Flatworms

1. Acoelomate Bilateral AnimalsAcoelomate Bilateral Animals

The term worm is loosely employed in The term worm is loosely employed in
biology and is applied to very different biology and is applied to very different
animals including the segmented worms animals including the segmented worms
(Annelids), roundworms (Annelids), roundworms
(pseudocolomates and a variety of (pseudocolomates and a variety of
acoelomate bilateral animals.acoelomate bilateral animals.

2. Acoelomate Bilateral AnimalsAcoelomate Bilateral Animals

There are three phyla of acoelomate There are three phyla of acoelomate
bilateral animals:bilateral animals:

Platyhelminthes: flatwormsPlatyhelminthes: flatworms

Nemertea: ribbon wormsNemertea: ribbon worms

Gnathostomulida: jawed wormsGnathostomulida: jawed worms

3. Acoelomate Bilateral AnimalsAcoelomate Bilateral Animals

By far the most important in diversity and By far the most important in diversity and
economic importance is the phylum economic importance is the phylum
Platyhelminthes, which includes a variety Platyhelminthes, which includes a variety
of parasitic forms such as the flukes and of parasitic forms such as the flukes and
tapeworms.tapeworms.

4. Acoelomate Bilateral AnimalsAcoelomate Bilateral Animals

Unlike the radiate animals all of these organisms Unlike the radiate animals all of these organisms
are mobile and have evolved cephalization with are mobile and have evolved cephalization with
their sense organs concentrated at the head their sense organs concentrated at the head
end. There is also the beginning of a ladder-end. There is also the beginning of a ladder-
type nervous system.type nervous system.

In addition, they are bilaterally symmetrical.In addition, they are bilaterally symmetrical.

5. Acoelomate Bilateral AnimalsAcoelomate Bilateral Animals

All of these animals are triploblastic, but lack a All of these animals are triploblastic, but lack a
coelom. Instead, they have a solid body filled coelom. Instead, they have a solid body filled
with parenchyma cells.with parenchyma cells.

They have evolved organs and in some cases They have evolved organs and in some cases
organ systems. The simplest excretory or organ systems. The simplest excretory or
osmoregulatory systems and circulatory systems osmoregulatory systems and circulatory systems
are found in members of these groups.are found in members of these groups.

6. Phylum PlatyhelminthesPhylum Platyhelminthes

Members of the Platyhelminthes typically have Members of the Platyhelminthes typically have
dorsoventrally flattened bodies that are usually dorsoventrally flattened bodies that are usually
slender and leaflike or ribbonlike.slender and leaflike or ribbonlike.

There are four classes in the Platyhelminthes. There are four classes in the Platyhelminthes.
The Turbellaria are free living whereas as The Turbellaria are free living whereas as
members of the Monogenea, Trematoda and members of the Monogenea, Trematoda and
Cestoda are parasitic.Cestoda are parasitic.

7. NutritionNutrition

The digestive system includes a mouth, pharynx, The digestive system includes a mouth, pharynx,
and blind intestine.and blind intestine.

In the free-living Turbellarians the pharynx can In the free-living Turbellarians the pharynx can
be everted from the mouth. be everted from the mouth.

Food is sucked into the intestine where a Food is sucked into the intestine where a
combination of extracellular and intracellular combination of extracellular and intracellular
digestion takes place. digestion takes place.

8. NutritionNutrition

Undigested food exits via the pharynx. Undigested food exits via the pharynx.

In the Cestoda the digestive tract is absent In the Cestoda the digestive tract is absent
and all nutrients are absorbed across the and all nutrients are absorbed across the
tegument (the syncytial membrane/body tegument (the syncytial membrane/body
covering found in all parasitic covering found in all parasitic
Platyhelminthes).Platyhelminthes).

9. Excretion/OsmoregulationExcretion/Osmoregulation

The osmoregulatory system consists of a series The osmoregulatory system consists of a series
of canals that end in flame cells or of canals that end in flame cells or
protonephridia).protonephridia).

This system appears mainly intended to remove This system appears mainly intended to remove
excess fluid, but retain essential ions. It is most excess fluid, but retain essential ions. It is most
well developed in freshwater Turbellarians, but well developed in freshwater Turbellarians, but
reduced or absent in marine species, which do reduced or absent in marine species, which do
not have to remove excess water.not have to remove excess water.

