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

2. Pseudocoelomate Animals
All have a pseudocoelom, a cavity surrounding the gut.
Unlike a true coelom, a pseudocoelom is not a cavity surrounded by mesoderm. Instead it is a persistent blastocoel and lacks a peritoneum.

3. Pseudocoelomate Animals
All pseudocoelomates have a body wall of muscles and epidermis that surrounds the pseudocoel.
All pseudocoelomates except for the Acanthocephala have a complete gut.

4. Pseudocoelomate Phyla
The pseudoceolomate phyla are listed below. In the interests of time we will discuss only the first three:
Rotifera: wheel animals
Acanthocephala: spiny-headed worms
Nematoda: roundworms
Gastrotricha
Entoprocta
Nematomropha
Kinorhyncha
Priapulida
Loricifera

5. Phylum Rotifera
Rotifers are named for their characteristic ciliated crown or corona, which when it beats looks like a rotating wheel.
Rotifers are tiny animals (most are µm long and the largest only 3mm long) the majority of which live in freshwater and are benthic inhabitants (live on the bottom).
About 2000 species have been described.

6. 9.1

7. Phylum Rotifera
The beating of the cilia in the corona draws in plankton-containing water for food.
The mouth opens to a modified muscular pharynx known as a mastax, which is a structure unique to rotifers.
The mastax has a set of complex jaws, which are used to grasp and chew food.

8. Phylum Rotifera
One group of rotifers, the Bdelloid rotifers, are very unusual in that there are no males, hermaphrodites, or evidence of meiosis.
Molecular evidence suggests that there has been only asexual reproduction in this group for several million years.

9. Phylum Rotifera
Because of the problem of accumulation of deleterious mutations in lineages of exclusively asexually reproducing animals (a process known as Muller’s ratchet) it is unclear how the bdelloids have been able to dispense with sexual reproduction entirely.
Other rotifers reproduce using a combination fo sexual and asexual reproduction.

10. Phylum Acanthocephala
Acanthocephalans are commonly known as spiny-headed worms because of the spiny eversible proboscis they use to attach to the gut of their host.
All 1100 species of Acanthocephalan are endoparasitic and most parasitize fish, birds and mammals.

11. 9.3

12. Phylum Acanthocephala
The body wall is covered with numerous minute depressions which enormously increase the surface are of the tegument and facilitates (as in cestodes) the absorption of food from the host’s gut.
As is true in cestodes, Acanthocephalans lack a gut.

13. Phylum Acanthocephala
Acanthocpehalans have a lifecycle in which a vertebrate is the definitive host and an invertebrate the intermediate host.
Acanthocephalans, as other parasites do, modify the behavior of the intermediate host to enhance the chances of its being eaten.

14. Phylum Acanthocephala
For example, acanthocephalans that parasitize Gammarus, a small freshwater crustacean, cause the Gammarus to alter its behavior in the presence of ducks, a common predator.
Instead of diving to the bottom when a duck appears, the Gammarus swims into the light and grasps tightly onto a piece of vegetation, greatly increasing its chances of being eaten.

15. Phylum Acanthocephala
The change in behavior appears to be caused by the Acanthocephalan pumping a serotonin-boosting molecule into the Gammarus’ brain.
This causes the Gammarus to think it’s having sex and cling as it would if mating. Interestingly, the parasite’s manipulation also causes female Gammarus to mimic the males mating behavior.

16. Phylum Acanthocephala
Another Acanthocephalan that parasitizes pill bugs causes them to reverse their normal behavior and avoid humid, dark areas.
Instead they wander in the open where they are much more vulnerable to birds, the acanthocephalans definitive host.
The parasite’s manipulations are very effective. Although fewer than 1% of pill bugs are typically infected with acanthocephalan parasites 30% of pill bugs delivered to nestlings are infected.

17. Phylum Nematoda
The nematodes are by far the most important group of pseudoceolomates both in terms of numbers (about 10,000 species) and their impact on humans.
Most nematodes are under 5cm and many are microscopic. However, some parasitic forms can be over a meter in length.

18. Phylum Nematoda
Nematodes use their pseudocoelom as a hydrostatic skeleton.
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.

19. 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.

20. 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.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. Ascaris lumbricoides: large roundworm of humans
It’s estimated that worldwide as many as 1.4 billion people are infected with Ascaris lumbricoides 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.

27. 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.

28. Ascaris lumbricoides: large roundworm of humans
In the intestines the worms cause abdominal symptoms and allergic reactions and may produce an intestinal blockage.

29. 9.8
Male (top) and female Ascaris lumbricoides

30. Hookworms
Hookworms are named for the dorsal curve in their anterior end.
Hookworms are quite small, the commonest species Necator americanus is only 11mm long. However, because they feed on blood a heavy infection can produce severe anemia.

31. Hookworms
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.

32. 9.9
Section through hookworm
attached to dog intestine

33. Hookworms
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.

34. Hookworms
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.

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

36. 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.

37. 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.

38. 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.

39. 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.

41. 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.

42. 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.

43. 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.

44. 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.

45. Trichina worms
Trichinella spiralis 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.

46. Trichina worms
Trichinella when it hatches from an ingested cysts 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.

47. 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.

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

49. Trichina worms Humans are an inadvertent host of Trichinella.
Normal life cycles involve pigs and rats.
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.

50. Filarial worms
At least 8 species of filarial worms infect humans especially in tropical regions.
Approximately 250 million people infected with Wuchereia bancrofti or Brugia malayi
These worms live in the lymphatic system and females can be 10cm long.

51. Filarial worms
Females 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.

52. Filarial worms
In some people exposed to persistent infections with these parasites, 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.

53. Elephantiasis of leg
caused by filarial worms
9.12

54. Filarial worms
The most common filarial worm in the U.S. is the cause of heartworm.
The microfilariae are transmitted by mosquitoes.

55. Diriofilaria immitis
Dog heartworm
9.13

56. River blindness River blindness is also caused by filarial worms.
In this case the insect that transmits the disease is a blackfly.
million people are infected worldwide (mainly central Africa and parts of South America) and more than 300,000 have been made blind.

57. 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.

58. River blindness
The worst problem is caused when problem is when the parasites die. The immune system produces a severe inflammation, which if it occurs in the eye it can cause blindness.
People infected at birth with river blindness commonly become blind by their 40s.