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Instructor: Almonther I. Alhamedi
The Islamic University of Gaza
Department of Biology
E-mail : [email protected]
Web page :http://site.iugaza.edu.ps/mhamedi
Platyhelminthes, or flatworms, or ribbon worms are the simplest
animals to have primary bilateral symmetry.
These phyla have only one internal space, a digestive cavity, with
the region between the ectoderm and endoderm filled with
mesoderm in the form of muscle fibers and mesenchyme
(parenchyma). Since they lack a coelom or a pseudocoelom, they
are termed acoelomate Bilateria, and because they have three
well-defined germ layers, they are termed triploblastic.
Acoelomate bilateral animals show more specialization and
division of labor among their organs than do radiate animals
because the mesoderm makes more elaborate organs possible.
Thus the acoelomates are said to have reached the organ system
level of organization.
They belong to the protostome division of the Bilateria and have
spiral cleavage.
1. Acoelomate Bilateria developed the basic bilateral plan of
organization that has been widely exploited in the animal
2. Mesoderm developed into a well-defined embryonic germ layer
(triploblastic), making available a great source of tissues,
organs, and systems.
3. Along with bilateral symmetry, cephalization was
established. Some centralization of the nervous system evident.
4. Along with the sub epidermal musculature, there is also a
mesenchymal system of muscle fibers.
5. They are the simplest animals with an excretory system.
6.Nemerteans are the simplest animals to have a circulatory
system with blood and a one-way alimentary canal.
7. Body flattened dorsoventrally; oral and genital apertures
mostly on ventral surface.
8. Epidermis may be cellular or syncytial (ciliated in some);
rhabdites in epidermis of most Turbellaria; epidermis a
syncytial tegument in Monogenea, Trematoda, Cestoda,
and some Turbellaria.
9. Muscular system primarily of a sheath form and of
mesodermal origin; layers of circular, longitudinal, and
sometimes oblique fibers beneath the epidermis.
10.Nervous system consisting of a pair of anterior
ganglia with longitudinal nerve cords connected by
transverse nerves and located in the mesenchyme in
most forms; similar to cnidarians in primitive forms.
11. Simple sense organs; eyespots in some
10. Excretory system of two lateral canals with branches
bearing flame cells (protonephridia); lacking in some
primitive forms.
Turbellarians are mostly free-living worms that range in length
from 5 mm or less to 50 cm.
Their mouth is on the ventral side.
Most are Unlike trematodes and cestodes, they have simple
life cycles.
Very small planaria swim by means of their cilia. Others move by
gliding. Muscle contractions also permit turning, twisting and folding of
the body.
Turbellarians are carnivores and prey on other animals or eat dead
animal remains.
Planarians have a muscular pharynx that they can insert into their
prey and then pump to bring in food fragments
These animals have a highly divided gut to greatly increase the
surface area for digestion and absorption
They have well developed sensory structures, including eyespots,
mechanoreceptors, and chemoreceptors
Intestinal pattern of two orders of turbellarians. A, Tricladida.
B, Polycladida.
Structure of a planarian. A, Reproductive and osmoregulatory systems,
shown in part. Inset at left is enlargement of flame cell. B, Digestive
tract and ladder-type nervous system. Pharynx is shown in resting
position. C, Pharynx extended through ventral mouth.
Cross section of planarian through pharyngeal region, showing
relationships of body structures.
The parasitic flatworms of class Trematoda, also
called flukes.
have oral suckers, sometimes supplemented by hooks, with
which they attach to their vertebrate hosts.
Trematodes of the order Digenea have complex life cycles
involving two or more hosts.
The larval worms occupy small animals, typically snails
and fish, and the adult worms are internal parasites of
Larval stages of some medically important species include
miracidium, redia, cercaria, and metacercaria.
Most trematodes are endoparasites.
Blood flukes differ from most other flukes in having the two
branches of the digestive tube united into a single tube in
the posterior part of the body.
Males are broader and heavier and have a large, ventral
groove, the gynecophoric canal, posterior to the ventral
sucker. The gynecophoric canal embraces the long, slender
Three species account for most of the schistosomiasis in
S. mansoni, which lives primarily in venules draining the
large intestine.
S. japonicum, which is found mostly in venules of the small
S. haematobium, which lives in venules of the urinary
The life cycle of blood flukes is similar in all species.
Eggs are discharged in human feces or urine; if they get into water, they
hatch as ciliated miracidia, which must contact the required kind of snail
within a few hours to survive.
In the snail, they transform into sporocysts, which produce another
generation of sporocysts. Daughter sporocysts give rise to cercariae
directly, without formation of rediae.
Cercariae escape from the snail and swim about until they contact the
skin of a human. They penetrate the skin, shedding their tails in the
process, and reach a blood vessel where they enter the circulatory system.
There is no metacercarial stage. The young schistosomes make their way
to the hepatic portal system of blood vessels and undergo a period of
development in the liver before migrating to their characteristic sites. As
eggs are released by adult females, they are somehow extruded through
the wall of venules and through the gut or bladder lining, to be voided
with feces or urine, according to species.
Bulinus truncatus is the intermediate host .
Adult male and female Schistosoma mansoni in copulation.
The male has a long gynecophoric canal that holds the female (the
darkly stained individual).
A- S. mansoni
B- S. haematobium
C- S. japonicum
Schistosoma Schistosoma Schistosoma
Schistosoma Schistosoma
Fasciola hepatica, also known as the common liver
fluke or sheep liver fluke.
Belong to subclass digenia.
The disease caused by the fluke is called fascioliasis.
F. hepatica is distributed worldwide, and causes great
economic losses in sheep and cattle. It has been known as
an important parasite of sheep and cattle for hundreds of
To complete its life cycle, F. hepatica requires a freshwater
snail as an intermediate host, such as family Lymnaeidae,
in which the parasite can reproduce asexually.
Tapeworms live in the gastrointestinal tract feeding on the
passing food.
The most common tapeworm species in humans are fish,,
beef and pork tapeworms. Fish, beef and pork tapeworms
grow many meters long.
A tapeworm attaches to the intestinal wall with its head,
They usually have long flat bodies in which there is a
linear series of sets of reproductive organs. Each set is
called a proglottid (portion of tapeworm that containing
aset of reproductive organ)and usually has at its anterior
and posterior ends zones of muscle weakness, marked
externally by grooves.
No digestive system is present.
Vitellaria : A group of glands that secrete
yolk around the egg in those
invertebrates, such as worms, whose
eggs do not contain yolk.
Cysticercus: a tapeworm larva that consists of a fluid-filled
sac containing an invaginated scolex and is situated in the
tissues of an intermediate host
taenia saginata scolex
Oncosphere: the larva of the tapeworm contained within the external
embryonic envelope and armed with six hooks.
Echinococcus granulosus, a dog tapeworm,
which may be dangerous to humans.
The adult tapeworm lives in intestine of a dog
or other carnivore.

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