10. Nervous system and sense organsNervous system and sense organs

Flatworms possess a simple brain and one to Flatworms possess a simple brain and one to
five pairs of longitudinal nerve cords that are five pairs of longitudinal nerve cords that are
cross connected to form a ladder-like cross connected to form a ladder-like
arrangement.arrangement.

There has been a tendency towards reduction of There has been a tendency towards reduction of
the number of pairs of nerve cords and the number of pairs of nerve cords and
increased development of the ventral pair. A increased development of the ventral pair. A
similar evolutionary pathway may have led to similar evolutionary pathway may have led to
the development of the ventral nerve cord found the development of the ventral nerve cord found
in annelids and arthropods.in annelids and arthropods.

11. Nervous system and sense organsNervous system and sense organs

Neurons are specialized for different tasks e.g. Neurons are specialized for different tasks e.g.
sensory and motor functions, which is an sensory and motor functions, which is an
important advance in the evolution of nervous important advance in the evolution of nervous
systems.systems.

There are a number of different sensory cells There are a number of different sensory cells
(e.g. rheoceptors and statocysts) found in (e.g. rheoceptors and statocysts) found in
flatworms and tactile and chemoreceptive cells flatworms and tactile and chemoreceptive cells
are abundant.are abundant.

12. Nervous system and sense organsNervous system and sense organs

In freshwater Planarians concentrations of In freshwater Planarians concentrations of
sensory cells form two ear-like structures sensory cells form two ear-like structures
(the auricles) found on the side of the (the auricles) found on the side of the
head.head.

Light sensitive eyespots or ocelli are Light sensitive eyespots or ocelli are
common in all classes but Cestoda. common in all classes but Cestoda.

13. ReproductionReproduction

Reproduction in the Platyhelminthes can Reproduction in the Platyhelminthes can
be asexual or sexual. However, most are be asexual or sexual. However, most are
hermaphroditic and cross fertilize.hermaphroditic and cross fertilize.

In parasitic forms sexual and asexual In parasitic forms sexual and asexual
reproduction may alternate in different reproduction may alternate in different
stages of the life historystages of the life history

14. Classification of PlatyhelminthesClassification of Platyhelminthes

There are four classes in the There are four classes in the
Platyhelminthes:Platyhelminthes:

Class Turbellaria: free-living flatworms.Class Turbellaria: free-living flatworms.

Class Turbellaria: endoparasitic flukesClass Turbellaria: endoparasitic flukes

Class Monogenea: parasitic flukes that are Class Monogenea: parasitic flukes that are
mainly ectoparasitesmainly ectoparasites

Class Cestoda: tapewormsClass Cestoda: tapeworms

15. Class TurbellariaClass Turbellaria

Class Turbellaria contains about 3000 species. Class Turbellaria contains about 3000 species.
There is considerable debate about the There is considerable debate about the
classification of the class and it is likely that the classification of the class and it is likely that the
class is not monophyletic. class is not monophyletic.

Most species are marine and benthic. However, Most species are marine and benthic. However,
some are also found in fresh water as well as in some are also found in fresh water as well as in
moist temperate and tropical terrestrial habitats. moist temperate and tropical terrestrial habitats.

16. 8.2
Figure 14.10
Marine turbellarian

17. Dugesia tigrina, a freshwater turbellarian
© Mauricio A. Muñoz

18. Class TurbellariaClass Turbellaria

Most Turbellarians are predators of invertebrates Most Turbellarians are predators of invertebrates
smaller than themselves. Other species are smaller than themselves. Other species are
herbivores or scavengers. herbivores or scavengers.

Turbellarians move by swimming, creeping or Turbellarians move by swimming, creeping or
crawling. They combine muscular contractions crawling. They combine muscular contractions
with ciliary movement to move.with ciliary movement to move.

19. Class TrematodaClass Trematoda

There are about 9000 species of There are about 9000 species of
trematodes all of which are parasitic. trematodes all of which are parasitic.
Most parasitize vertebrates. Most parasitize vertebrates.

