to enhance the chances of its being host to enhance the chances of its being eaten.eaten.


For example, acanthocephalans that parasitize For example, acanthocephalans that parasitize Gammarus,Gammarus, a small freshwater crustacean, a small freshwater crustacean, cause the cause the GammarusGammarus to alter its behavior in the to alter its behavior in the presence of ducks, a common predator. presence of ducks, a common predator.

Instead of diving to the bottom when a duck Instead of diving to the bottom when a duck appears, the appears, the GammarusGammarus swims into the light and swims into the light and grasps tightly onto a piece of vegetation, greatly grasps tightly onto a piece of vegetation, greatly increasing its chances of being eaten. increasing its chances of being eaten.


The change in behavior appears to be caused The change in behavior appears to be caused by the Acanthocephalan pumping a serotonin-by the Acanthocephalan pumping a serotonin-boosting molecule into the boosting molecule into the GammarusGammarus’ brain.’ brain.

This causes the This causes the GammarusGammarus to think it’s having to think it’s having sex and cling as it would if mating. Interestingly, sex and cling as it would if mating. Interestingly, the parasite’s manipulation also causes female the parasite’s manipulation also causes female GammarusGammarus to mimic the males mating behavior. to mimic the males mating behavior.


Another Acanthocephalan that parasitizes pill Another Acanthocephalan that parasitizes pill bugs causes them to reverse their normal bugs causes them to reverse their normal behavior and avoid humid, dark areas. behavior and avoid humid, dark areas.

Instead they wander in the open where they are Instead they wander in the open where they are much more vulnerable to birds, the much more vulnerable to birds, the acanthocephalans definitive host. acanthocephalans definitive host.

The parasite’s manipulations are very effective. The parasite’s manipulations are very effective. Although fewer than 1% of pill bugs are typically Although fewer than 1% of pill bugs are typically infected with acanthocephalan parasites 30% of infected with acanthocephalan parasites 30% of pill bugs delivered to nestlings are infected. pill bugs delivered to nestlings are infected.

Phylum NematodaPhylum Nematoda

The nematodes are by far the most The nematodes are by far the most important group of pseudoceolomates important group of pseudoceolomates both in terms of numbers (about 10,000 both in terms of numbers (about 10,000 species) and their impact on humans.species) and their impact on humans.

Most nematodes are under 5cm and many Most nematodes are under 5cm and many are microscopic. However, some parasitic are microscopic. However, some parasitic forms can be over a meter in length.forms can be over a meter in length.


Nematodes use their pseudocoelom as a Nematodes use their pseudocoelom as a hydrostatic skeleton. hydrostatic skeleton.

The body has a thick cuticle (made The body has a thick cuticle (made primarily of collagen) secreted by the primarily of collagen) secreted by the underlying epidermis, which resists the underlying epidermis, which resists the high hydrostatic pressure exerted by the high hydrostatic pressure exerted by the fluid in the pseudocoelom.fluid in the pseudocoelom.


Beneath the epidermis is a layer of longitudinal muscles.Beneath the epidermis is a layer of longitudinal muscles.

Muscles in nematodes are not arranged in antagonistic Muscles in nematodes are not arranged in antagonistic pairs, the antagonistic role is played by the cuticle.pairs, the antagonistic role is played by the cuticle.

Contraction of a longitudinal muscle on one side is Contraction of a longitudinal muscle on one side is transmitted through the hydrostatic skeleton and transmitted through the hydrostatic skeleton and stretches the cuticle on the opposite side of the body.stretches the cuticle on the opposite side of the body.

When the muscle relaxes, the cuticle contracts and the When the muscle relaxes, the cuticle contracts and the body returns to its resting position.body returns to its resting position.


Nematodes have a complete gut with a mouth, Nematodes have a complete gut with a mouth, muscular pharynx, intestine, rectum, and anus.muscular pharynx, intestine, rectum, and anus.

Most nematodes are dioecious and males are Most nematodes are dioecious and males are smaller than females. smaller than females.

Fertilization is internal and juveniles go through Fertilization is internal and juveniles go through several developmental stages, each time molting several developmental stages, each time molting or shedding their cuticle.or shedding their cuticle.

Free-living nematodesFree-living nematodes

Free-living nematodes live in the sea, in fresh Free-living nematodes live in the sea, in fresh water, and in the soil. They occur worldwide in water, and in the soil. They occur worldwide in all environments and most live in the interstitial all environments and most live in the interstitial spaces of sediments and soils.spaces of sediments and soils.

