1. Acoelomate Bilateral Animals
The term worm is loosely employed in
biology and is applied to very different
animals including the segmented worms
(Annelids), roundworms
(pseudocolomates and a variety of
acoelomate bilateral animals.
2. Acoelomate Bilateral Animals
There are three phyla of acoelomate
bilateral animals:
Platyhelminthes: flatworms
Nemertea: ribbon worms
Gnathostomulida: jawed worms
3. Acoelomate Bilateral Animals
By far the most important in diversity and
economic importance is the phylum
Platyhelminthes, which includes a variety
of parasitic forms such as the flukes and
tapeworms.
4. Acoelomate Bilateral Animals
Unlike the radiate animals all of these organisms
are mobile and have evolved cephalization with
their sense organs concentrated at the head
end. There is also the beginning of a ladder-
type nervous system.
In addition, they are bilaterally symmetrical.
5. Acoelomate Bilateral Animals
All of these animals are triploblastic, but lack a
coelom. Instead, they have a solid body filled
with parenchyma cells.
They have evolved organs and in some cases
organ systems. The simplest excretory or
osmoregulatory systems and circulatory systems
are found in members of these groups.
6. Phylum Platyhelminthes
Members of the Platyhelminthes typically have
dorsoventrally flattened bodies that are usually
slender and leaflike or ribbonlike.
There are four classes in the Platyhelminthes.
The Turbellaria are free living whereas as
members of the Monogenea, Trematoda and
Cestoda are parasitic.
7. Nutrition
The digestive system includes a mouth, pharynx,
and blind intestine.
In the free-living Turbellarians the pharynx can
be everted from the mouth.
Food is sucked into the intestine where a
combination of extracellular and intracellular
digestion takes place.
8. Nutrition
Undigested food exits via the pharynx.
In the Cestoda the digestive tract is absent
and all nutrients are absorbed across the
tegument (the syncytial membrane/body
covering found in all parasitic
Platyhelminthes).
9. Excretion/Osmoregulation
The osmoregulatory system consists of a series
of canals that end in flame cells or
protonephridia).
This system appears mainly intended to remove
excess fluid, but retain essential ions. It is most
well developed in freshwater Turbellarians, but
reduced or absent in marine species, which do
not have to remove excess water.
10. Nervous system and sense organs
Flatworms possess a simple brain and one to
five pairs of longitudinal nerve cords that are
cross connected to form a ladder-like
arrangement.
There has been a tendency towards reduction of
the number of pairs of nerve cords and
increased development of the ventral pair. A
similar evolutionary pathway may have led to
the development of the ventral nerve cord found
in annelids and arthropods.
11. Nervous system and sense organs
Neurons are specialized for different tasks e.g.
sensory and motor functions, which is an
important advance in the evolution of nervous
systems.
There are a number of different sensory cells
(e.g. rheoceptors and statocysts) found in
flatworms and tactile and chemoreceptive cells
are abundant.
12. Nervous system and sense organs
In freshwater Planarians concentrations of
sensory cells form two ear-like structures
(the auricles) found on the side of the
head.
Light sensitive eyespots or ocelli are
common in all classes but Cestoda.
13. Reproduction
Reproduction in the Platyhelminthes can
be asexual or sexual. However, most are
hermaphroditic and cross fertilize.
In parasitic forms sexual and asexual
reproduction may alternate in different
stages of the life history
14. Classification of Platyhelminthes
There are four classes in the
Platyhelminthes:
Class Turbellaria: free-living flatworms.
Class Turbellaria: endoparasitic flukes
Class Monogenea: parasitic flukes that are
mainly ectoparasites
Class Cestoda: tapeworms
15. Class Turbellaria
Class Turbellaria contains about 3000 species.
There is considerable debate about the
classification of the class and it is likely that the
class is not monophyletic.
Most species are marine and benthic. However,
some are also found in fresh water as well as in
moist temperate and tropical terrestrial habitats.
18. Class Turbellaria
Most Turbellarians are predators of invertebrates
smaller than themselves. Other species are
herbivores or scavengers.
Turbellarians move by swimming, creeping or
crawling. They combine muscular contractions
with ciliary movement to move.
19. Class Trematoda
There are about 9000 species of
trematodes all of which are parasitic.
Most parasitize vertebrates.
Adaptations for parasitism include suckers
and hooks for attachment, glands to
produce cyst material and increased
reproductive capacity.
21. Class Trematoda
Structurally trematodes are similar to
turbellarians having a well developed
digestive system and similar nervous,
excretory, and reproductive systems.
However, a major difference is the
tegument .
22. Tegument
The tegument (found in all parasitic
Platyhelminthes) is a nonciliated, cytoplasmic
syncytium that overlays layers of muscle.
The syncytium represents extensions of cells
that are located below the muscle in the
parenchyma.
The tegument protects the parasite against its
host (e.g. against digestive enzymes).
24. Digenean Trematodes
There are three subclasses of
Trematodes, but two are small, poorly
studied groups.
The third group, the Digenea, however is a
large group of major medical and
economic importance.