Adaptations for parasitism include suckers Adaptations for parasitism include suckers
and hooks for attachment, glands to and hooks for attachment, glands to
produce cyst material and increased produce cyst material and increased
reproductive capacity.reproductive capacity.

20. Sheep liver fluke

21. Class TrematodaClass Trematoda

Structurally trematodes are similar to Structurally trematodes are similar to
turbellarians having a well developed turbellarians having a well developed
digestive system and similar nervous, digestive system and similar nervous,
excretory, and reproductive systems. excretory, and reproductive systems.
However, a major difference is the However, a major difference is the
tegumenttegument..

22. TegumentTegument

The tegument (found in all parasitic The tegument (found in all parasitic
Platyhelminthes) is a nonciliated, cytoplasmic Platyhelminthes) is a nonciliated, cytoplasmic
syncytium that overlays layers of muscle.syncytium that overlays layers of muscle.

The syncytium represents extensions of cells The syncytium represents extensions of cells
that are located below the muscle in the that are located below the muscle in the
parenchyma.parenchyma.

The tegument protects the parasite against its The tegument protects the parasite against its
host (e.g. against digestive enzymes).host (e.g. against digestive enzymes).

23. 8.5
Figure 14.05

24. Digenean TrematodesDigenean Trematodes

There are three subclasses of There are three subclasses of
Trematodes, but two are small, poorly Trematodes, but two are small, poorly
studied groups.studied groups.

The third group, the Digenea, however is a The third group, the Digenea, however is a
large group of major medical and large group of major medical and
economic importance.economic importance.

25. Digenean trematodesDigenean trematodes

The flukes have a complex life cycle in The flukes have a complex life cycle in
which a snail is the first (or intermediate) which a snail is the first (or intermediate)
host and a vertebrate the final (or host and a vertebrate the final (or
definitive host).definitive host).

The definitive host is one in which the The definitive host is one in which the
fluke reproduces sexually.fluke reproduces sexually.

26. Digenean trematodesDigenean trematodes

In some species there may be 2 or 3 In some species there may be 2 or 3
intermediate hosts before the definitive intermediate hosts before the definitive
host is reached.host is reached.

Trematodes inhabit a variety of sites in Trematodes inhabit a variety of sites in
their hosts including the digestive tract, their hosts including the digestive tract,
respiratory tract, circulatory system, respiratory tract, circulatory system,
urinary tract, and reproductive tract.urinary tract, and reproductive tract.

27. Digenean trematodesDigenean trematodes

Digenean life cycles are very complex and Digenean life cycles are very complex and
the fluke passes through numerous the fluke passes through numerous
stages.stages.

28. Digenean trematodesDigenean trematodes

A typical example would include the A typical example would include the
following stages:following stages:

AdultAdult

EggEgg (or shelled embryo) shed into water (or shelled embryo) shed into water

MiracidiumMiracidium: a free swimming, ciliated larva : a free swimming, ciliated larva
that finds and penetrates a snail intermediate that finds and penetrates a snail intermediate
hosthost

29. Digenean trematodesDigenean trematodes

SporocystSporocyst: reproduces asexually in : reproduces asexually in
intermediate host producing more sporocysts intermediate host producing more sporocysts
or another asexually reproducing stage called or another asexually reproducing stage called
a a redia.redia.

Redia Redia produce more redia or cercariae. produce more redia or cercariae.
Cercariae leave the intermediate host and Cercariae leave the intermediate host and
swim. Then they penetrate the skin of swim. Then they penetrate the skin of
another intermediate host or the definitive another intermediate host or the definitive
host.host.

30. Digenean trematodesDigenean trematodes

CercariaeCercariae that enter an intermediate host that enter an intermediate host
encyst in muscle and wait to be consumed by encyst in muscle and wait to be consumed by
the definitive host.the definitive host.

CercariaeCercariae that enter the definitive host make that enter the definitive host make
their way to their desired home and develop their way to their desired home and develop
into an adult fluke which reproduces sexually into an adult fluke which reproduces sexually
and produces eggs.and produces eggs.

31. ClonorchisClonorchis liver fluke liver fluke

Clonorchis is the most important liver fluke to infect Clonorchis is the most important liver fluke to infect
humans. Common in much of Asia (including China, humans. Common in much of Asia (including China,
Japan and southern Asia).Japan and southern Asia).

