Phylum Platyhelminthes- characteristics, classification, examples

Platyhelminthes (flatworms) definition

Platyhelminthes are triploblastic, bilaterally symmetrical, dorsoventrally flattened, acoelomate flatworms with organ grade of construction without a definite anus, circulatory, skeletal or respiratory system but with Protonephridial excretory system and mesenchyme filling the space between the various organ of the body.

Phylum Platyhelminthes (flatworms)

Figure: Phylum Platyhelminthes. Source: WebMD and Wikipedia.

Phylum Platyhelminthes (flatworms) characteristics

  • They are free-living, commensal or parasitic.
  • They are bilaterally symmetrical and dorsoventrally flattened, triploblastic worm.
  • Bilaterally symmetrical with the definite polarity of head and tail ends.
  • Triploblastic i.e. body derived from three embryonic germ layers; ectoderm, mesoderm, and endoderm.
  • Dorsoventrally fattened i.e. well-developed ventral surface with mouth and gonopore.
  • Their body generally shapes as a worm but varies from moderately elongated flattened to long ribbon-like and leaf-like.
  • They are small to moderate in size varying from microscopic to extremely elongated form measuring up to 10-15 meters.
  • Their body is unsegmented except in class Cestoda.
  • The majority of them are white, colorless and some derive color from ingested food while free-living form are grey, brown-black or brilliantly colored.
  • Their anterior end of the body is differentiated into the head.
  • Mouth and genital pores on the ventral surface are well marked in turbellarians but less marked in cestodes and trematodes.
  • Their parasitic form has adhesive structures like hooks, spines and suckers, and adhesive secretions.
  • The body is covered with cellular or syncytial, frequently ciliated epidermis; while trematodes cestodes, lacks epidermis and their body covered with cuticle.
  • Exo- and endoskeleton are completely absent, hence the body is generally soft. The hard part consists of cuticle, spines, thorns, hooks, teeth.
  • They are acoelomate i.e. without any body cavity.
  • Space between various organs filled with special mesodermal tissues, the mesenchyme, and parenchyma.
  • Their digestive system is branched and incomplete without an anus and totally absent in acoela and cestode.
  • They lack skeletal, respiratory and circulatory systems.
  • The excretory system includes a lateral canal and a single or pair of protonephridia with flame cells or bulbs. Absent in some primitive form.
  • Their nervous system is primitive, ladder-like. The main nervous system consists of a pair of ganglia or brain and one or three pairs of longitudinal nerve cords connected by transverse nerves.
  • Their sense organs are simple. A common occurrence in tubellaria but greatly reduced in parasitic form. Chemo- and tangoreceptors commonly in the form of ciliated pits and grooves.
  • They are mostly monoecious (hermaphrodite).
  • Their reproductive system is highly evolved or complex in most of the forms.
  • Asexual reproduction occurs by fission in many freshwater turbellaria.
  • In the majority of form, eggs are devoid of yolk. They are produced separately in the yolk or vitelline glands.
  • Fertilization is internal but cross-fertilization in trematodes and self-fertilization in cestodes.
  • Their life cycle is complicated involves one or more hosts.
  • Parthenogenesis and polyembryony commonly occur trematodes and tapeworms.
  • Some tapeworm propagates by endogenous or exogenous budding.
  • The flatworm is either free-living or ecto-or endocommensals or parasitic.

Phylum Platyhelminthes (flatworms) Classification

The classification is from Hyman, L.H., (1951) up to suborder only with certain modifications.

Class 1- Turbellaria (L., turbella= a little string)

  • Mostly free-living but some ectocommensals and endocommensals or parasitic called
  • Terrestrial marine or freshwater.
  • Body unsegmented and covered with ciliated cellular or syncytial epidermis, containing mucus-secreting cells and rod-shaped body called
  • Mouth ventral. intestine preceded by the muscular pharynx.
  • Adhesive organs(suckers) abundantly present.
  • Sense organ i.e. Tango, chemo, and photoreceptors common in free-living forms.
  • The excretory system consists of protonephridia, the flame cells.
  • Mostly reproduction sexual, asexual and by regeneration.
  • Life cycle simple.

Order 1- Acoela

  • Small, exclusively marine, less than 2 mm.
  • Ventral mouth; no muscular pharynx and no intestine.
  • The excretory system is totally absent.
  • No flame cells, definite gonads, gonoducts, and yolk glands.
  • Mostly free-living, found under stones or bottom mud, algae, some live in the intestine of sea-urchins and sea-cucumbers.
  • Some colored or brown by symbiotic algae.
  • Examples: Convoluta, Ectocotyle, Afronta.

