Planarians

Here are ten basic facts about Platyhelminthes:

1. They have three layers of cells.

2. They are bilateral in symmetry.

3. They lack organs for respiration and therefore the cells allow gases to to diffuse out in

from their surroundings.

4. They use sexual and asexual reproduction.

5. They have a partial digestive system.

6. They have an excretory system.

7. Flatworms are predominantly hermaphraditic.

8. They have a muscular system.

9. Flatworms are composed of free living forms and parasites.

10. They are motile.

Trematodes

There are somwhere around 9,000 species of trematodes. Most of them have a leaf type shape. All of them are parasitic and often the adults parasitize vertebrates. On their body is a certain kind of epidermal arrangement by which the main cell bodies are deep and seperated from the cytoplasm that is next to a layer of muscle near the exterior. This particular epidermal arrangement is called tegument. Some species of trematoda have a very strong pharynx in theanterior sucker. It is used to suck in cells, blood, and fluids.Trematodes can be compared to turbellarians in that they have a well developed alimentarycanal. Their muscular, excretory, and reproductive systems are also relatively complete. Also, the way that trematoda have adapted has caused a loss in quite a bit of the digestive system. The food that the trematoda needs to live off of has already been digested for them.Most of the trematoda class have a very complex life cycle. The larval stages parasitize many or a single species that are different from the adult hosts. Trematoda are mostly endoparasites and include several parasites that have a huge effect on the human population. The human liver flukes and blood flukes that cause schistosomiasis are good examples of how they could affect the human population.Trematodes require an intermediate host in their life cycle with vertebrates being the definitive host. Larval stages may occur in either invertebrate or vertebrate hosts. There are three groups of trematodes:· Monogenea, which typically are external parasites of fish with direct life cycles

· Aspidogastrea, these are endoparasites with the entire ventral surface as an adhesive organ

· Digenea, these are endoparasites with simpler adhesive organs and life cycles involving one or more intermediate hosts (indirect life-cycle). Most Digenean trematodes inhabit the alimentary canal of vertebrates and many of the associated organs, such as the liver, bile duct, gall bladder, lungs, bladder and ureter. These organs are rich in cavities containing food such as blood, mucus, bile and intestinal debris. The Digenean trematodes have a complex life cycle, with rare exceptions, always involve a mollusk host. There may be six larval stages – the miracidium, sporocyst, redia, cercaria, mesocercaria (rare) and the metacercaria (the majority have 4 or 5 stages).

Trematode eggs have a smooth hard shell and the majority of them are operculate.

Cestoda

All of the class cestoda are endoparasitic tapeworms. This class consists of around 5,000 species

and most of these worms need two hosts to live. The adult worms require a vertebrate. Also,

there are a number of species of tapeworm that inhabit humans. Tapeworms are like trematodes

and monogeneans in that they are all accustomed to the parasitic lifestyle. They are without

cilia but have an outer tegument. The scolex is a knob-shaped head which contains hooks and

suckers which help to attach onto a host. Tapeworms will grow by making body sections called

proglottids which follow right behind the scolex, which would mean that the oldest proglottids

are at the rear end. These worms can grow up to 12 meters.

Tapeworms are without mouths, digestive tracts, and sense organs. The way they obtain their

nutrients is by absorbing them through their folded teguments. These folds in the tegument allow

more surface area for absorbtion. The tapeworm excretory system rids the proglottid of wastes.

In every proglottid there are both male and female reproductive systems. Although self-

fertilization can occur, cross-fertilization is typical. The eggs from the tapeworm develop into

embryos which have a hard outer shell. The eggs do not hatch until they have been consumed by

a suitable intermediate host.

Cestoda are common in people as adults. The larval stages are mostly found in swine, but have

also been found in mammals and birds.

Major Attributes:

Endoparasitic.

Body is covered by tegument.

Anterior end is a scolex.

Body segments called proglottids.

Hermaphroditic.

Turbellaria

The class turbellaria is made up mostly of marine flatworms that are free living. One of the most

well known turbellarian species is the planarian. It is not marine, but can be found in ponds and

streams. There are twelve orders and more than 4,500 species of turbellaria. The five general

groups of turbellaria are primarily based on the differences of how the digestive cavity is formed.

The acoels are the most primitive turbellarians and lack a digestive cavity. Acoelomates make

up most of the planarians. They are three-layered and have an outside ectoderm layer, a middle

mesoderm layer, and an inside endoderm layer.

Planarians are able to sense the intensity and direction of light with two anterior eyespots tha

thave photosensitive cells. There are two anterior clusters of nerve cells called ganglia that form

a simple brain. The ganglia attain information from sensory cells and send impulses to the rest of the body. Also, planarians seem to have the ability to learn. Some learned responses are able to be stored chemically.

Some planarians can be carnivorous although most are scavengers. Their feeding mechanism is

rather simple. It extends to a muscular tube, the pharynx, out of its mouth. The pharynx leads

into a cavity with only one opening that branches throughout the entire body. This type of cavity

is called a gastrovascular cavity. This cavity is used for digestion and to transport food to the

entire body.