Adult flukes live in the bile passages and shelled Adult flukes live in the bile passages and shelled
miricidia pass out in feces. The miricidia enter snails miricidia pass out in feces. The miricidia enter snails
eventually leave the snails as cercariae and find a fish eventually leave the snails as cercariae and find a fish
where they encyst.where they encyst.

If fish is eaten raw or poorly cooked the person becomes If fish is eaten raw or poorly cooked the person becomes
infectedinfected

32. 8.8
Figure 14.12

33. How do flukes manipulate their How do flukes manipulate their
hosts?hosts?

Many parasites infect an intermediate host that Many parasites infect an intermediate host that
needs to be eaten by the definitive host for the needs to be eaten by the definitive host for the
parasite to complete its lifecycle.parasite to complete its lifecycle.

There are many instances of parasites altering There are many instances of parasites altering
their intermediate hosts behavior to make it their intermediate hosts behavior to make it
more vulnerable a predator (the definitive host). more vulnerable a predator (the definitive host).
Such behavior is widespread in flukes.Such behavior is widespread in flukes.

34. How do flukes manipulate their How do flukes manipulate their
hosts?hosts?

In the Carpenteria Salt Marshes in In the Carpenteria Salt Marshes in
southern California lives the fluke southern California lives the fluke
EuhaplorchisEuhaplorchis californiensis. californiensis.

It has a life cycle that includes two It has a life cycle that includes two
intermediate hosts, first the California intermediate hosts, first the California
Horn Snail and then the California killifish Horn Snail and then the California killifish
and a final host which can be any of a and a final host which can be any of a
variety of fish eating birds.variety of fish eating birds.

35. How do flukes manipulate their How do flukes manipulate their
hosts?hosts?

The fluke leaves its definitive host as an egg in The fluke leaves its definitive host as an egg in
bird droppings which are eaten by the fluke’s bird droppings which are eaten by the fluke’s
first intermediate host the snail. snail host. first intermediate host the snail. snail host.

The fluke then castrates the snail (preventing it The fluke then castrates the snail (preventing it
from diverting energy into eggs and away from from diverting energy into eggs and away from
the parasite).the parasite).

The fluke then reproduces asexually and sheds The fluke then reproduces asexually and sheds
cercariae into the water.cercariae into the water.

36. How do flukes manipulate their How do flukes manipulate their
hosts?hosts?

The cercariae seek out the next The cercariae seek out the next
intermediate host the killifish and latch intermediate host the killifish and latch
onto the fish’s gills.onto the fish’s gills.

Each cercaria works its way into a blood Each cercaria works its way into a blood
vessel and explores until it finds a nerve vessel and explores until it finds a nerve
which it then follows until it reaches the which it then follows until it reaches the
fish’s brain.fish’s brain.

37. How do flukes manipulate their How do flukes manipulate their
hosts?hosts?

The cercariae don’t penetrate the brain The cercariae don’t penetrate the brain
but sit on top of it. Then they wait for the but sit on top of it. Then they wait for the
fish to be eaten by a bird.fish to be eaten by a bird.

Once eaten by a bird they break out of the Once eaten by a bird they break out of the
fish’s head and move into the bird’s gut fish’s head and move into the bird’s gut
where they produce eggs that continue where they produce eggs that continue
the cyclethe cycle

38. How do flukes manipulate their How do flukes manipulate their
hosts?hosts?

The cercariae sitting on the bird’s brain The cercariae sitting on the bird’s brain
apparently don’t sit passively waiting.apparently don’t sit passively waiting.

Killifish when swimming occasionally Killifish when swimming occasionally
shimmy and jerk around flashing their shimmy and jerk around flashing their
bellies. Those infected with cercariae are bellies. Those infected with cercariae are
four times more likely to do so than non-four times more likely to do so than non-
infected fish.infected fish.

39. How do flukes manipulate their How do flukes manipulate their
hosts?hosts?

In field experiments in which penned fish In field experiments in which penned fish
were made available to foraging birds were made available to foraging birds
infected fish were infected fish were 30 times30 times (!) more likely (!) more likely
to be eaten than uninfected fish.to be eaten than uninfected fish.

