WORMS Platyhelminthes Notes Flatworms and Tapeworms What is

WORMS Platyhelminthes Notes Flatworms and Tapeworms

What is a flatworm? • Phylum Platyhelminthes • The least complex worms belong to this phylum. • These flatworms are acoelomates (which means they have no body cavities) and they have bilateral symmetry. • They have thin, solid bodies. • They live in oceans and freshwater and also in moist habitats on land. • The most well-known members of this phylum are the parasitic tapeworms and flukes, which cause disease in other animals, including humans. • The most commonly studied flatworms are planarians.

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

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

Nervous System • The planarian nervous system includes two nerve cords that run the length of the body. • It also includes eyespots that can detect the presence or absence of light.

Nervous System • It also has sensor cells that can detect chemicals and movement in the water. • At the anterior end of the nerve cord is a small swelling called a ganglion. • This receives messages from the eyespots and sensory pits and communicates to the rest of the body.

B. Reproduction (planarian) • Planarians are hermaphrodites. • During sexual reproduction, individual planarians exchange sperm, which travel along tubes to reach the eggs. • Fertilization occurs internally. • The zygotes are released in capsules into the water, where they hatch into tiny planarians. • Asexual reproduction involves regeneration, which is the replacement or regrowth of missing body parts.

C. Feeding/Digestion (planarian) • Planarians feed on dead or slow-moving organisms. • It extends a tube-like, muscular organ called a pharynx out of its mouth. • Enzymes released by this structure begin digesting the food outside the planarian’s body. • Food particles are sucked into the digestive tract, where they are broken up.

D. Feeding/Digestion (parasitic flatworms) • These flatworms are adapted to obtaining nutrients from inside the bodies of one or two hosts. • These parasites have mouthparts with hooks that keep the worm firmly attached inside the host. • Because they are surrounded by nutrients, they do not need to move to find food.

E. Tapeworm Bodies • The body of a tapeworm is made up of a knob-shaped head called a scolex. • The body also has detachable, individual sections called proglottids. • This section contains muscles, nerves, flame cells, and male/female reproductive organs. • The flame cells help remove excess water. • Each proglottid can contain up to 100, 000 eggs. • Some adult tapeworms that live in animal intestines can be more than 10 meters in length.

F. Fluke Life Cycle • A fluke is a parasitic flatworm that spends part of its life in internal organs of a vertebrate (ex. Human, sheep). • It feeds on cells, blood, and other fluids from the host.

Fluke Life Cycle • Flukes have complex life cycles that can include one, two, or more hosts.

Flukes • Blood flukes from the genus Schistosoma cause a disease in humans called schistosomiasis. • This is common in countries where rice is grown. • Farmers have to work in standing water in rice fields during planting and harvesting.

Classes in Phylum Platyhelminthes • Class Turbellaria: planarians – Convoluta – Macrostomum lineare – Stylochus zebra

Class Turbellaria • Convoluta

Class Turbellaria • Macrostomum lineare

Class Turbellaria • Stylochus zebra

Phylum Platyhelminthes • Class Monogenea – Gyrodactylus elegans – Diplozoan paradoxum

Class Monogenea • Gyrodactylus elegans

Class Monogenea • Diplozoan paradoxum

Class Monogenea • Polystoma

Phylum Platyhelminthes • Class Trematoda – Schistosoma mansoni – Non-schistosomes

Class Trematoda • Schistosoma mansoni

Phylum Platyhelminthes • Class Cestoidea