The representatives of the phylum Platyhelminthes are commonly known as the flatworms or tapeworms. The word ‘Platyhelminthes’ is derived from the Greek word, ‘platy’ meaning flat and ‘helminth’ meaning worm. They are simple soft-bodied, bilaterian, unsegmented invertebrate animals. The Phylum Platyhelminthes makes up the 4th largest phylum among the animal kingdom. But among the acoelomate organisms, the phylum Platyhelminthes constitutes the largest phylum with more than about 20,000 known species throughout the world. Among them, around 80% live as parasitic life on humans and other animals and few are free-living.
The parasitic forms cause some trouble to the host animals, feed on host`s tissues and make certain diseases such as Schistosomiasis, or snail fever, Taeniasis, etc. while the free-living flatworms are scavengers or predators. Generally, free-living species live in water and some in shaded, humid terrestrial ecosystems, such as leaf litter. The members of this phylum have diverse sizes which range from microscope to 3 feet long.
General Characteristics of Platyhelminthes
Classification of Platyhelminthes
The Phylum Platyhelminthes is classified into the following classes:
Class-1: Turbellaria (Latin, turbella = a string)
Class Turbellaria contains about 3,000 species, of which, majority of these species live in marine environments, some are found in freshwater environments and few live in tropical terrestrial and moist temperate environments.
Examples: Pseudobiceros bedfordi, Pseudoceros dimidiatus
Class Turbellaria includes the following orders:
It is one of the largest groups of flatworms and most of the members live in the aquatic environment and they lead exclusively parasitic life (ectoparasites) of aquatic vertebrates.
Example: Diplozoon paradoxum, Gyrodactylus adspersi
Class-3: Cestoda (Greek, kestos = girdle, eidos = form)
Class Cestoda is divided into the following two subclasses:
Example: Gyrocotyle rugosa, Amphilina foliacea
The subclass Cestodaria consists of the following orders:
Example: Taenia solium, Diphyllobothrium latum
Class-4:Trematoda (Greek, trema = hole, eidos = form)
The members of this class are commonly known as flukes. There are more than about 20,000 known species of class Trematoda which are all parasitic in nature.
The class Trematoda is divided into the following two subclasses:
Example: Multicotyle purvisi, Lobatostoma manteri
Subclass-2: Digenea (Greek, Dis – double, Genos – race)
Example: Lecithochirium sp., Pycnoporus heteroporus
Subclass Digenea includes several orders that include:
The members of the phylum Platyhelminthes are known as flatworms which can adapt to an enormous variety of habitats. Flatworms live as endoparasites in the intestines and digestive tracts of the human body. Several species such as Cestodes (tapeworms) and digeneans (flukes) can cause diseases in livestock and human beings. In many countries, serious losses of stocks in fish farms are occurred by the monogeneans. The genus Schistosoma bores skin of human and causes diseases, known as Schistosomiasis, or bilharzia or snail fever. Besides these, many species of Platyhelminthes play an important role for healthy streams, lakes, and ponds. They also offer food for animals such as dragonflies, when they are young.
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