40. How do flukes manipulate their How do flukes manipulate their
hosts?hosts?

Research on how the flukes alter the fish’s Research on how the flukes alter the fish’s
behavior has shown that the flukes behavior has shown that the flukes
produce powerful molecular signals called produce powerful molecular signals called
fibroblast growth factors. fibroblast growth factors.

These interfere with the growth of nerves These interfere with the growth of nerves
and may be the mechanism the flukes and may be the mechanism the flukes
use.use.

41. SchistosomiasisSchistosomiasis

Schsitosomiasis is an infection with blood Schsitosomiasis is an infection with blood
flukes and is one of the most important flukes and is one of the most important
major infectious diseases on the planet.major infectious diseases on the planet.

More then 200 million people are infected More then 200 million people are infected
worldwide with these flukes which they worldwide with these flukes which they
acquire swimming or walking in water in acquire swimming or walking in water in
which the intermediate snail host liveswhich the intermediate snail host lives

42. Schistosome
life cycle.

43. SchistosomiasisSchistosomiasis

When a schistosome cercaria swims it takes When a schistosome cercaria swims it takes
care to avoid UV light which can damage it, but care to avoid UV light which can damage it, but
is very sensitive to the scent of humans.is very sensitive to the scent of humans.

When it senses molecules from human skin it When it senses molecules from human skin it
swims rapidly and jerks around looking for the swims rapidly and jerks around looking for the
person. When it makes contact it releases person. When it makes contact it releases
chemicals that soften the skin and burrows in chemicals that soften the skin and burrows in
shedding its tail at the same time.shedding its tail at the same time.

44. SchistosomiasisSchistosomiasis

The fluke searches until it finds a capillary and The fluke searches until it finds a capillary and
enters it. enters it.

The capillary is only barely wide enough for the The capillary is only barely wide enough for the
fluke and it moves along using its pair of fluke and it moves along using its pair of
suckers. Eventually, it reaches a larger blood suckers. Eventually, it reaches a larger blood
vessel in which it can float until it reaches the vessel in which it can float until it reaches the
lungs and enters an artery and eventually makes lungs and enters an artery and eventually makes
its way to the liver.its way to the liver.

45. SchistosomiasisSchistosomiasis

Once in the liver, the fluke feeds on blood Once in the liver, the fluke feeds on blood
and begins to mature and develops and begins to mature and develops
ovaries or testes depending on its sex.ovaries or testes depending on its sex.

The fluke grows dozens of times larger in The fluke grows dozens of times larger in
the course of a few weeks and then the course of a few weeks and then
begins to search for a mate.begins to search for a mate.

46. SchistosomiasisSchistosomiasis

The fluke produces chemicals to attract The fluke produces chemicals to attract
members of the opposite sex.members of the opposite sex.

Females are slender and delicate, Females are slender and delicate,
whereas males are much bigger and have whereas males are much bigger and have
a spiny trough or groove into which the a spiny trough or groove into which the
female fits and locks in.female fits and locks in.

47. 8.9a and b
Figure 14.13

48. SchistosomiasisSchistosomiasis

Once paired up the pair mature sexually Once paired up the pair mature sexually
and travel from the liver to a permanent and travel from the liver to a permanent
home that is species-specific. home that is species-specific.

In In SchistosomaSchistosoma mansonimansoni it is near the it is near the
large intestine, in large intestine, in S.S. haemotobiumhaemotobium it is the it is the
bladder, and in bladder, and in S.S. nasale,nasale, a blood fluke of a blood fluke of
cows, it is the nose.cows, it is the nose.

49. SchistosomiasisSchistosomiasis

Once established the pair remain in situ Once established the pair remain in situ
for the rest of their lives.for the rest of their lives.

The male consumes blood and feeds the The male consumes blood and feeds the
female most of it, which she turns into female most of it, which she turns into
eggs, which pass out of the host and can eggs, which pass out of the host and can
begin the life cycle again.begin the life cycle again.

50. Class MonogeneaClass Monogenea

The monogenetic flukes were previously The monogenetic flukes were previously
classified as on order of the Trematoda, but classified as on order of the Trematoda, but
recent work suggests they are more closely recent work suggests they are more closely
related to cestodes (tapeworms).related to cestodes (tapeworms).