Vast numbers of nematodes occur. One square Vast numbers of nematodes occur. One square meter of sea bottom mud has been estimated to meter of sea bottom mud has been estimated to hold 4.4 million nematodes and 90,000 were hold 4.4 million nematodes and 90,000 were counted on a single decomposing apple.counted on a single decomposing apple.


The slender, tapered body of nematodes The slender, tapered body of nematodes equips them to live in interstitial spaces. equips them to live in interstitial spaces.

Most free-living nematodes are less than Most free-living nematodes are less than 2.5mm in length and often are 2.5mm in length and often are microscopic. The largest soil dwelling microscopic. The largest soil dwelling nematodes may be 7mm long and the nematodes may be 7mm long and the largest marine forms a whopping 5cm.largest marine forms a whopping 5cm.


Most free-living nematodes are Most free-living nematodes are carnivorous.carnivorous.

However, some feed on algae and fungi However, some feed on algae and fungi and some are detritivores. Others feed on and some are detritivores. Others feed on plants, especially the roots.plants, especially the roots.


Many root feeding nematodes are major Many root feeding nematodes are major agricultural pests. These species pierce agricultural pests. These species pierce root cells and suck out their contents.root cells and suck out their contents.

Nematodes are estimated to destroy 12% Nematodes are estimated to destroy 12% of the world’s cash crops annually.of the world’s cash crops annually.

Parasitic nematodesParasitic nematodes

There are a great many species of There are a great many species of parasitic nematodes and they attack parasitic nematodes and they attack virtually all groups of animals and plants.virtually all groups of animals and plants.

Parasitic forms include ascarids, Parasitic forms include ascarids, hookworms, Guinea worms, trichina hookworms, Guinea worms, trichina worms, pinworms, and filarial worms.worms, pinworms, and filarial worms.

Ascaris lumbricoides: large Ascaris lumbricoides: large roundworm of humansroundworm of humans

It’s estimated that worldwide as many as 1.4 It’s estimated that worldwide as many as 1.4 billion people are infected with billion people are infected with AscarisAscaris lumbricoideslumbricoides which lives in the small intestine. which lives in the small intestine.

Females may be a foot long and produce Females may be a foot long and produce 200,000 eggs a day. 200,000 eggs a day.

Infection occurs when parasite eggs are eaten Infection occurs when parasite eggs are eaten with uncooked food or when soiled fingers are with uncooked food or when soiled fingers are put into the mouth.put into the mouth.


The larvae penetrate the intestinal wall The larvae penetrate the intestinal wall and travel through the blood stream to the and travel through the blood stream to the lungs where they break out of the alveoli lungs where they break out of the alveoli (often causing pneumonia). (often causing pneumonia).

Then they make their way up the trachea Then they make their way up the trachea where they are swallowed and eventually where they are swallowed and eventually settle in the small intestine.settle in the small intestine.


In the intestines the worms cause In the intestines the worms cause abdominal symptoms and allergic abdominal symptoms and allergic reactions and may produce an intestinal reactions and may produce an intestinal blockage.blockage.

9.8

Figure 15.05a

Male (top) and female Ascaris lumbricoides

HookwormsHookworms

Hookworms are named for the dorsal Hookworms are named for the dorsal curve in their anterior end. curve in their anterior end.

Hookworms are quite small, the Hookworms are quite small, the commonest species commonest species NecatorNecator americanusamericanus is only 11mm long. However, because is only 11mm long. However, because they feed on blood a heavy infection can they feed on blood a heavy infection can produce severe anemia.produce severe anemia.

HookwormsHookworms

Large plates in the hookworm’s mouth are used Large plates in the hookworm’s mouth are used to cut the intestinal lining of the host. to cut the intestinal lining of the host.

The parasite then pumps blood through its gut, The parasite then pumps blood through its gut, partially digesting it before excreting it.partially digesting it before excreting it.

Because hookworms suck more blood than they Because hookworms suck more blood than they use, they can cause debilitating anemia. In use, they can cause debilitating anemia. In children a hookworm infection can stunt growth children a hookworm infection can stunt growth and cause a general lack of energy.and cause a general lack of energy.

9.9

Figure 15.06

Section through hookworm attached to dog intestine

HookwormsHookworms

Hookworms do not permanently attach in Hookworms do not permanently attach in one spot, but move around the gut and one spot, but move around the gut and reattach when they are ready to feed.reattach when they are ready to feed.