25. Digenean trematodes
The flukes have a complex life cycle in
which a snail is the first (or intermediate)
host and a vertebrate the final (or
definitive host).
The definitive host is one in which the
fluke reproduces sexually.
26. Digenean trematodes
In some species there may be 2 or 3
intermediate hosts before the definitive
host is reached.
Trematodes inhabit a variety of sites in
their hosts including the digestive tract,
respiratory tract, circulatory system,
urinary tract, and reproductive tract.
27. Digenean trematodes
Digenean life cycles are very complex and
the fluke passes through numerous
stages.
28. Digenean trematodes
A typical example would include the
following stages:
Adult
Egg (or shelled embryo) shed into water
Miracidium : a free swimming, ciliated larva
that finds and penetrates a snail intermediate
host
29. Digenean trematodes
Sporocyst : reproduces asexually in
intermediate host producing more sporocysts
or another asexually reproducing stage called
a redia.
Redia produce more redia or cercariae.
Cercariae leave the intermediate host and
swim. Then they penetrate the skin of
another intermediate host or the definitive
host.
30. Digenean trematodes
Cercariae that enter an intermediate host
encyst in muscle and wait to be consumed by
the definitive host.
Cercariae that enter the definitive host make
their way to their desired home and develop
into an adult fluke which reproduces sexually
and produces eggs.
31. Clonorchis liver fluke
Clonorchis is the most important liver fluke to infect
humans. Common in much of Asia (including China,
Japan and southern Asia).
Adult flukes live in the bile passages and shelled
miricidia pass out in feces. The miricidia enter snails
eventually leave the snails as cercariae and find a fish
where they encyst.
If fish is eaten raw or poorly cooked the person becomes
infected
33. How do flukes manipulate their
hosts?
Many parasites infect an intermediate host that
needs to be eaten by the definitive host for the
parasite to complete its lifecycle.
There are many instances of parasites altering
their intermediate hosts behavior to make it
more vulnerable a predator (the definitive host).
Such behavior is widespread in flukes.
34. How do flukes manipulate their
hosts?
In the Carpenteria Salt Marshes in
southern California lives the fluke
Euhaplorchis californiensis.
It has a life cycle that includes two
intermediate hosts, first the California
Horn Snail and then the California killifish
and a final host which can be any of a
variety of fish eating birds.
35. How do flukes manipulate their
hosts?
The fluke leaves its definitive host as an egg in
bird droppings which are eaten by the fluke’s
first intermediate host the snail. snail host.
The fluke then castrates the snail (preventing it
from diverting energy into eggs and away from
the parasite).
The fluke then reproduces asexually and sheds
cercariae into the water.
36. How do flukes manipulate their
hosts?
The cercariae seek out the next
intermediate host the killifish and latch
onto the fish’s gills.
Each cercaria works its way into a blood
vessel and explores until it finds a nerve
which it then follows until it reaches the
fish’s brain.
37. How do flukes manipulate their
hosts?
The cercariae don’t penetrate the brain
but sit on top of it. Then they wait for the
fish to be eaten by a bird.
Once eaten by a bird they break out of the
fish’s head and move into the bird’s gut
where they produce eggs that continue
the cycle
38. How do flukes manipulate their
hosts?
The cercariae sitting on the bird’s brain
apparently don’t sit passively waiting.
Killifish when swimming occasionally
shimmy and jerk around flashing their
bellies. Those infected with cercariae are
four times more likely to do so than non-
infected fish.
39. How do flukes manipulate their
hosts?
In field experiments in which penned fish
were made available to foraging birds
infected fish were 30 times (!) more likely
to be eaten than uninfected fish.
40. How do flukes manipulate their
hosts?
Research on how the flukes alter the fish’s
behavior has shown that the flukes
produce powerful molecular signals called
fibroblast growth factors.
These interfere with the growth of nerves
and may be the mechanism the flukes
use.
41. Schistosomiasis
Schsitosomiasis is an infection with blood
flukes and is one of the most important
major infectious diseases on the planet.
More then 200 million people are infected
worldwide with these flukes which they
acquire swimming or walking in water in
which the intermediate snail host lives
43. Schistosomiasis
When a schistosome cercaria swims it takes
care to avoid UV light which can damage it, but
is very sensitive to the scent of humans.
When it senses molecules from human skin it
swims rapidly and jerks around looking for the
person. When it makes contact it releases
chemicals that soften the skin and burrows in
shedding its tail at the same time.
44. Schistosomiasis
The fluke searches until it finds a capillary and
enters it.
The capillary is only barely wide enough for the
fluke and it moves along using its pair of
suckers. Eventually, it reaches a larger blood
vessel in which it can float until it reaches the
lungs and enters an artery and eventually makes
its way to the liver.
45. Schistosomiasis
Once in the liver, the fluke feeds on blood
and begins to mature and develops
ovaries or testes depending on its sex.
The fluke grows dozens of times larger in
the course of a few weeks and then
begins to search for a mate.
46. Schistosomiasis
The fluke produces chemicals to attract
members of the opposite sex.
