(Gr., Platys- flat + helminth- worm)
Aristotle mentioned tapeworms. 
Gegenbaur (1859AD) placed flatworms in a separate group and gave the term Platyhelminthes
Phylum Platyhelminthes includes about 13,000 species.

Examples of Phylum Platyhelminthes are very very important for Medical and Applied Science Entrance Exams of Nepal. So memorize the below noted VVI Examples.
  • Dugesia (Planaria)
  • Fasciola hepatica (Liver fluke)
  • Schistosoma (Blood fluke)
  • Paragonimus westermani (Lung fluke)
  • Polystoma (Bladder Fluke)
  • Taenia solium (Pork tapeworm)
  • Taenia saginata (Beef tapeworm)
  • Echinococcus granulosus (dog tapeworm)
  • Diphylobothrium latum (Fish tapeworm)
  • Hymenolepsis nana (Dwarf tapeworm)

General characters of Phylum Platyhelminthes

1. Mostly parasites (ecto or endoparasites), few free-living (Planaria), and rarely commensals.
2. First animals having a tissue-organ grade of body organization i.e., cells aggregate to form tissues and tissues made organs and definite polarity of anterior (head) and posterior (tail) ends (Cephalization), triploblastic and bilaterally symmetrical.
3. Dorso-ventrally flattened leaf or ribbon or tape-like body, appendages are absent in flatworms, usually well defined ventral surface bearing mouth and gonopore
4. Epidermis cellular or syncytial, frequently ciliated. Absent in some.
If flatworms are free-living (Turbellarians), body with ciliated epidermis. If flatworms are parasitic (Trematodes and Cestodes), the body with a cuticle is called a tegument.
5. Acoelomate (Space between various organs is filled with special mesodermal tissue (ground tissue), the mesenchyme or parenchyma.)
6. Microscopic to 10-15 meters long.
7. Unsegmented, if present pseudosegmented (class Cestoda).
8. Adhesive structures like suckers, hooks, and spines are present in parasitic forms.
9. Digestive system is branched and incomplete without an anus. But Digestive System or Alimentary Canal is completely absent in Cestoda.
10. Aerobic respiration (Free-living forms), anaerobic respiration (parasitic forms), gaseous exchange through the general body surface.
11. Skeletal, respiratory, and circulatory systems are absent.
12. Excretory system consists of lateral canals and protonephridia (flame cells or solenocytes). They are ammonotelic.
13. Nervous system is primitive and ladder-like. Comprises a pair of anterior ganglia with one to three pairs of longitudinal nerve cords connected by transverse nerves.
14. Simple sense organs. Eyespots or photoreceptors in free-living forms.
15. Mostly hermaphrodite, reproduce sexually in parasitic forms but freshwater turbellarians reproduce asexually by fission. Regeneration power is also in some flatworms (e.g., Planaria). In flukes, presence of a pair of testes and a single ovary while in tapeworm many testes and single ovary present.
16. Fertilization internal, cross in trematodes or self in cestodes. Cleavage- spiral
17. Life cycle is complicated involving many larvae and hosts. Development is direct or indirect. Usually indirect in endoparasites.

Note: Dugesia was formerly called Planaria.

Advancement over coelenterates

Triploblastic (mesoderm instead of mesoglea), organ-system level of body organization, well developed nervous and excretory systems, gonoducts and copulatory organs, definite head and tail, bilateral symmetry, more complex and efficient gastro-vascular cavity, well developed sensory organs, separate vitelline glands for the production of the yolk as the ovaries produce only ova.

Unique features

Parenchymatous tissue, flame cells for excretion and osmoregulation, ladder-like nervous system, self-fertilization in tapeworms, well-marked regeneration in Dugesia or Planaria.

Classification of Phylum Platyhelminthes

On the basis of habitat, presence or absence of segments, and alimentary canal and cilia in the epidermis, phylum Platyhelminthes is divided into three classes- Turbellaria, Trematoda, and Cestoda.

S.N Turbellaria (Planarians) (L; turbella=a stirring) Trematoda (Flukes) (Gr; tremta = hole; eidos = form) Cestoda (Tapeworms) (Gr; kestos = girdle; eidos= form)
1. Mostly free-living (non-parasitic), terrestrial, freshwater, or marine, some are commensals Ecto and endoparasites Exclusively endoparasites
2. Body unsegmented and leaf-like Body unsegmented, dorsoventrally flattened (leaf-like) Body pseudosegmented, ribbon-like
3. The alimentary canal is incomplete without an anus. The alimentary canal is branched without the anus. The alimentary canal is absent. (Predigested nutrients from the host’s gut through their general body surface.)
4. Suckers or hooks are absent. Suckers are present. Suckers and hooks are present.
5. Epidermis ciliated, cellular, or syncytial The body is covered by a cuticle (tegument). The body is covered by a cuticle (tegument).
6. Development is usually direct, with spiral cleavage, no larval forms Development direct in ectoparasites and indirect in endoparasites. Larval forms (MSRCM) Development is indirect. Larval forms- onchosphere, hexacanth (6 hooked larvae), and cysticercus (bladder worm)
7. Mostly hermaphrodite Usually hermaphrodite or unisexual Every segment is hermaphrodite.
8. Examples: Dugesia (Planaria), Bipalium, Convoluta, Mesostoma Examples:
Fasciola hepatica (Liver fluke)
Schistosoma (Blood fluke)
Paragonimus westermani (Lung fluke)
Polystoma (Bladder Fluke)
Taenia solium (Pork tapeworm)
Taenia saginata (Beef tapeworm)
Echinococcus granulosus (Dog tapeworm)
Diphylobothrium latum (Fish tapeworm)
Hymenolepsis nana (Dwarf tapeworm)

Dugesia (Planaria)

· Free-living turbellarian found in the freshwater ponds, lakes, streams, and shallow rivers (aquatic).
· They are gregarious (live in groups) and nocturnal, cannibalistic
· Black to brown in colour and bilateral symmetry.
· The body has a triangular anterior head with two lateral projections called auricles and blacklight sensitive two dorsal eyes. 1.5 cm in length
· Mouth opens on the mid-ventral surface near the middle of the animal.
· Pharynx is a tubular structure that can be everted beyond the mouth.
· Genital pore lies at the posterior region of the pharynx. Three forked intestine
· The flattened ventral surface of the body is covered with cilia for locomotion and Rhabdites (Mucus secreting cells on epidermis), omnivorous (dead organisms, microscopic plants as well as small molluscs and crustaceans)
· Remarkable power of regeneration
· If an individual is cut transversely into two parts, the anterior part will regenerate a new tail and a posterior part will develop a new head. It involves 2 processes.
Epimorphosis- forming of missing part and morphollaxis- fitting of original part with regenerated parts.
· It shows lateral polarity. If the head is cut longitudinally into 2 or several parts along a middle line, each part grows into a complete head thus forming a several-headed Planaria. This is called heteromorphism.
· A section cut from the middle of the body will regenerate both ahead at the anterior end and a tail at the posterior end.
· Pieces of one Planarian can also be grafted onto another.
· Hermaphrodite, multiplies asexually by binary fission.
· Direct development and spiral cleavage.

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