Phylum Platyhelminthes - Cestodes Quiz

Phylum Platyhelminthes - Classes

• Turbellaria: Free-living lifestyle
• Trematoda: Parasitic lifestyle, internal parasites
• Monogenea: Parasitic lifestyle, external parasites
• Cestoda: Parasitic lifestyle, internal parasites

Cestodes - Parasitic Strategies

• Strobila: The body of the worm, composed of a chain of proglottids, from behind the scolex.
• Scolex: A specialized structure for attachment to the host's intestinal epithelium. It's an anchoring organ.
• Proglottid: A reproductive section of the strobila. Each contains male and female reproductive systems. It has no digestive system but has microtriches similar to microvilli to increase the surface area for nutrient absorption.

Cestodes - Life Cycle

• Gravid proglottids: Shed in feces and are the source of shelled larvae.
• Shelled larvae (oncospheres): Develop from gravid proglottids, move through intermediate host, and grow into adult tapeworms.
• Cyst (cysticercus): A cyst develops in the intermediate host muscle (bladder worm).
• Definitive host ingestion: When definitive host ingests the cyst, the scolex invaginates from the cyst wall, attaches to the intestine, and the worm matures.
• Immense reproductive capacity: Individuals of large species can produce up to 1,000,000 eggs per day.
• Maturation: Adult worms grow inside the intestines/intermediate host of the host.
• Gamete exchange: Gamete exchange may occur between proglottids of different worms, or within the same worm.

Trematodes - Parasitic Strategies

• Life Cycle Stages (Trematodes): Typically 5 developmental stages

• The Life cycle usually includes 2 or more intermediate hosts

• The species specificity for the first intermediate host is usually high

• Specificity (Trematodes): Specificity for other intermediate and definitive hosts is low.

• Adulthood (Trematodes): Adulthood is only achieved in a definitive vertebrate host

• Asexual Replication (Trematodes): The life cycle includes asexual replication of developmental stages, mostly in the first intermediate host.

• Egg release: Eggs are usually released in feces; eggs must reach water to hatch.

• Miracidium: Free-living ciliated larval stage, penetrates the epidermis of the first intermediate host (usually a snail).

• Penetration gland: Specialized gland in the miracidium that penetrates the host.

• Sporocyst: The miracidium develops into a sporocyst when inside the intermediate host.

• Sporocysts (Trematodes): Sporocysts reproduce asexually, producing sporocysts OR rediae, thus yielding many progeny from a single egg.

• Embryo (Trematodes): From germ balls, (developing redia or sporocysts).

• Rediae: A sac-like structure that contains developing cercariae. Rediae also reproduce asexually, producing more cercariae, further increasing the progeny from the original egg.

• Cercariae: Free swimming larvae that emerge from the snail host. Their task is to penetrate the second intermediate host - often a fish.

• Metacercariae: Cercariae encyst in the tissues of the second intermediate host, forming metacercariae. These are eaten by the definitive host.

• Definitive Host (Trematodes): The metacercariae are eaten by the definitive host, the adult worm forms within, and maturation takes place.

• Example (Trematodes): Clonorchis sinensis - Chinese liver fluke.