3. Phylum Platyhelminthes
• Platyhelminthes (plat”e-hel-min’thez) (Gr. platys, flat + helmins, worm)
• Over 34,000 species
• Size from 1 mm or less to 25 m.
• Flattened dorsoventrally, triploblastic, acoelomate, bilaterally symmetrical.
• Parenchyma: mesoderm tissues include a loose tissue.
• Unsegmented bodies (members of the class Cestoidea are strobilated)
• Gut: Incomplete usually present but absent in Cestoidea.
• Cephalized: an anterior cerebral ganglion and usually longitudinal nerve cords
• Excretory/osmoregulatory structures: Protonephridia
• Monoecious: complex reproductive systems.
• Nervous system: pair of anterior ganglia located in the mesenchyme.
5. Class Turbellaria
• Named for the turbulence that their beating cilia create in the water.
• Free-living bottom dwellers in freshwater and marine environments.
• Over 3,000 species
• Are predators and scavengers.
• Digestion and Nutrition: From Assignment
• Exchanges with the Environment: From Assignment
• Nervous and Sensory Functions: From Assignment
• Reproduction and Development: From Assignment
6. Class Trematoda
• Trematoda: (Gr. trematodes, perforated form) are collectively called flukes.
• Flukes are flat and oval to elongate, and range from less than 1 mm to 6 cm
in length.
• Parasites
• The digestive tract: mouth, a muscular pumping pharynx and branched
pouches called cecae.
• Tegument: an outer layer of epidermis.
• Two Subclasses: Subclass Aspidogastrea and subclass Digenea
• Of the subclasses of Trematoda, Aspidogastrea are small and poorly known
groups, but Digenea (Gr. dis, double, + genos, race) is a large group with
many species of medical and economic importance.
9. Subclass Digenea (Gr. di, two + genea, birth)
• Digenea: Require at least two different hosts to complete their life cycles,
• Intermediate host: First host (mollusc) and In some species a second, and
sometimes even a third, intermediate host may intervenes. Harbor immature
stages of animal
• Definitive host: The host in which sexual reproduction occurs, sometimes called
the final host (a vertebrate). Harbor mature stages of animal.
• Adhesive organs: two large suckers.
• Oral sucker: Anterior sucker, surrounds the mouth.
• Ventral sucker (acetabulum): located below the oral sucker on the middle
portion of the body.
• General Life Cycle: An adult -> egg -> miracidium -> sporocyst -> redia -> cercaria
-> metacercaria stages
• What is polyembryony? Why it is important?
10.
11. Some Important Trematode Parasites of Humans
• The Chinese liver fluke Clonorchis sinensis is a common parasite of humans in
Asia, where more than 30 million people are infected. The adult lives in the bile
ducts of the liver, where it feeds on epithelial tissue and blood
• The metacercaria develops into an adult in a human who eats raw or poorly
cooked fish, a delicacy in Asian countries and gaining in popularity in the Western
world (e.g., sushi, sashimi, ceviche).
• Fasciola hepatica is called the sheep liver fluke (because it is common in sheep-
raising areas and uses sheep or humans as its definitive host. The adults live in
the bile duct of the liver.
• Humans may become infected with Fasciola hepatica by eating a freshwater plant
called watercress that contains the encysted metacercaria.
• Schistosomes are blood flukes infect more than 200 million people throughout
the world.