Order 2- Rhabdocoela

  • Small (less than 3mm) freshwater, marine, and terrestrial form.
  • Simple pharynx and sac-like intestine without diverticula.
  • Nervous system with 2 main longitudinal trunks.
  • Protonephridia excretory system.
  • Eye usually present.
  • The reproductive system comprises few compact gonads, gonoducts and a cuticularized structure instead of penis papilla present. Yolk gland present or absent.
  • Marine, freshwater or terrestrial. Free-living, commensal or parasitic form
  • Examples: Catenula, Microstomum, Macrostomum, Mesostoma.

Suborder 1. Notandropora

  • Exclusively freshwater forms.
  • Simple pharynx.
  • The excretory system consists of a single median protonephridia.
  • Testes single compact mass, penis unarmed.
  • No yolk gland.
  • Asexual fission occurs with the formation of the chain of zooids.
  • Examples: Catenula,

Suborder 2. Opisthandropora

  • Freshwater or marine form.
  • The excretory system consists of paired nephridia.
  • Testes compact, penis armed with a stylet.
  • No yolk gland.
  • Asexual reproduction with a chain of zooids.
  • Examples: Macrostomum, Microstomum.

Suborder 3. Lecithopora

  • Freshwater, marine or terrestrial forms.
  • Bulbose pharynx.
  • The excretory system consists of paired nephridia.
  • Separate ovaries and yolk glands.
  • Reproduction is exclusively sexual.
  • Mostly free-living, some commensals or parasitic form.
  • Examples: Anoplodium, Mesostoma.

Suborder 4. Temnocephalida

  • Freshwater ectocommensals form.
  • The anterior end of the body provided with 2-12 tentacles.
  • Posterior end of the body provided with 1-2 adhesive discs.
  • Dolii form pharynx.
  • Simple gonopore.
  • Examples: Temnocephala, Monodiscus.

Order 3- Alloecoela

  • Moderate-sized between 1 and 10mm.
  • Mostly marine, freshwater and brackish water form.
  • Pharynx simple, Bulbose or plicate; intestine straight or branched (short diverticula).
  • The excretory system consists of paired protonephridia having 2 or 3 main branches and nephridiopores.
  • Nervous system with 3or 4 pairs of longitudinal nerve cords provided with transverse connectives.
  • The reproductive system consists of numerous testes and a pair of ovaries.
  • Penis papilla is mostly present.
  • Some are ectoparasitic or ectocommensals in the habit.
  • Examples: Prorhynchus, Plagiostomum, Geocentrophora.

Suborder 1. Archophora

  • Marine form.
  • Plicate pharynx.
  • Primitive female reproductive system, no female ducts.
  • Male copulatory apparatus simple opening posteriorly.
  • Examples: Proporoplana (only examples).

Suborder 2. Lecithoepitheliata

  • Marine, freshwater or terrestrial form.
  • Simple or Bulbose pharynx.
  • Penis with the cuticular stylet.
  • Simple or none female ducts.
  • No yolk glands.
  • Nutritive cells surround ova.
  • Examples: Prorhynchus, Geocentophora.

Suborder 3. Cumulata

  • Freshwater or marine form.
  • Bulbose or plicate pharynx.
  • Intestine usually devoid of diverticula.
  • Unarmed penis.
  • The female reproductive system consists of germovitellaria or separate ovaries and yolk glands.
  • Examples: Hypotrichina.

Suborder 4. Seriata

  • Mostly marine and freshwater form.
  • Plicate pharynx.
  • Intestine usually with lateral diverticula.
  • The female reproductive system consists of separate ovaries and yolk glands.
  • Statocyst is mostly present.
  • Examples: Otoplana, Bothrioplana.

Order 4- Tricladida

  • Large-sized turbellarians (2 to 60cm long).
  • Marine, freshwater or terrestrial forms.
  • Mouth mid-ventral.
  • Pharynx plicate usually directed backward.
  • Intestine with 3 branches, each with many diverticula.
  • Eyes usually present.
  • Protonephridia as lateral networks with many nephridiopores.
  • The male reproductive system consists of 2 or numerous testes; a penis papilla present.
  • The female reproductive organ consists of a pair of ovaries with yolk glands and a copulatory brusa.
  • Single gonopore.
  • Examples: Gunda, Dugesia, Bdelloura, Geoplana.