Females are slender and delicate,
whereas males are much bigger and have
a spiny trough or groove into which the
female fits and locks in.
48. Schistosomiasis
Once paired up the pair mature sexually
and travel from the liver to a permanent
home that is species-specific.
In Schistosoma mansoni it is near the
large intestine, in S. haemotobium it is the
bladder, and in S. nasale, a blood fluke of
cows, it is the nose.
49. Schistosomiasis
Once established the pair remain in situ
for the rest of their lives.
The male consumes blood and feeds the
female most of it, which she turns into
eggs, which pass out of the host and can
begin the life cycle again.
50. Class Monogenea
The monogenetic flukes were previously
classified as on order of the Trematoda, but
recent work suggests they are more closely
related to cestodes (tapeworms).
Monogeneans are typically external parasites of
fish that clamp onto the gills using a hooked
organ called an opisthaptor . Some also
parasitize frogs and turtles.
52. Class Monogenea
Unlike the trematodes Monogeneans have
only a single host.
The egg hatches into a ciliated larva which
seeks out its host in the water.
53. Class Cestoda (tapeworms)
Tapeworms are parasites of the vertebrate
digestive tract and about 4000 species of
are known.
Almost all tapeworms require at least two
hosts with the definitive host being a
vertebrate although intermediate hosts
can be invertebrates.
54. Class Cestoda
Members of the Class Cestoda (tapeworms) are
quite different in appearance from the other
members of the Platyhelminthes.
They have long, flat, tape-like bodies composed
of a scolex for attaching to their host and a
chain of many reproductive units or proglottids
called a strobila. New proglottids form behind
the scolex and the strobila may become
extremely long.
56. Tapeworm scolex
Hooks
Suckers
The scolex is equipped with a combination of suckers
and hooks that enable it to grip onto its host’s
intestines.
57. Class Cestoda
Tapeworms live in the intestines and
because they are immersed in digested
food lack a digestive system of their own
simply absorbing food across their
tegument.
58. Class Cestoda
To facilitate the absorption of food a
tapeworm’s tegument has huge numbers
of tiny projections called microtriches ,
which are broadly similar to the microvilli
of the vertebrate intestine.
They similarly increase the surface area of
the tegument for absorption.
60. Class Cestoda
Tapeworms are usually monoecious (have both
male and female reproductive organs).
A proglottid is fertilized by another proglottid in
the same or a different strobila.
Shell-encased embryos form in the uterus and
exit the proglottid via a uterine pore or the entire
proglottid may detatch and pass out of the host.
62. Human tapeworms
Humans are definitive hosts to several
tapeworms including the beef tapeworm
Taenia saginata, pork tapeworm T. solium,
and fish tapeworm Diphyllobothrium latum.
63. Human tapeworms
The lifecycles of these parasites are similar.
Shelled larave are shed into the environment.
These are consumed by the intermediate host
and the larvae (oncospheres) hatch, bury into
blood vessels and make their way to skeletal
muscle where they encyst becoming so called
“bladder worms” or cysticerci.
64. Human tapeworms
The encysted larva develops an invaginated
scolex and waits, perhaps for years, for its host
to be eaten.
If the meat is uncooked the cysticercus extends
its scolex, attaches to the wall of the intestine
and within 2-3 weeks matures and begins
growing and producing eggs. A tapeworm may
be many meters long and live for years.
66. Humans as intermediate hosts
Humans may become intermediate hosts for
tapeworms with potentially disastrous
consequences if they consume shelled larvae in
contaminated food.
In an evolutionarily unfamiliar environment,
cysticerci may encyst in inappropriate locations
such as the brain which is frequently fatal.
68. Phylum Nemertea (Rhynchocoela)
Ribbonworms
The nemerteans (ribbon worms) are long,
marine predatory worms and there are
about 1000 species known.
Unlike members of the Platyhelminthes
nemerteans have a complete gut with a
mouth and anus and a true circulatory
system
69. Phylum Nemertea (Rhynchocoela)
Ribbonworms
Prey is captured using a long muscular proboscis
armed with a barb called a stylet..
The proboscis lies in an interior cavity called the
rhynchocoel and muscular pressure on fluid in the
rhynchocoel causes the proboscis to be quickly
everted.
The prey is wrapped in the sticky, slime-covered,
proboscis and stabbed repeatedly with the stylet.
Neurotoxins in the slime incapacitate the prey.
70. Figure 14.24a
Figure 14.24b
8.18
Internal structure of female ribbon worm
(above).
Nemertean with proboscis extended (right)
71. Figure 14.25
8.19
Baseodiscus mexicanus a nemertean from
the Galapagos Islands
72. Phylum Gnathostomulida
The first Gnatostomulid was not
discovered until 1928 and only about 80
species are known.
They are tiny (0.5-1mm long) wormlike
animals that live in the interstitial spaces
of sand and silt.
73. Phylum Gnathostomulida
Because they lack a pseudocoel,
circulatory system, and anus
gnathostomulids were first classed as
turbellarians.
More recently it has been suggested that
they are more closely related to the phyla
Rotifera and Acanthocephala.
