Platyhelminthes

Platyhelminthes is made up of two words i.e. Platy which means flat and helminthes which mean worms. Gegenbaur coined the term Platyhelminthes.

General characteristics of phylum Platyhelminthes:

• Most of them are parasites while some are free living in sea water, fresh water or moist soil.
• Their body is dorsoventrally flattened and bilaterally symmetrical and soft leaf-like.
• Organ system grade of body organization.
• Triploblastic
• Body is covered with distinct head with a cellular syncytial one layered partly ciliated epidermis while, in parasitic Trematodes and Cestodes the body is covered with cuticle, the epidermis is absent.
• They possess adhesive structures like hooks, spines, suckers and adhesive secretions for attachment.
• Parthenogenesis and polyembryony usually occur in trematodes and tapeworms.
• Most of them are white or colorless, some derive color from the ingested food while the free living form shows brown, gray, black or brilliant color.
• Endoskeleton and Exoskeleton are absent.
• The digestive system is totally absent in some and some consists of mouth, pharynx and intestine. The anus is absent.
• The circulatory and respiratory system is absent whereas the excretory system is present.
• A simple nervous system is present and sense organs are greatly reduced in parasitic forms.
• Asexual reproduction occurs by fission and sexual by the fusion of gametes.
• Fertilization is internal.
• Eyespots are present as a sensory organ.

Classification:

Platyhelminthes consists of about 10,000 to 15,000 species. They are divided into three classes on the basis of habitat, body shape and mouth position.

Turbellaria:

• They are mostly free-living, aquatic, carnivorous, worms while few are commensals or parasitic.
• Body is dorsoventrally flattened, unsegmented, covered with ciliated, cellular or syncytial epidermis containing secretory cells. The cuticle is absent.
• The life cycle is simple.
• Hooks and suckers usually absent.
• Example: Planaria, Bipalium.

Trematoda:

• The body is covered with thick cuticle layer. There are no epidermis and cilia.
• The body is unsegmented and dorsoventrally flattened.
• They remain attached to their host body by means of suckers and sometimes by hooks.
• The life cycle is complex.
• Examples: Fasciola hepatica.

Cestoda:

• The body is covered with cuticle, ciliated, the epidermis is absent.
• They are found as endoparasitic in the intestine of vertebrates.
• Digestive system and sense organs are absent.
• Lifecycle is complicated and involves two or more hosts.
• Example: Taenia solium, Convoluta.