Suborder 1.  Maricola

  • Exclusively marine form.
  • A pair of eyes and auricular grooves present.
  • Typical penis papilla sometimes armed with the stylet.
  • Rounded copulatory brusa present.
  • Only sexual reproduction takes place.
  • Examples: Bdelloura,

Suborder 2. Paludicola

  • Mostly freshwater, rarely brackish water forms.
  • Eyes 2 to many or completely absent.
  • Brusa usually presents anterior to the penis.
  • Mostly asexual reproduction.
  • Examples: Planaria or Dugesia.

Suborder 3. Terricola

  • Terrestrial, tropical and subtropical forms.
  • Elongated body mostly.
  • 2 to many eyes.
  • Brusa is mostly absent.
  • Male and female antra usually separate.
  • Asexual reproduction may also occur.
  • Examples: Bipalium, Geoplana.

Order 5- Polycladida

  • Moderate -sized turbellarians (2 to 20 mm).
  • Marine, many bottom dwellers or littoral zones.
  • Plicate pharynx, intestine highly branched.
  • The nervous system consists of numerous radially arranged nerve cords.
  • Numerous eyes.
  • Male and female gonopore separate.
  • No yolk glands.
  • Testes and ovaries are numerous and scattered.
  • Examples: Leptoplana, Notoplana, Cestoplana, Planocera, Thysanozoon.

Suborder 1. Acotylea

  • Usually vertical curtain-like pharynx.
  • Suckers absent behind the gonopore.
  • Nuchal type tentacles.
  • Eyes never occur as a pair of clusters on the anterior margin.
  • Examples: Euplana, Leptoplana, etc.

Suborder 2. Cotylea

  • Tubular pharynx.
  • Sucker present behind the female pore.
  • A pair of marginal tentacles bearing eyes or a cluster of eyes at the anterior margin.
  • Examples: Thysanozoon, Yungia.

Class 2- Trematoda (Gr., trematodes= having pore)

  • Ectoparasitic or endoparasitic commonly called
  • Body unsegmented dorsoventrally flattened leaf-like.
  • Teguments thick but without cilia and rhabdites.
  • Body undivided and covered with cuticle.
  • Suckers and sometimes hooks present.
  • Digestive tract incomplete consists of the anterior mouth, simple pharynx and two forked or many branches intestine; anus absent.
  • 3 pairs of the longitudinal nerve cord.
  • Protonephridial excretory system consisting of flame cells.
  • Mostly hermaphrodites(monoecious).
  • Single ovary, 2 to many testes.
  • Development direct (in ectoparasites) or indirect (in endoparasites) with alternation of hosts.

Order 1. Monogenea

  • Mostly ectoparasites in cold-blooded aquatic vertebrates.
  • Oral suckers either weak or absent.
  • Anterior end provided with a pair of adhesive structures.
  • Posterior end provided with an adhesive disc usually with hooks.
  • Excretory pores paired situated anteriorly on the dorsal side.
  • Male and female gonopore usually separate.
  • Vagina one or two. Uterus is small with a few shelled eggs.
  • Only one host in the life cycle.
  • Free-swimming ciliated larva called
  • Examples: Diplozon, Polystoma, Gyrodactylus, Dactylogyrus.

Order 2. Digenea

  • Endoparasites of vertebrates and invertebrates.
  • 2 suckers without hooks; oral sucker around the mouth and ventral sucker or acetabulum.
  • Single posterior excretory pore.
  • No vagina. The uterus usually long with many shelled eggs.
  • The life cycle complicated involving many larval stages.
  • One to more intermediate hosts in the life cycle.
  • Larval forms reproduce asexually before metamorphosis.
  • Examples: Fasciola, Bucephalus, Opisthorchis, Paragonimus, Schistosoma.

Order 3. Aspidocotylea (=Aspidogastraea)

  • No oral suckers.
  • Large ventral suckers subdivided into several suckers without hooks.
  • Only one testis in the male system.
  • Endoparasites in the gut of fishes and reptiles.
  • Examples: Aspidogaster, Cotylapsis, Stichocotyle.