Monogeneans are typically external parasites of Monogeneans are typically external parasites of
fish that clamp onto the gills using a hooked fish that clamp onto the gills using a hooked
organ called an organ called an opisthaptoropisthaptor. Some also . Some also
parasitize frogs and turtles.parasitize frogs and turtles.

51. 8.11
Figure 14.16

52. Class MonogeneaClass Monogenea

Unlike the trematodes Monogeneans have Unlike the trematodes Monogeneans have
only a single host.only a single host.

The egg hatches into a ciliated larva which The egg hatches into a ciliated larva which
seeks out its host in the water.seeks out its host in the water.

53. Class Cestoda (tapeworms)Class Cestoda (tapeworms)

Tapeworms are parasites of the vertebrate Tapeworms are parasites of the vertebrate
digestive tract and about 4000 species of digestive tract and about 4000 species of
are known.are known.

Almost all tapeworms require at least two Almost all tapeworms require at least two
hosts with the definitive host being a hosts with the definitive host being a
vertebrate although intermediate hosts vertebrate although intermediate hosts
can be invertebrates.can be invertebrates.

54. Class CestodaClass Cestoda

Members of the Class Cestoda (tapeworms) are Members of the Class Cestoda (tapeworms) are
quite different in appearance from the other quite different in appearance from the other
members of the Platyhelminthes.members of the Platyhelminthes.

They have long, flat, tape-like bodies composed They have long, flat, tape-like bodies composed
of a of a scolexscolex for attaching to their host and a for attaching to their host and a
chain of many reproductive units or chain of many reproductive units or proglottids proglottids
called acalled a strobila. strobila. New proglottids form behind New proglottids form behind
the scolex and the strobila may become the scolex and the strobila may become
extremely long. extremely long.

55. 8.12
Figure 14.18

56. Tapeworm scolex
The scolex is equipped with a combination of suckers The scolex is equipped with a combination of suckers
and hooks that enable it to grip onto its host’s and hooks that enable it to grip onto its host’s
intestines.intestines.
Hooks
Suckers

57. Class CestodaClass Cestoda

Tapeworms live in the intestines and Tapeworms live in the intestines and
because they are immersed in digested because they are immersed in digested
food lack a digestive system of their own food lack a digestive system of their own
simply absorbing food across their simply absorbing food across their
tegument.tegument.

58. Class CestodaClass Cestoda

To facilitate the absorption of food a To facilitate the absorption of food a
tapeworm’s tegument has huge numbers tapeworm’s tegument has huge numbers
of tiny projections called of tiny projections called microtrichesmicrotriches, ,
which are broadly similar to the microvilli which are broadly similar to the microvilli
of the vertebrate intestine. of the vertebrate intestine.

They similarly increase the surface area of They similarly increase the surface area of
the tegument for absorption.the tegument for absorption.

59. 8.13
Figure 14.17

60. Class CestodaClass Cestoda

Tapeworms are usually monoecious (have both Tapeworms are usually monoecious (have both
male and female reproductive organs).male and female reproductive organs).

A proglottid is fertilized by another proglottid in A proglottid is fertilized by another proglottid in
the same or a different strobila. the same or a different strobila.

Shell-encased embryos form in the uterus and Shell-encased embryos form in the uterus and
exit the proglottid via a uterine pore or the entire exit the proglottid via a uterine pore or the entire
proglottid may detatch and pass out of the host.proglottid may detatch and pass out of the host.

61. 8.14
Figure 14.20

62. Human tapewormsHuman tapeworms

Humans are definitive hosts to several Humans are definitive hosts to several
tapeworms including the beef tapeworm tapeworms including the beef tapeworm
Taenia saginataTaenia saginata, pork tapeworm , pork tapeworm T. soliumT. solium, ,
and fish tapeworm and fish tapeworm Diphyllobothrium latumDiphyllobothrium latum..

63. Human tapewormsHuman tapeworms

The lifecycles of these parasites are similar. The lifecycles of these parasites are similar.

Shelled larave are shed into the environment. Shelled larave are shed into the environment.

