Phylum-Platyhelminthes: Definition, General Characters, Identifying Characters, Examples

In 1859 the scientist Gegenbaur proposed the name phylum platyhelminthes. Originally the animals of this group were placed in the taxon vermis by Linnaeus in 1735 along with other worms. In 1876 Minot recognized Platyhelminthes as a separate phylum. The animals of this group are commonly known as flatworms or tapeworms.

The term Platyhelminthes is coming from the Greek words platy and helminthes,

” Platy = flat and Helminthes = worm “

The phylum Platyhelminthes includes the parasitic and free-living flatworms having a bilaterally symmetrical body. The animals do not have coelom or body cavity in their body.

 

Definition:

Platyhelminthes are the group of organisms having bilaterally symmetrical triploblastic acoelomate body organization containing protonephridia as an excretory system without anus, circulatory and respiratory system.

General characters:

1. Bilaterally symmetrical triploblastic body.
2. Body worm-like or tape-like and dorsoventrally flattened.
3. Coelom or body cavity is absent in the body.
4. The Platyhelminthes are generally endoparasitic and they have hooks and suckers to get attached to the host.
5. Usually an incomplete gut is present without an anal opening.
6. Their excretory organs are flame cells.
7. The flame cells are branched and connected to the excretory duct.
8. The outer skin contains cuticles or cilia.
9. Exoskeleton or endoskeleton structures are absent.
10. The body does not have a specific circulatory system and the respiratory system.
11. In the phylum Platyhelminthes, most of the animals are hermaphrodites. The reproductive system is complex and advanced.
12. Multiple larval stages are present in the life cycle.

Fig: Planaria sp.

Identifying characters of Platyhelminthes:

1. Body structure: Body dorsoventrally flattened and bilaterally symmetrical.
2. The Coelom: The coelom is absent in the body. Mesenchymal tissues are present between the organs.
3. Muscle layers: Muscles are arranged in layers and clusters.
4. Excretory system: Their excretory system is comprised of protonephridia and excretory organs are flame cells.
5. Hooks and suckers: They have hooks and suckers to get attached to the host.

Fig: Fasciola sp.

Examples:

There are about 20000 living species in this phylum. Some common animals of phylum Platyhelminthes are Planaria, Hoploplana, Polystoma, Fasciola, Geoplana, Anaperus, Taenia, Echinococcus, Paragonium, Convoluta, Schistosoma, etc. 

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