Class 3- Cestoda (Gr., ketos, gridle+ eidos, form)

  • Endoparasitic in the intestine of vertebrates.
  • Commonly called tapeworm.
  • Body divided into many segmented (proglottids) but rarely undivided, elongated, flat, ribbon-like.
  • Tegument without microvilli.
  • Body without epidermis and cilia but covered with cuticle.
  • Anterior end (scolex) is provided with adhesive structures (hooks, suckers) except in cestodaria.
  • Mouth and digestive systems totally absent.
  • The excretory system consists of protonephridia with typical terminal flame
  • The nervous system usually comprises a pair of ganglia and 2 lateral longitudinal nerve cords.
  • Each mature segment or proglottids monoecious, with male and female organs.
  • Life cycle complicates usually involving 2 or more hosts.
  • Embryos with hooks.

Subclass 1. Cestodaria

  • Endoparasitic in the coelom or intestine of vertebrates.
  • Body unsegmented, leaf-like without scolex and strobila (monozoic).
  • No alimentary canal.
  • Only one set of the monoecious reproductive system.
  • Larva lycophore with 10 hooks.

Order 1. Amphilinidea

  • Endoparasitic forms in the coelom of fishes.
  • Body flattened, oval or elongated.
  • No sucker.
  • Scolex absent.
  • Protrusible pharynx.
  • Anterior end bears frontal glands.
  • Male and vaginal pores situated posteriorly.
  • The uterus is very much coiled opening near the anterior end.
  • Examples: Amphilina.

Order 2. Gyrocotylidea

  • Endoparasitic forms in the intestine of fishes.
  • Body elongated and flattened.
  • An anterior sucker and a posterior rosette-shaped adhesive organ present.
  • Anterior end bears eversible proboscis.
  • Uterine, male and vaginal pores are together situated in the anterior half of the body.
  • Uterus short straight runs directly to pores.
  • Examples: Gyrocotyle.

Subclass 2. Eucestoda

  • Endoparasitic form in the intestine of fishes.
  • Body long, ribbon-like.
  • The body is divided into scolex, neck, and strobila with many proglottids (polyzoic).
  • Scolex expanded bearing adhesive structures.
  • Mostly with several sets of monoecious reproductive organs.
  • Larva with 6 hooks.

Order 1. Tetraphyllidea

  • Endoparasitic forms; exclusively in the intestine of elasmobranch fishes.
  • Scolex with 4 leaf-like bothria (sessile suckers) often provided with
  • Testes are anterior to ovaries.
  • Vitelline glands scattered.
  • Cirrus armed with spines and hooks.
  • Common genital atrium marginal.
  • Examples: Phyllobothrium, Myzophyllobothrium.

Order 2. Diphyllidea

  • Parasitic in the intestine of elasmobranch fishes.
  • Scolex with 2 bothria and spiny head stalk.
  • Strobila consists of not more than 20 proglottids.
  • Examples: Echinobothrium.

Order 3. Trypanorhyncha

  • Parasitic in the spiral valve of the digestive tract of elasmobranch fishes.
  • Moderately sized body.
  • Scolex with 4 bothria and 4 protrusible spiny proboscides.
  • Vitellaria in cortical parenchyma placed in a continuous layer.
  • Testes extend behind the ovary posteriorly.
  • Lateral gonopores; ventrally open uterus.
  • Examples: Haplobothrium, Tetrarhynchus.

Order 4. Pseudophyllidea

  • Parasitic in the intestine of teleost fishes and terrestrial vertebrates.
  • Body segmented into strobila or unsegmented.
  • Scolex with 2 to 6 shallow bothria (Suckers) rarely without adhesive organs.
  • Bilobed ovary, testes numerous, follicular and scattered in the mesenchyma of proglottids.
  • Vitellaria follicular, numerous.
  • Midventral gonopores.
  • Examples: Bothriocephalus, Dibothriocephalus.

Order 5. Taenioidea or Cyclophyllidea

  • Parasitic in the intestine of reptiles, birds, and mammals.
  • Large-sized tapeworm.
  • Scolex bears 4 larges in cupped suckers (acetabula) often with an apical rostellum armed with hooks.
  • Ovary two or many lobed; uterine opening absent.
  • Gonopores on one or both margins.
  • The excretory system consists of 4 longitudinal vessels.
  • Vitellaria (yolk gland) single and compact.
  • Examples: Taenia, Echinococcus, Hymenolepis, Moniezia.


  1. Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
  2. Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.

Phylum Platyhelminthes

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