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Phylum Platyhelminthes
and other Acoelomate
Animals, Chapter 8,
Zoology
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Three germ layers (triploblastic)
Bilateral symmetry and cephalization
Acoelomate; with no internal body cavity other than the digestive
tract. The spaces between the organs are filled with mesodermal
tissue called parenchyma
Nervous system consists of a pair of anterior organs called ganglia
that are similar to a brain
Excretory system of two lateral running canals connected to structures
called flame cells
No circulatory or respiratory organs; oxygen and carbon dioxide
exchange occurs by diffusion
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Many flatworms like the turbellarians are free-living
organisms that dwell on the bottom of the ocean or
freshwater habitats or moist places on land.
All of the flatworms that belong to class Trematoda,
Monogenea, (flukes) and Cestoda (tapeworms) are
parasitic. Some are ectoparasites (live on the outside of
their host) and others are endoparasites (live on the inside
of their host).
Many have complex lifestyles that involve more than one
host. The first host is often an invertebrate and the second
a vertebrate.
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The digestive system of some flatworms is incomplete,
having only one opening that serves as a mouth and an
anus.
Turbellarians have a muscular mouth and pharynx that
protrudes from the ventral surface.
The pharynx is inserted into prey or decayed organic
matter, digestive enzymes are secreted, and then the
pharynx brings the food into the gastrovascular cavity
where it is digested.
Digestion in tapeworms (cestodes) occurs externally by
diffusion.
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The excretory system of flatworms consists of a complex
network of cells called flame cells
Flame cells (or protonephridia) are similar to kidneys in other
organisms.
The flame cells consist of a hollow cup filled with cilia that
beat and pump water and nitrogenous wastes out the body.
When the cilia beat they resemble flames, giving them their
name.
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Flatworms are the first animal we have discussed that
exhibits cephalization.
Cephalization means they have a head (anterior) and tail
(posterior) region.
Cephalization allows animals to move (crawl, swim, fly etc.)
in a certain direction to search for resources.
In order to move in a specific direction the animal requires
a central nervous system.
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The central nervous system in flatworms is a region of nerve
tissue found at the anterior end called a ganglion.
The ganglion attaches to a pair of ventral nerve cords that
run the length of the body.
The nerve cords are sensitive to touch, chemical detection,
equilibrium, water current direction and they can control
muscular movement.
The ganglion are also attached to eye spots that are
sensitive to light.
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Reproduce both sexually and asexually.
Sexual reproduction usually occurs by cross fertilization
(trading sperm) with other individuals.
Most flatworms are monoecious.
Only a few flatworms are dioecious.
Asexual reproduction usually occurs by fission in which the
organism separates into two halves and regenerate into
two adults.
In some flatworms, asexual reproduction can occur as
thousands of juvenile offspring.
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Free-living, soft flattened bodies
Bodies are covered in cilia
Mouth located on ventral surface near the center of the body
Mostly hermaphroditic
Some can reproduce asexually by fission
Examples: Dugesia (planaria), Microstomum, Planocera
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Parasitic lifestyle with two hosts.
First (intermediate) host is a mollusc.
Second (definitive) host is a vertebrate.
Sexual reproduction occurs in the definitive host.
Occasionally a third intermediate host plays a role in the
life cycle.
Parasitism can occur in almost every system of the host’s
body including the digestive, respiratory, circulatory,
excretory, and reproductive systems.
Examples: Fasciola, Clonorchis, Schistosoma
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A typical life cycle would include the following life cycle
stages; adult, egg, miracidium, sporocyst, redia, cercaria,
and metacercaria.
Egg is released from host's excrement and must reach a
source of water to be released.
In the water the egg releases a free-swimming, ciliated
larva called the miracidium
The miracidium penetrates and enters the tissues of a
mollusc (usually a snail), and transforms into a sporocyst.
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Sporocysts then reproduce asexually to produce redia.
Redia reproduce asexually to produce cercaria.
Cercaria emerge from the mollusc and penetrate a
second intermediate host or they can develop as a
metacercaria on vegetation.
Metacercaria are ingested by a definitive host and
develop into adult flukes.
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Some of the most serious parasites in humans are flukes.
Genus Clonorchis is a liver fluke that is common in China, south Asia,
and Japan and can cause cirrhosis of the liver and death.
Genus Schistosoma is a blood fluke that causes an infection called
schistosomiasis. It is one of the deadliest diseases in the world with
nearly 200 million people infected in Africa, South America, and the
Middle East.
Symptoms of severe dysentery, anemia, enlarged liver, inflammation
of the bladder, and brain damage.
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One type of fluke that
infects birds causes a
painful irritation in
humans called
swimmer's itch.
In the case of this fluke
occurring in humans,
the life cycle becomes
terminated since the
host in unsuitable.
Children with
Schistosomiasis
Swimmer's itch
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Ectoparasites that attach to the gills of fish.
Hook-like mouth called an opisthaptor.
Occasionally they are found in frogs and turtles, but mostly
on the skin or gills of fish.
Common but cause little damage to their host.
They can become a serious threat when fish become
crowded, like in the case of fish farming.
Life cycle simple with only a single host
Examples: Dactylogyrus, Polystoma, Gyrodactylus
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Image on the right found on the gills of a fish
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Long flat bodies with a hook-like mouth called a scolex used
for attachment to the wall of the intestine of its host.
The rest of the body is followed by regular repeating units of
reproducing sections called proglottids. The main body of a
tapeworm is a chain of proglottids called a strobila.
No digestive tract; nutrients are absorbed across the skin by
diffusion.
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All cestodes require at least two hosts.
Adults are parasitic in the digestive tract of vertebrates.
Often one of the intermediate hosts is an invertebrate.
Almost ALL vertebrate species can become infected, but adult
tapeworms do little harm to their hosts.
Humans most often get infected by consuming raw or rare beef, pork,
or fish.
Children often become infected by putting their fingers in their mouth
after touching infected animals.
Occasionally cysts can develop in the brain leading to severe brain
damage and death.
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Ribbon Worms and Jaw Worms are similar to Phylum Platyhelminthes
in several ways, but have significant differences which place them in
their own phylums
They are both acoelomate and bilaterally symmetrical.
Ribbon worms have flame cells and ganglia.
Some ribbon worms can regenerate like planarians.
However, in ribbon worms, the digestive tract is complete with both a
mouth and an anus.
Ribbon worms also have a circulatory system
Jaw worms are a newly discovered species and have yet to be
properly classified. They somewhat resemble rotifers.
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Ribbon Worms
Jaw Worms