Class Trematoda aka Parasitic Flukes


Most trematodes are hemaphroditic,with the exception of schistosomes. Trematodes reproduce using self-fertilization, cross-fertilization, and parthenogenesis. Assexual reproduction occurs in the first intermediate host.

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Female Parts

The female reproductive system is made up of a single ovary connected to a short oviduct and yolk duct. A blind pouch is used as a seminal receptacle and is usually located just off the oviduct. Extending towards the head is a single uterus that can be modified as a vagina.

Male Parts

The male reproductive system consists of a variable number of testes that link up to an eversible cirrus. The cirrus sac houses the prostatic glands and contracts causing the eversion of the cirrus.

Complex Lifecycle of the Trematode

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1) Oval-shaped egg with a covering called an operculum- Eggs are deposited into the water through the feces of the definitive host.

2) Free-swimming larva covered in cilia called miracidium- Swim out of the operculum in freshwater.

3) Sporocyst- Once the miracidium find an appropriate intermediate host (usually a snail because of chemical attractions), it penetrates the host, loses its cilia and develops into a sporocyst (produces more sporocysts assexually or produces a number of rediae).

4) Rediae- produce more rediae assexually or produce tailed rediae called cercariae.

5) Cercariae- cercariae leave the intermediate host and seek out a second intermediate host or a definitive host. They penetrate this host and develop into metacercariae. Definitive hosts are always vertebrates.

6) Metacercariae- juvenile flukes

7) Adult Flukes- grow from the metacercarieae

8) Snail- common intermediate host

Trematode Habitat

Trematodes live in various types of environments, including the digestive system and associated organs. Because they typically have more than one host, trematodes must adapt to changing environmental factors such as temperature and acidity.