PLATYHELMINTHES

Instructor: Almonther I. Alhamedi

The Islamic University of GazaDepartment of Biology

E-mail : . .mhamedi@iugaza edu psWeb page: http:// . . . /site iugaza edu ps mhamedi

GENERAL CHARACTERISTICS 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.

GENERAL CHARACTERISTICS 1. Acoelomate Bilateria developed the basic bilateral plan

of organization that has been widely exploited in the animal kingdom.

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.

GENERAL CHARACTERISTICS

GENERAL CHARACTERISTICS 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.

CLASS TURBELLARIA 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.

GENERAL CHARACTERISTICS Nutrition

• 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

Senses• 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, Digestivetract 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.

CLASS TREMATODA 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 vertebrates.

Larval stages of some medically important species include miracidium, redia, cercaria, and metacercaria.

Most trematodes are endoparasites.

SCHISTOSOMA: BLOOD FLUKES 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 female

Three species account for most of the schistosomiasis in humans:

S. mansoni, which lives primarily in venules draining the large intestine.

S. japonicum, which is found mostly in venules of the small intestine.

S. haematobium, which lives in venules of the urinary bladder.

LIFE CYCLE 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

A

B

C

Schistosomajaponicum

Schistosoma

mansoni

Schistosomahaematobiu

m

Schistosomaintercalatu

m

Schistosoma

mekongi

FASCIOLA HEPATICA 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 years.

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.

CLASS CESTODA 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, scolex. 

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.