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“Means flatworm”
Characteristics
1. There are about 20,000 species and more
than 85% are parasitic.
2. Flatworms live in marine, freshwater and
moist terrestrial environments.
3. They are triploblastic,
which means they have
three germ layers
( endoderm, mesoderm,
and ectoderm)
4.They are the most primitive animals to have bilateral
symmetry and minimal cephalization, where the sense
organs become localized in the anterior end of the animal.
5. They have no coelom ( body cavity) so they are considered to
be an acoelomate. They have a digestive tube with the space
between the ectoderm and the endoderm filled with a fibrous
cellular packing material called parenchyma.
6. Because they show more specialization and division of labor
they have reached the organ system level of development
7. Digestive system is incomplete because there is only one
opening; one class, Cestoda ( the tapeworms) have no
digestive system.
8. Excretory system of two lateral canals with flame cells
9. No circulatory or Gas exchange systems
10. They have some simple sense organs such as eyespots
(ocelli) in some and chemoreceptors in most flatworms,
especially the auricles of planarians. Some have statocysts for
gravity and some have rheotaxis for water current direction.
11. Most forms are
monoecious, usually
with well-developed
gonads and internal
fertilization. Some have
direct development,where
the juvenile form appears
to be a smaller version of
the adult. Some have
indirect development with
complicated life cycles
involving several hosts
12. Nervous system with 1-5 pairs of longitudinal nerve cords and a
ganglion to form the beginnings of a brain
13. Skeleton is hydrostatic
There are four diverse classes of flatworms
1. class Turbellaria means “ commotion-like”
2. class Trematoda means “ perforated form”
3. class Monogena means “ single kind”
4. class Cestoda means “ girdle form”
Characteristics of Turbellaria
 A. most are free living and non-parasitic
 B. Range in size from 5mm to 50cm
 C. Locomotion by cilia or glide on a slime trail
D. most well know species of this class is Planaria which are extensively
used in the laboratory
1. Planaria are mainly carnivorous and feed mostly on small
crustaceans and insects. They wrap their body around prey and
evert a pharnyx through the mouth located on the ventral side
of the body, to suck up their prey.
2. Planaria have considerable powers of regeneration
Ex. A piece of planaria cut from the middle of
the body will grow both a new head and a
new tail
3. Planaria are freshwater and some are found in moist soil
usually living under rocks or rotting logs in tropical rain
forests.
Characteristics of class
Trematoda ( flukes)
A. There are about 11,000 species
B. All trematodes are endoparasitic
C. Most adult trematodes are digestive trac parasites of vertebrates
D. Have structural adaptations for parasitism
1. Penetration glands or glands to produce cyst material
2. Organ for adhesions such at hooks and suckers
3. Increased reproductive capacity
E. sense organs are poorly developed or absent
Sub class Digena
means “ double race”
A. Have indirect life cycle with first intermediate host being a
mollusc ( usually a snail)
B. The definitive or final host is always a vertebrate
C. General life cycle of
Trematodes (flukes)
1. The egg passes in feces and must reach the water
2. Egg hatches into a free swimming Miracidium
3. Miricidium finds a snail and penetrates the tissue to
form a sporocyst
4. Inside the snail, the sporocyst produces many rediae
asexually
5. the many rediae produce many more cercariae with
tails, asexually
6. Cercariae emerge from the snail and either
penetrate a second intermediate host or encyst on
vegetation to become metacercariae, which are
juvenile flukes
7. The adult grows from a metacercariae when that
stage is eaten by the right definitive host
Sheep liver fluke (Faciola hepatica)
Adult fluke lives in the liver of sheep and the eggs are passed
In the feces of the sheep. After two generations inside snails, the
Cercariae emerge to encyst on vegetation and wait to be eaten by
other sheep.
Human Liver Fluke
(clonorchis sinensis)
Common in E. Asia, especially in China, Japan, and
S.E. Asia. Other animals that are also infected by this
fluke are Cats, Dogs, and Pigs. Vary in length from
10mm to 30mm. Usually found in bile passageways of
humans and other fish eating mammals. Eggs are
shed into water with feces but do not hatch until they
are eaten by certain snails. Cercariae escape into the
water and swim until they find one certain kind of fish
and bore into its muscle, encyst as Metacercariae.
