Phylum platyhelminthes are the only group of animals having no extinct form and having successful genetic continuity of their population. The word ‘parasite’ is often added after ‘Helminth’ but vast animals belonging to this group are free-living. This group is further privileged to have the organs in the body for the first time and also are the first animals to have three germinal layers-ectoderm, mesoderm (mesenchyme) and endoderm. The group is divided into two phyla – Phylum Platyhelminthes and Phylum Nemathelminthes. The latter is included under Phylum Aschelminthes now.
Platyhelminthes are acoelomate, bilaterally symmetrical and dorsoventrally flattened. Their leaf shaped or ribbon shaped body has a high surface area to volume ratio with all regions of the body close to the surface. This allows for the supply of oxygen and the escape of metabolites for diffusion. Phylum Platyhelminthes is divided into 3 classes-Turbellaria, Trematoda and Cestoda. Turbellaria includes free-living flatworms (planarians). Trematodes comprise flukes which are ectoparasite and endoparasite and cestodes are completely endoparasite. Cestodes show best parasitic adaptations. Both trematodes and cestodes are responsible for various diseases in man and his animals.
DIAGNOSTIC CHARACTERS OF PHYLUM PLATYHELMINTHES
- Organ grade of organization.
- Acoelomate bilaterial without definitive anus.
- Triploblastic (ectoderm, endoderm and mesoderm).
- Mesoderm gives rise to various organs such as excretory, reproductive, etc.
- Skeletal, circulatory and respiratory systems absent.
- Free-living, or parasitic causing various diseases.
CLASSIFICATION OF PHYLUM PLATYHELMINTHES
CLASS I TURBELLARIA
- Mostly free-living.
- Body is covered with epidermis.
- Mouth ventral.
Order 1 Acoela
- Intestine absent. Mouth directly opens into mesenchyme.
- Small marine turbellarians.
Example :- Convolula, Haplodiscus.
Order 2 Rhabdocoela
- Intestine sac-like.
- Gonads, Yolk gland and protonephridia present.
Example :- Stenostomum. Catenula, Microstomum.
Order 3 Alloeocoela
- Intestine diverticulated.
- Protonephridia paired.
Example :- Hofstenia, Plagiostomum.
Order 4 Tricladida
Order 4 Polycladida
- Intestine contains many branches.
- Male and female gonopores separate.
Example :- Stylochus, Thysanozoon, Leptoplana.
CLASS II TREMATODA
- Commonly called as flukes.
- Body dorsoventrally flattened and covered with tegument. (Old name cuticle no longer in use).
- Ectoparasites or endoparasites. No free-living forms.
- Life cycle monogenetic or digenetic with or without intermediate hosts.
- Suckers for adhesion.
Order 1 Monogenea
- Ecto or endoparasites or commensals.
- Attachment organ is opisthapter.
- No intermediate host in life cycle.
Sub-order 1 Monopisthocotylea
- Single opisthator.
- Oral sucker absent or poorly developed.
Example :- Gyrodactylus, Dactylogyrus.
Sub-order 2 Polyopisthocotylea
- Opisthaptor contains many suckers.
- Ecto or endoparasites.
Example :- Diplozoon, Polystoma.
Order 2 Digenea
- Endoparasitic flukes.
- Life cycle is completed in two or more hosts hence digenetic.
- Oral and ventral suckers present. Single excretory pore.
Family 1 Paramphistomidae
- Acetabulum on posterior end.
- Parasites of vertebrates.
Example :- Paramphistomum, Gastrothylax, Cotylophoron
Family 2 Opisthorchidae
- Found in bile ducts of fish-eating mammals including man.
- Eggs bulb-shaped.
Example :- Opisthorchis or Clonorchis.
Family 3 Fasciolidae
- Commonly called as liver flukes.
- Internal organs branched.
Example :- Fasciola hepatica, F. gigantica, F. magna, F. americana.
Family 4 Schistosomatidae
- Commonly called as blood flukes.
- Female is carried by male in its gynecophoric canal. Male female separate.
Example :- Schistosoma mansoni, S. japonicum and S. haematobium.
CLASS III CESTODA
- Commonly called as tape worms.
- Body ribbon or tape like.
- Exclusively endoparasites.
- Body covered by tegument.
- Intestine completely absent.
- Body segmented and divided into scolex, neck and segments.
- Vagina has evolved in females.
Sub-class A :- Cestodaria
- Undivided cestodes (Monozoic)
- Larva has 10 hooks.
- Scolex absent
Order 1 Amphilinidea
- Male pore and vagina posterior. No suckers.
- Intestinal parasites of fishes.
Example :- Amphilina
Order 2 Gyrocotylidea
- Anterior sucker present.
- Uterus straight.
- Endoparasites of fishes.
Example :- Gyrocotyle
Sub-class B :- Eucestoda
- Segmented Tapeworms (Polyzoic).
- Larva has 6 hooks (hexacanth)
Order 1 Tetraphyllidea
- Scolex with 4 ear-like bothridia.
- Intestinal parasites of elasmobranch fishes.
Example :- Phyllobothrium.
Order 2 Pseudophyllidea
- Scolex with 2 bothria.
- Intestinal parasites of fishes and vertebrates.
Example :- Dibothriocephalus latum.
Order 3 Protocephala
- Scolex with 4 cup-shaped suckers.
- Intestinal parasites of fish and amphibians.
Example :- Protocephalus.
Order 4 Cyclophyllidea or Taenioidea