Phylum Platyhelminthes
and other Acoelomate
Animals, Chapter 8,
Zoology
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Three germ layers (triploblastic), first animal discussed in this class with
true organs and tissues.
Bilateral symmetry and cephalization.
Acoelomate, which means the spaces between the organs are filled
with mesoderm tissue called parenchyma.
Nervous system consists of a pair of anterior organs called ganglia
that are basically a simple brain.
Excretory system consists of two lateral canals connected to structures
called flame cells
No circulatory or respiratory organs; respiration 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, and Cestoda are parasitic. Some are
ectoparasites and others are endoparasites.
Many have complex lifestyles that involve more than one
host. The first host is usually an invertebrate and the
second host is usually 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
food into the gastrovascular cavity where it is digested.
Digestion in tapeworms (cestodes) occurs by diffusion.
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The excretory system of flatworms consists of a complex
network of cells called flame cells
Flame cells (also known as 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 of the flames cells beat they resemble flames.
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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 (brain, ganglia etc.).
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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 full length of the body.
The nerve cords are sensitive to touch, chemical detection,
balance and 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 hermaphrodites.
Most flatworms are monoecious (both sexes in one animal).
Only a few flatworms are dioecious (separate sexes).
Asexual reproduction occurs by fission. The organism
separates into two halves that regenerate into two adults.
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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. Schistosomiasis 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 tapeworms 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 larval cysts (called bladder worms) 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 separate 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