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