Classification: Phylum Platyhelminthes - The Tribe of Flatworms.
Other features: bilaterally symmetrical, triploblastic acoelomate, gut with one opening.
- Dorso-ventrally flattened.
- Flame cells are the excretory structures.
Bilateral Symmetry: can be divided in half such that each side is a mirror image of the other.
Free-living Flatworm: Dugesia - the freshwater planarian. Study the diagram of its external features.
Parasitic Flatworm: Fasciola hepatica - the Liver Fluke. It is an obligate parasite.
- A parasite is a living organism that lives with and feeds off another, the host, causing it harm.
An 'obligate' parasite does not have a free-living feeding stage.
- Diagrams: external structure of liver fluke, reproductive system of liver fluke.
- Outline of Life Cycle
Sexually mature adults »» Fertilised eggs »» Miracidium (first larva) »» Sporocyst (second larva) »» Redia (third larva) »» Secondary Redia »» Cercaria (fourth larva) »» Encysted Cercaria »» Sexually immature adults.
Study the 'shapes' of the larval stages.
The Miracidium and encysted cercaria are external free-living but non-feeding stages.
The sexually immature adults feed in the liver tissue, the sexually mature adult live in the bile ducts.
Sporocyst, redia and cercaria are produced in the secondary host.
- A larva is a sexually immature pre-adult animal.
Some biologists restrict the term larva to those juvenile animals that are distinctively different to the adult and use
the term nymph for sexually immature juveniles similar in structure to the adult.
- Sheep and cattle are the common 'primary hosts'. The mature adult parasites reproduce sexually in them.
Limnaea, the dwarf pond snail, is the secondary host. The larvae reproduce sexually there.
- Effect of Liver Fluke on Primary Host.
Failure to thrive, reduced fertility, reduced milk yield, extensive liver damage, paralysis, death.
- Liver Fluke Control
Eliminate the secondary host: spray molluscicides, drain the land, use ducks and geese to feed on them.
Kill the immature and mature adult flukes: dose the primary host.
Eliminate contact with encysted cercaria: fence off wet areas, remove livestock to dry ground.
Prevent the fertilised eggs from hatching: lime the land.
- Liver Fluke Adaptations
- Sexually mature adults live in the bile ducts: early easy exit of fertilised eggs from host.
- Suckers: attach sexually mature adults to bile duct wall and so can remain in a favourable site.
- Cuticle of Adults: protection of adults from the host's acquired immune system.
- Hermaphroditism: cross-fertilisation if two or more flukes present, sexual reproduction is possible even if only one fluke is present, increased population as each individual produces fertilised eggs.
- 20,000 fertilised eggs per day per fluke: increases the chances of secondary host being found.
- Shell of fertilised egg: protection from digestion in primary host.
- Miracidia have cilia and are chemotactic: increased chance of finding a secondary host.
- Spine of Miracidia: easy entry of the miracidia into the secondary host.
- Use of Secondary Host: to increase parasite population so increasing chances of locating a primary host.
Ten thousand cercaria are produced for each miracidium that enters a snail.
- Encystment of Cercaria: survive longer so greater chance of being ingested by a primary host.
A more in-depth look at Phylum Platyhelminthes including photographs.
Encyclopedia.com is a good on-line resource and its offering on Platyhelminthes is reasonable with a section on anatomy which is well worth a look.