When ever a mammal eats raw infected fish, the
metacercarial cyst dissolves in the intestine and the
young fluke migrates up the bile duct. There they
become adults and may live for 15-30 years. A heavy
infestation of liver flukes can destroy the liver and
result in death. This can be controlled by destroying
snails and thoroughly cooking all fish.
Life cycle of a Human
Liver fluke
Human Blood Flukes
(Schistosoma)
Causes a disease called Schistosomiasis which
infects over 200 million people and is very
common in Africa, So. America and the middle
and far east. Eggs hatch from feces, in water,
and form a miricidium( ciliated larvae) that
penetrate a certain species of snail. After two
generations in the snail, they form Cercaria which
swim in water until they contact bare human skin.
They pierce the skin and shed their tails, swim up
blood vessels to the liver where they develop and
then migrate to the appropriate blood vessel, the
large intestine, the small intestine, or the bladder
causing abscesses, bloody diarhea, ulcers and
abdominal pain.
Swimmers itch: (Schistosome dermatitis)
Various species of schistosomes can penetrate the skin of
humans and cause intense itching and a rash. These
schistosomes do no real harm because humans are not the
definitive host. Many northern American lakes are heavily
infested.
III. Characteristics of class Monogena
A. Monogeans are all ectoparasites, primarily of
frogs and the gills of fish
B. Have a direct life cycle in a single host
C. The oncomiracidium ( ciliated larvae)
attaches to a host’s gills with posterior hooks
which become the attachment organ of the
adult is called the opisthapor.
D. These parasites can become serious pests of
fish farming
IV. Characteristics of class Cestoda
A. They have a flattened and
segmented body.
B. Each segment is called
a proglottid and contains
a set of reproductive organs.
A chain of proglottids is
called a strobila. Proglottids
originate in an area called
the germanitive zone which
is behind the head.
C. Nearly all tapeworms are monoecious
D. They have no digestive track, living off the
intestinal contents of the host which they digest
through the epidermis.
E. They have no stomach or intestine
F. Have a specialized structure called
the scolex, or holdfast, which is
located on the head portion
and is the organ of attachment.
It is formed with hooks and
suckers for attachment to the
intestinal wall.
G. Almost all cestodes require two
hosts, the adult lives in the
intestine of the definitive host
H. There are almost 1000 species of
tapeworm known, infecting almost
All types of vertebrates
Beef tapeworm (Taenia saginata)
This tapeworm is found as an adult in the
intestine of humans. The juvenile form is
found in muscle tissue of cattle. Mature
adults can reach over 30 ft. in length with
over 200 proglottids. The scolex has four
suckers for attachment. Gravid proglottids
are passed in fecal material and crawl out and
dry and rupture. Embryos are viable on grass
for five months and are picked up by cattle
grazing. Infected people can expel many
proglottids daily. Humans become infected by
eating rare cooked roast beef, steaks and
barbecue.
Beef tapeworm life cycle
Pork tapeworm (Taenia solium)
 This tapeworm lives in the small intestine of
humans and the juvenile form is found in the
muscles of pigs. The life cycle is the same as
the beef tapeworm except humans become
infected by eating improperly cooked pork.
This infection is much more dangerous
because if the proglottids or eggs are
accidentally ingestee, the liberated embryos
called Cystcerci can develop and migrate to
several organs in the human body. This
condition is called Cysteriosis. Common sites
of infection are the eyes and the brain, which
results in blindness or death.
Pig tapeworm life cycle
Pork tapeworm
larvae in brain
Fish tapeworm
(diphyllobothrium latum)
This tapeworm is found in the intestine of
humans , dogs, cats, and other mammals.
Immature stages are in crustaceans and fish.
This is the longest tapeworm to infect humans
with a length of over 65 ft. this tapeworm can
cause a serious anemia. Fish tapworm
infections can occur anywhere in the world
where people commonly eat raw fish. In the
United States infections, are most common in
the great lakes region.
Unilocular Hydatid
( Echinococcus gramulosus)
This tapeworm infects dogs, and humans
among other species of mammal are the
intermediate hosts. The juvenile stage is
a cyst which grown slowly but for a long
time, up to 20 years and can reach the
size of a basketball. It is filled with
thousands of scolexes. Each scolex will
produce a tapeworm when eaten by a
canine. The only treatment is surgical
removal of the hydatid cyst.