The word Platyhelminthes is derived from Greek which means “the flat”.
The name Platyhelminthes has been given to the group of flatworms and the body of these animals is soft and dorsoventrally compressed.
General Characteristics of Phylum Platyhelminthes
Grade of organization: They are multicellular organisms and are triploblastic acoelomates i.e. they contain ectoderm and endoderm plus the development of mesoderm.
Shape and Symmetry: The Phylum Platyhelminthes have bilateral symmetry and these organisms have an unsegmented body. The size ranges from a few millimeters (planaria) to several meter long animals(tapeworm).
Habitat: Some members of Phylum Platyhelminthes are free-living, some live in freshwater and some are found in marine water.
Special Body cells:
Flame cells: Flame cells are specialized excretory cells found in the flatworms. These are bulb-like cells. A flame cell has a tuft of cilia, whose beating propels interstitial fluid into the tubular system. The beating of cilia looks like a flickering flame; therefore, these cells are termed flame cells.
Sac-like digestive system: The body space is mostly occupied by a branching sac type digestive system. The digestive system is poorly developed in some species or maybe absent as in the tape-worms.
Organs and Organ systems: They have sense organs at the anterior end but respiratory and circulatory systems are not present.
Nervous system: There is a well developed nervous system but it may be in the form of a network f nerves or ganglia.
Movement: The free-living kinds are motile. They shoe movement by cilia present in their undersides (Planaria). In parasitic types the motion is limited.
Body in Phylum Platyhelminthes: The physique is dorsoventrally flattened and is unsegmented and looks like a leaf.
Mode of nutrition: With few exceptions, the Platyhelminthes are all parasites, largely endoparasites, i.e., reside within their hosts. The most frequent cases include Taenia solium (tapeworm), Fasciola hepatica (liver fluke), and Schistosoma (blood fluke). The parasites are more prevalent in tropics. A few of them cause diseases in people.
Reproduction in Phylum Platyhelminthes
In Platyhelminthes reproduction takes place by asexual as well as sexual means:
Asexual reproduction is by fission where the organism constricts from the centre into two fragments, all fragments will regenerate the lost component.
The sexually reproducing species are hermaphrodite, i.e., both female and male reproductive organs exist in the same individual. Larval kind is occasionally present.
Parasitic platyhelminths undergo quite intricate life cycles, and often include many larval stages in different organisms — the intermediate hosts; these hosts might be invertebrate or vertebrate.
Classification of Phylum Platyhelminthes
The Turbellaria includes chiefly free-living, marine species, though some species reside in freshwater or moist temperate environments. Hookers and suckers are not present.
Examples: Planaria, Otoplana
The cestodes, or tapeworms, are internal parasites, chiefly of vertebrates. Hooks and suckers are found for sucking digested substance from host species therefore digestive tract are also absent.
The trematodes, or flukes, are parasites of mollusks and a variety of different organisms, such as humans. They also have suckers as they are parasites and obtain food by sucking substances from the host.
Example: Liver fluke
Examples of Phylum Platyhelminthes
Dugesia (Planaria): A free-living flatworm with cillias.
Fasciola (Liver luke): An endoparasite in sheep and sometimes in humans. It lives in the bile duct of its hosts.
Taenia (Tapeworm): An endoparasite of humans, cattle, and pig.