Hookworms have evolved sophisticated Hookworms have evolved sophisticated anti-clotting factors that keep platelets anti-clotting factors that keep platelets from clumping and forming a clot while the from clumping and forming a clot while the hookworm is feeding. hookworm is feeding.

HookwormsHookworms

When the hookworm releases, a clot forms When the hookworm releases, a clot forms and the tissue can recover. and the tissue can recover.

By using this approach instead of By using this approach instead of producing a crude blood thinner to ensure producing a crude blood thinner to ensure blood flow, hookworms prevent hemophilia blood flow, hookworms prevent hemophilia developing in their hosts, which would be developing in their hosts, which would be fatal for the hookworm.fatal for the hookworm.

HookwormsHookworms

The life cycle of hookworms is very similar The life cycle of hookworms is very similar to that of ascarids and infection occurs in to that of ascarids and infection occurs in the same way by contact with eggs in soil the same way by contact with eggs in soil or food.or food.

Guinea wormsGuinea worms

Guinea worm infections (also referred to as Guinea worm infections (also referred to as Dracunculiasis) are now confined to sub-Dracunculiasis) are now confined to sub-Saharan Africa. Adults are threadlike nematode Saharan Africa. Adults are threadlike nematode worms that can grow to 1 meter in length.worms that can grow to 1 meter in length.

The adult lives in humans and the intermediate The adult lives in humans and the intermediate host is tiny crustaceans.host is tiny crustaceans.

Humans become infected when they drink water Humans become infected when they drink water containing the crustaceans.containing the crustaceans.


The immature worm penetrates the gut The immature worm penetrates the gut wall and wanders through the body, wall and wanders through the body, maturing and growing.maturing and growing.

After about a year the female makes her After about a year the female makes her way to the surface of the skin (usually in way to the surface of the skin (usually in the legs) causing very painful blistering. the legs) causing very painful blistering.


To ease the pain, sufferers immerse their To ease the pain, sufferers immerse their feet in water. This bursts the blisters and feet in water. This bursts the blisters and the female worm then protrudes from the the female worm then protrudes from the sore and lays her eggs, thus continuing sore and lays her eggs, thus continuing the life cycle.the life cycle.


There is no cure for Guinea worms and There is no cure for Guinea worms and the only way to remove one is to slowly the only way to remove one is to slowly over the course of weeks wind the worm over the course of weeks wind the worm out on a stick.out on a stick.

If the worm breaks, If the worm breaks,

a serious bacterial a serious bacterial

infection results.infection results.

Interestingly, the traditional symbols for medicineand 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 theGuinea worm removal technique.


Guinea worm infection is avoidable with Guinea worm infection is avoidable with relatively simple precautions such as relatively simple precautions such as preventing people walking in drinking preventing people walking in drinking water sources and boiling or filtering water water sources and boiling or filtering water before drinking it.before drinking it.


Since the mid 1980’s a campaign to Since the mid 1980’s a campaign to eradicate Guinea worms coordinated by eradicate Guinea worms coordinated by the U.N. and the Carter Center has had the U.N. and the Carter Center has had tremendous success.tremendous success.

In 1986, an estimated 3.5 million people In 1986, an estimated 3.5 million people were infected, but by 2000 the number of were infected, but by 2000 the number of cases had been reduced to about 75,000.cases had been reduced to about 75,000.


Guinea worms have been eliminated from Guinea worms have been eliminated from Pakistan, India, and Iran and infections Pakistan, India, and Iran and infections greatly reduced over much of sub-Saharan greatly reduced over much of sub-Saharan Africa.Africa.

The major barrier to elimination at this The major barrier to elimination at this point is the ongoing conflict in southern point is the ongoing conflict in southern Sudan where the majority of cases now Sudan where the majority of cases now occur.occur.

Trichina wormsTrichina worms

TrichinellaTrichinella spiralisspiralis is a tiny nematode that is a tiny nematode that causes the potentially fatal disease causes the potentially fatal disease trichinosis. trichinosis.

Humans typically become infected by Humans typically become infected by eating undercooked pork. Trichinella lives eating undercooked pork. Trichinella lives in cysts formed in individual muscle cells in cysts formed in individual muscle cells of the host.of the host.


Trichinella when it hatches from an ingested Trichinella when it hatches from an ingested cysts in its host’s gut drills through the wall of the cysts in its host’s gut drills through the wall of the gut where females produce living young. gut where females produce living young.

