Platyhelminthes NotesFlatworms and

Tapeworms

What is a flatworm?Phylum Platyhelminthes1. The least complex worms belong to

this phylum. 2. These flatworms are acoelomates

(they have no body cavities) and they have bilateral symmetry.

3. They have thin, solid bodies.

What is a flatworm?4. They live in oceans and freshwater

and also in moist habitats on land. 5. The most well-known members of this

phylum are the parasitic tapeworms and flukes, which cause disease in other animals, including humans.

6. The most commonly studied flatworms are planarians.

A. Nervous System (planarian)1. Most of the nervous system is located in

the head.

2. Some flatworms have a nerve net, and others have the beginnings of a central nervous system.

A. Nervous System (planarian)

3. The planarian nervous system includes two nerve cords that run the length of the body.

4. It also includes eyespots that can detect the presence or absence of light.

A. Nervous System (planarian)

Nervous System5. It also has sensor cells that can detect

chemicals and movement in the water. 6. At the anterior end of the nerve cord is a

small swelling called a ganglion. 7. This receives messages from the

eyespots and sensory pits and communicates to the rest of the body.

B. Reproduction (planarian)1. Planarians are hermaphrodites. 2. During sexual reproduction,

individual planarians exchange sperm, which travel along tubes to reach the eggs.

3. Fertilization occurs internally.

B. Reproduction (planarian)4. The zygotes are released in

capsules into the water, where they hatch into tiny planarians.

5. Asexual reproduction involves regeneration, which is the replacement or regrowth of missing body parts.

C. Feeding/Digestion (planarian)

1. Planarians feed on dead or slow-moving organisms.

2. It extends a tube-like, muscular organ called a pharynx out of its mouth.

3. Enzymes released by this structure begin digesting the food outside the planarian’s body.

4. Food particles are sucked into the digestive tract, where they are broken up.

D. Feeding/Digestion (parasitic flatworms)

1. These flatworms are adapted to obtaining nutrients from inside the bodies of one or two hosts.

2. These parasites have mouthparts with hooks that keep the worm firmly attached inside the host.

3. Because they are surrounded by nutrients, they do not need to move to find food.

E. Tapeworm Bodies1. The body of a

tapeworm is made up of a knob-shaped head called a scolex.

2. The body also has detachable, individual sections called proglottids.

E. Tapeworm Bodies

3. This section contains muscles, nerves, flame cells, and male/female reproductive organs.

E. Tapeworm Bodies4. The flame cells help remove excess

water. 5. Each proglottid can contain up to

100,000 eggs. 6. Some adult tapeworms that live in

animal intestines can be more than 10 meters in length.

F. Fluke Life Cycle1. A fluke is a parasitic flatworm that

spends part of its life in internal organs of a vertebrate (ex. Human, sheep).

2. It feeds on cells, blood, and other fluids from the host.

Fluke Life Cycle3. Flukes have complex life cycles that can

include one, two, or more hosts.

More on Flukes1. Blood flukes from the genus Schistosoma

cause a disease in humans called schistosomiasis.

2. This is common in countries where rice is grown.

3. Farmers have to work in standing water in rice fields during planting and harvesting.

Classes in Phylum Platyhelminthes

1. Turbellaria2. Monogenea3. Cestoda4. Trematoda5. Aspidogastrea