Classification of the phylum Platyhelminthes

The phylum Platyhelminthes include free-living as well as parasitic animals. The free-living flatworms live in springs, sands, streams or in bodies of salt water. On the other hand, parasitic form of each species has its own particular habitat. They are widely distributed among human beings and other vertebrates.

Definition: A group of flat or ribbon like animals having bilateral symmetry, without any segmentation, coelom, blood vascular system and anus and whose internal organs are surrounded by parenchyma tissues, are known as platyhelminthes.

Classification: The classification system described here follows the system which as appeared in Invertebrate Zoology Vol. I, by Parker and Haswell and edited by Marshall and Willim (1974).

General Characters of Phylum- Platyhelminthes

i)                    Bilaterally symmentrical body, dorsoventrally flattened, triploblastic, and without any body skeleton.

ii)                   Unsegmented body, laceolet or ribbon like, body covered with cilia or cuticle.

iii)                 Anterior end with a head and sucker.

iv)                 With the sucker and hooks they remain anchored to some organs.

v)                  Coelom absent but the interior of the body is filled up with parenchyma derived from mesenchyme.

vi)                 Alimentary canal either imcomplete or branched, anus absent.

vii)               Definite respiratory and blood vascular system absent.

viii)              Excretory canal with flame cells.

ix)                 Reproduction asexual and sexual.

x)                  Hermaphrodite and life cycle is complicated with sexual larval stages.

Phylum Platyhelminthes has been divided into 5 classes namely-

Ø      Class I- Turbellaria

Ø      Class II- Digenia

Ø      Class III- Aspidogastrea

Ø      Class IV- Monogenea

Ø      Class V- Cestoda

Classification with reasons and examples

Ø                  Class I- Turbellaria


i) Most members are free-living, some are symbiotic, some are commensals, some are parsites.

ii)  Free-living forms may be marine, fresh water or terrestrial in habit.

iii) Unsegmented body without any sucker.

iv)  Ectodermal cilia mucous and rhabdite may be present. Mucous glands may be found in mesenchyme.

v) Mouth is anterior and ventral anus absent.

vi) Intestine may or may not be present.

Class Turbellaria has two subclasses: as-

Subclass 1. Archopora


i.      All are marine.

ii.      Ovary divided.

Example- Convulata

Subclass 2. Neophora


i.      Most of them are aquatic, some are terrestrial.

ii.      Ovary undivided.

Examples- Planaria notoplana

Ø                  Class II- Digenea:


i.      Leaf-like or lanceolet body form.

ii.      To complete the life cycle two hosts are required, all are endoparasites.

iii.      Two suckers, one near the mouth and other posterior to it.

iv.      Bipartite intestine.

v.      Many larval forms in the life history.

vi.      Cilia in the ectoderm absent.

This class has no sub-class.

Examples: Fasciola hepatica, F. buski

Ø                  Class III- Aspidogastrea:


i.      All are small and endoparasites.

ii.      Mouth funnel-like without sucker.

iii.      Intestine undivided.

iv.      Sucker present on the ventral side.

This class has no sub class.

Examples: Aspidogaster, Conchiola

Ø                  Class IV- Monogenea:


i.      Live as ectoparasite on the skin or gill of fishes.

ii.      Adult without cilia and rhabdite.

iii.      Pharynx and divided intestine present, except Gyrocotyle.

iv.      Mouth may be surrounded by sucker, sucker may be absent in many cases, posteroventral sucker present.

v.      A pair of anterolateral excretory pore present.

vi.      Free living and free swimming larvae possess 10-16 lateral hooks, life cycle simple.

This class has no sub class.

Examples: Entobdella, Polystome etc.

Class V- Cestoda :


i.      Elongated, flat, ribbon-like segmented body consisting of many segments, each segment is known as proglottid.

ii.      The head is like the pinhead and known as scolex.

iii.      Scolex with rostellum having hooks ad suckers.

iv.      Digestive system absent.

v.      Body is covered by a cuticle, sense organs absent.

vi.      They are endoparasites of vertebrate.

vii.      Hermaphrodite, life cycle complicated.

This class has been divided into 2 sub classes as-

Sub class I: Cestodaria


i.      Small body, with a single segment, scolex absent.

ii.      Sucker presents at the anterior side.

iii.      Larva contains 10 hooks.

iv.      Parasite of fishes.

Example- Amphilina

Sub Class II: Eucestoda


i.      Ribbon like body with many proglottids.

ii.      Scolex with 4 suckers and many hooks.

iii.      Larva hexacanth, i.e., with six hooks.

Examples- Taenia, Echinococcus etc.

3 thoughts on “Classification of the phylum Platyhelminthes”

  1. informations are very useful presented in a very simple way, thanks.
    But no informations about parenchyma tissue which i need more

  2. This was elaborate. However, I advocate the inclution of the life cycles of all the examples of flat worms. Thank you.

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