These juveniles travels in the circulatory system These juveniles travels in the circulatory system to a muscle.to a muscle.

The juvenile penetrates an individual muscle cell The juvenile penetrates an individual muscle cell and breaks the cell down so it can be remade.and breaks the cell down so it can be remade.


Trichinella, just as a virus does, manipulates the Trichinella, just as a virus does, manipulates the host cell’s DNA. It causes the cell to recruit a host cell’s DNA. It causes the cell to recruit a blood supply to supply food to the cell and also blood supply to supply food to the cell and also produce collagen to form a cyst around the cell.produce collagen to form a cyst around the cell.

The Trichinella juvenile awaits ingestion by The Trichinella juvenile awaits ingestion by another host. When ingested it emerges from its another host. When ingested it emerges from its cysts enters the mucosal lining of gut, develops cysts enters the mucosal lining of gut, develops into an adult and continues the life cycle.into an adult and continues the life cycle.


Adults usually do not persist long in the Adults usually do not persist long in the gut before being expelled by the host’s gut before being expelled by the host’s immune system.immune system.


Humans are an inadvertent host of Humans are an inadvertent host of Trichinella. Trichinella.

Normal life cycles involve pigs and rats.Normal life cycles involve pigs and rats.

In humans, infection with a few Trichinella In humans, infection with a few Trichinella parasites may cause no symptoms, but parasites may cause no symptoms, but heavy infections can cause intense muscle heavy infections can cause intense muscle pain and in some cases death.pain and in some cases death.

Filarial wormsFilarial worms

At least 8 species of filarial worms infect At least 8 species of filarial worms infect humans especially in tropical regions.humans especially in tropical regions.

Approximately 250 million people infected Approximately 250 million people infected with with Wuchereia bancroftiWuchereia bancrofti or or Brugia malayiBrugia malayi

These worms live in the lymphatic system These worms live in the lymphatic system and females can be 10cm long.and females can be 10cm long.


Females release live young microfilariae Females release live young microfilariae into the blood and lymphatic system.into the blood and lymphatic system.

The microfilariae are picked up by The microfilariae are picked up by mosquitoes where they develop, become mosquitoes where they develop, become infective and can infect another person.infective and can infect another person.


In some people exposed to persistent In some people exposed to persistent infections with these parasites, infections with these parasites, elephantiasis may develop.elephantiasis may develop.

This is caused by blockage of lymphatic This is caused by blockage of lymphatic ducts and inflammation. There may be ducts and inflammation. There may be excessive growth of connective tissue and excessive growth of connective tissue and enormous swelling of infected parts enormous swelling of infected parts including legs, arms and scrotum.including legs, arms and scrotum.

9.12

Figure 15.11

Elephantiasis of leg caused by filarial worms


The most common filarial worm in the U.S. The most common filarial worm in the U.S. is the cause of heartworm.is the cause of heartworm.

The microfilariae are transmitted by The microfilariae are transmitted by mosquitoes. mosquitoes.

9.13

Figure 15.12

Diriofilaria immitis Dog heartworm

River blindnessRiver blindness

River blindness is also caused by filarial worms. River blindness is also caused by filarial worms.

In this case the insect that transmits the disease In this case the insect that transmits the disease is a blackfly. is a blackfly.

18- 30 million people are infected worldwide 18- 30 million people are infected worldwide (mainly central Africa and parts of South (mainly central Africa and parts of South America) and more than 300,000 have been America) and more than 300,000 have been made blind.made blind.


When a black fly becomes infected, the worm When a black fly becomes infected, the worm larvae spread to its salivary glands. When it larvae spread to its salivary glands. When it bites someone the larvae pass into the skin. bites someone the larvae pass into the skin.

The larvae develop into adults and form nodules The larvae develop into adults and form nodules under the skin. The adults breed and produce under the skin. The adults breed and produce thousands of larvae, which spread all over the thousands of larvae, which spread all over the body - including the eyes. body - including the eyes.


The worst problem is caused when The worst problem is caused when problem is when the parasites die. The problem is when the parasites die. The immune system produces a severe immune system produces a severe inflammation, which if it occurs in the eye it inflammation, which if it occurs in the eye it can cause blindness.can cause blindness.

People infected at birth with river People infected at birth with river blindness commonly become blind by their blindness commonly become blind by their 40s.40s.

Documents

Pseudocoelomate Animals There are 9 pseudocoelomate phyla. These are a diverse lot most of which are small, and some microscopic, although a few are relatively