These are consumed by the intermediate host These are consumed by the intermediate host
and the larvae (oncospheres) hatch, bury into and the larvae (oncospheres) hatch, bury into
blood vessels and make their way to skeletal blood vessels and make their way to skeletal
muscle where they encyst becoming so called muscle where they encyst becoming so called
“bladder worms” or “bladder worms” or cysticerci.cysticerci.

64. Human tapewormsHuman tapeworms

The encysted larva develops an invaginated The encysted larva develops an invaginated
scolex and waits, perhaps for years, for its host scolex and waits, perhaps for years, for its host
to be eaten. to be eaten.

If the meat is uncooked the cysticercus extends If the meat is uncooked the cysticercus extends
its scolex, attaches to the wall of the intestine its scolex, attaches to the wall of the intestine
and within 2-3 weeks matures and begins and within 2-3 weeks matures and begins
growing and producing eggs. A tapeworm may growing and producing eggs. A tapeworm may
be many meters long and live for years.be many meters long and live for years.

65. 8.15
Figure 14.19

66. Humans as intermediate hostsHumans as intermediate hosts

Humans may become intermediate hosts for Humans may become intermediate hosts for
tapeworms with potentially disastrous tapeworms with potentially disastrous
consequences if they consume shelled larvae in consequences if they consume shelled larvae in
contaminated food.contaminated food.

In an evolutionarily unfamiliar environment, In an evolutionarily unfamiliar environment,
cysticerci may encyst in inappropriate locations cysticerci may encyst in inappropriate locations
such as the brain which is frequently fatal.such as the brain which is frequently fatal.

67. 8.16
Figure 14.21
Cysticerci in human brain

68. Phylum Nemertea (Rhynchocoela) Phylum Nemertea (Rhynchocoela)
RibbonwormsRibbonworms

The nemerteans (ribbon worms) are long, The nemerteans (ribbon worms) are long,
marine predatory worms and there are marine predatory worms and there are
about 1000 species known.about 1000 species known.

Unlike members of the Platyhelminthes Unlike members of the Platyhelminthes
nemerteans have a complete gut with a nemerteans have a complete gut with a
mouth and anus and a true circulatory mouth and anus and a true circulatory
systemsystem

69. Phylum Nemertea (Rhynchocoela) Phylum Nemertea (Rhynchocoela)
RibbonwormsRibbonworms

Prey is captured using a long muscular proboscis Prey is captured using a long muscular proboscis
armed with a barb called a stylet..armed with a barb called a stylet..

The proboscis lies in an interior cavity called the The proboscis lies in an interior cavity called the
rhynchocoel and muscular pressure on fluid in the rhynchocoel and muscular pressure on fluid in the
rhynchocoel causes the proboscis to be quickly rhynchocoel causes the proboscis to be quickly
everted.everted.

The prey is wrapped in the sticky, slime-covered, The prey is wrapped in the sticky, slime-covered,
proboscis and stabbed repeatedly with the stylet. proboscis and stabbed repeatedly with the stylet.
Neurotoxins in the slime incapacitate the prey.Neurotoxins in the slime incapacitate the prey.

70. 8.18
Figure 14.24b
Figure 14.24a
Internal structure of female ribbon worm
(above).
Nemertean with proboscis extended (right)

71. 8.19
Figure 14.25
Baseodiscus mexicanus a nemertean from
the Galapagos Islands

72. Phylum GnathostomulidaPhylum Gnathostomulida

The first Gnatostomulid was not The first Gnatostomulid was not
discovered until 1928 and only about 80 discovered until 1928 and only about 80
species are known.species are known.

They are tiny (0.5-1mm long) wormlike They are tiny (0.5-1mm long) wormlike
animals that live in the interstitial spaces animals that live in the interstitial spaces
of sand and silt.of sand and silt.

73. Phylum GnathostomulidaPhylum Gnathostomulida

Because they lack a pseudocoel, Because they lack a pseudocoel,
circulatory system, and anus circulatory system, and anus
gnathostomulids were first classed as gnathostomulids were first classed as
turbellarians.turbellarians.

More recently it has been suggested that More recently it has been suggested that
they are more closely related to the phyla they are more closely related to the phyla
Rotifera and Acanthocephala.Rotifera and Acanthocephala.

74. 8.20
Figure 14.27
Gnathostomula jenneri