The
word platyhelminthes was first proposed by Gaugenbaur in 1859 and include all
flatworms. The phylum platyhelimnthes characterized by having soft body,
unsegmented, dorsoventrally flattened worms having a bilateral symmetry with
organ status of organization, acoelomate and triploblastic. Many of these are
parasitic, the free living forms and some are generally aquatic, either marine
or fresh water. Some have lacking digestive system with a single opening- the
mouth, anus is absent while in others digestive system is absent. Platyhelminthes
do not have circulatory, respiratory and skeletal system. Excretion and
osmoregulation is conducted by protonephridia or flame cells where ammonia is
the main excretory waste product. There nervous system is of the simple type,
having a pair of cerebral ganglia and longitudinal nerves connected by
transverse commissures. Sense organs are poorly advanced, present only in the
free living forms. Essentially hermaphrodite with a complex reproductive
system, development is either direct or indirect with one or more larval
stages. Flatworms have ability of regenerate themselves. The phylum comprises almost
13,000 species (by Saxena R, 1957).

The word parasite is come from the two greek words,
“para” which means “beside” and “sitos” also means “food”
(by Barnes 1987). Parasites are those organisms which live on or inside of some
other living organisms, which is known as the host example Ascaris (parasite)
in Man (host) (by Uninotesteam, 2015). Phylum Platyhelminthes is divided into a
four classes, only classes turbellaria include free living forms while the
remain three classes that are trematodes, monogeneans and cestoda are parasitic
forms.

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Class
turbellaria

 Mostly are free living either terrestrial or
aquatic live in both fresh and marine water. Wide, flat leaf-like soft body
covered with a cellular or syncytial epidermis having secretory cells and
rhabdites. Turbellarians are generally
divided into five groups based primarily on variation in the form of the
digestive cavity, a structure that is readily recognizable through the
transparent body wall. The epidermis is partially ciliated at least on the
ventral surface. Mostly hermaphrodite reproduction can be either asexual or
sexual. Asexual reproduction is by fission or by strobilation. They have a simple
life cycle. Development is straight in most turbellarians however some
polyclads produce a free swimming larva. Mostly carnivorous and they have a
great power of regeneration.

Class monogenea

The all life of monogenetic fluke taken
place inside of the hosts especially on a single host regularly on the skin and
gills of fish, they do not live in human host. They
are largely parasitic on marine and freshwater fishes, amphibians, reptiles,
and cephalopod Mollusca. They have hooks
and opisthaptor which is the organ of attachment located on the posterior end
use to holds or grasp on to the fish. Numerous are ectoparasite, but some
of them conquer the body cavities like the mouth, gill chambers and urogenital
tract. Since they are attached to the skin of fast moving host, the monogeneans
have a dorso flattened body. Example: Polystoma, Gyrodactylus.

Class
trematode

Trematode
is a greek name where tremta means a hole, and eidos means form, also called
fluke. They are ecto- or endo- parasitic worms. Unlike the monogenea, the
trematodes need two to four host to complete its life cycle. They have a leaf
like body dorsoventrally flattened, unsegmented, there body sheltered with
thick cuticle. In this class ciliated epidermis with rhabdites absent. Have
oral suckers for holds to the body of the host. Hermaphrodite, life cycle is
either simple or complex with one or more larval stages and have ability of
producing more offspring. Example: Fasciola, Schistosoma.

Class
cestoda

Cestode
is a greek word, kestos means girdle and eidos means form, they are endoparasites
living in the intestine of vertebrates, commonly known as tapeworms. Few of
them are unsegmented species while others consist of a series of linearly arranged
reproductive segments called proglottids (not have true segment). Suckers and
hooks are the organs of attachment to the hosts. There is no mouth or digestive
system, food is absorbed through the cuticle by diffusion. Adults live in the
digestive tract of vertebrates, and larval forms encyst in the flesh of various
vertebrates and invertebrates. Each segment is a hermaphrodite with one or two sets
of male and female reproductive organs. Complicated life cycle usually
involving two or more hosts. In some species the ripe proglottids filled with
eggs, are shed and then develop into adult. In others the fertilized eggs leave
the adult host in the feces and if the eggs are consumed by the intermediate host
the life cycle continues. Tapeworm species that infest human intestines as adults
include Taenia saginata, T. solium, the dwarf tapeworm, Hymenolepsis nana, and
the fish tapeworm, Diphyllobothrium latum (Purves et al 1998).