Phylum : Platyhelminthes (Flat worms), its classification and characteristics

• multicellular, triploblastic, bilaterally symmetrical.
• first animals to have organ-system grade of body organization.
• body is dorsoventrally flattened so called flat worms.
• size varies from less than mm and more than 10-15 metres.
• mostly parasitic, few are free living and are found in water or damp soil.
• parasitic forms have hooks and suckers to stick to the host body.
• blind sac body plan.
• alimentary canal is absent or incomplete without anus.
• excretion occurs by flame cells, paired protonephridia or bulbs.
• mostly hermaphrodite.
• free living forms have photoreceptors, chemoreceptors and tango receptors.
• respiration occurs through general body surface.
• in many, fresh water asexual reproduction occurs by fission.
• life cycle is simple or complex with alternation of one or more hosts.
• development is direct in free living forms and indirect in parasitic forms.
• fertilization is internal.
• parthenogenesis and polyembryony usually occurs in Trematodes and tapeworms.

Classification

Flatworms are classified in three classes on the basis of habit. They are:

Class 1: Turbellaria

• free living flatworms found in sea or fresh water.
• commonly known as eddy worms.
• cuticle is absent.
• body is covered by ciliated epidermis.
• hooks and suckers are absent.
• tangoreceptors, chemoreceptors, photoreceptors are present.
• mostly hermaphrodite, some reproduce asexually.
• life cycle is simple.
• examples: Planaria (Dugesia), Convoluta, Otoplana, Bipalium.

 Image source: microbiologynotes

Class 2: Trematoda

• ecto or endo parasites.
• body is flat, unsegmented, commonly known as fluke.
• usually rounded or leaf-like.
• epidermis and cilia absent.
• suckers are present.
• life cycle is complex.
• usually hermaphrodite except few families of order Digenia.
• examples: Fasciola hepatica (liver fluke), Schistosome (blood fluke), Monocotyle, Cotylapsis

Image source: slideplayer

Class 3: Cestoda

• exclusively endoparasitic.
• flat, pseudo segmented, elongated and ribbon like.
• hooks and suckers are present.
• body covered with cuticle, ciliated epidermis absent.
• digestive system and sense organs are absent.
• pair of ganglia and two lateral longitudinal nerve cords present.
• life cycle is complex, involves two or more hosts.
• examples: Taenia solium (pork tapeworm), Echinococcus (dog tapeworm)

References:

i) https://animaldiversity.org/accounts/Platyhelminthes/classification/

ii) https://www.microscopemaster.com/flatworms.html

Phylum : Platyhelminthes (Flat worms), its classification and characteristics