Phylum Platyhelminthes – Classification, Definition, Characteristics, Examples

Definition of Platyhelminthes (flatworms) 

Platyhelminthes is a triploblastic, bilaterallysymmetrical, dorsoventrally-flattened, acoelomate, flatworms without an organ grade of construction. However, it has a Protonephridial exretory system and mesenchyme that fills the space between various organs.

Characteristics of Platyhelminthes (flatworms) 

  • They can be free-living, parasitic or commensal.
  • They are bilaterally symmetrical, dorsoventrally flattened and triploblastic.
  • Bilaterally symmetrical, with the definite Polarity of Head and Tail Ends.
  • Triploblastic, i.e. Body derived from three embryonic germ layer layers: ectoderm. mesoderm. and endoderm.
  • Dorsoventrally fattened, i.e. Ventral surface well-developed with mouth and gonopore.
  • The body of the worm is generally shaped like a worm, but it can vary from a moderately elongated flattened shape to a long ribbon-like or leaf-like.
  • They range in size from very small to quite large, measuring between a few and tens of meters.
  • Except in class Cestoda, their body is not segmented.
  • They are mostly white and colorless, while some are derived from food. Free-living forms are either grey, brown-black, or brightly colored.
  • The head is distinguished at the anterior end of their bodies.
  • The ventral surface of the ventral mouth and genital pores are clearly marked in turbellarians, but less so in cestodes or trematodes.
  • They have adhesive structures such as hooks, spines, suckers and adhesive secretions in parasitic form.
  • The body is covered by cellular, syncytial, often ciliated epidermis. Trematodes cestodes have no epidermis, and their bodies are covered with cuticle.
  • The body is generally very soft because it lacks the exo- and endoskeleton. Cuticle, spines and thorns are the hard parts. Hooks and teeth make up the rest of the hard part.
  • They are called acoelomate, i.e. They have no body cavities.
  • There is space between organs that are filled with mesodermal tissue, mesenchyme and parenchyma.
  • Their digestion system is incomplete and branched without an anus, and completely absent in acoela or cestode.
  • They lack the skeletal, respiratory, and circulatory systems.
  • An excretory system also includes a lateral channel and one or more protonephridia that contain flame cells or bulbs. In primitive form.
  • Their nervous system is ladder-like and primitive. One pair of ganglia, or brain, and one to three pairs of longitudinal nerve chords are the main nervous system. These cords are connected by transverse nerves.
  • Their sense organs, however, are very simple. This is a common problem in tubellaria, but it is much less common in parasitic forms. Chemo-, tangoreceptors are often found in the form ciliated grooves and pits.
  • They are monoecious (hermaphrodite) and mainly monoecious.
  • In most forms, their reproductive system is complex and highly developed.
  • In many freshwater turbellaria, asexual reproduction is possible by fission.
  • Eggs are usually devoid of yolk in the majority of forms. They are made separately in the vitelline or yolk glands.
  • Fertilization can be internal or cross-fertilization with trematodes, and self-fertilization within cestodes.
  • Their life cycle is complex and involves several hosts.
  • Polyembryony and parthenogenesis are often caused by trematodes or tapeworms.
  • Some tapeworms are able to reproduce by either endogenous or exogenous burping.
  • Flatworms can be either parasitic or free-living.

Phylum Platyhelminthes (flatworms) Classification

The classification comes from Hyman L.H. (1951), up to suborder with some modifications.

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

  • Most are free-living, but there are some parasitic and endocommensals as well.
  • Terrestrial freshwater or marine.
  • Unsegmented body covered with ciliated cell or syncytial epidermis.
  • Mouth ventral. The muscular pharynx precedes the intestine.
  • There are many adhesive organs (suckers).
  • Sense organ i.e. In free-living organisms, there are many photoreceptors, chemo, and tango.
  • The excretory system is composed of the flame cells and the protonephridia.
  • Reproduction sexual, asexual, and by regeneration.
  • Simple life cycle

Order 1- Acoela

  • Only marine in size, less than 2mm
  • Ventral mouth; no muscular or intestinal pharynx.
  • The excretory system has been completely eliminated.
  • There are no flame cells, gonoducts or definite gonads.
  • Most of them are free-living and can be found under rocks or bottom mud. Some live in the intestines of sea-urchins, sea-cucumbers, and others.
  • Some are colored or made brown by symbiotic alga.
  • Examples: Convoluta, Ectocotyle, Afronta.

Order 2- Rhabdocoela

  • Small freshwater, marine and terrestrial forms (less than 3mm).
  • Simple pharynx, simple intestine and sac-like intestinale without diverticula.
  • Nervous system with two main longitudinal trunks.
  • Protonephridia excretory systems.
  • Eyes are usually present
  • There are few compact gonads, no gonoducts and a cuticularized structure in the reproductive system. Yolk gland absent or present.
  • Terrestrial, freshwater, marine. Free-living, commensal, or parasitic form
  • Examples: Catenula, Microstomum, Macrostomum, Mesostoma.

Suborder 1. Notandropora

  • Only freshwater forms
  • Simple pharynx.
  • The excretory system is composed of one median protonephridia.
  • Testes single compact mass, penis unarmed.
  • There is no yolk gland.
  • The formation of the chain zooids is the result of sexual fission.
  • Examples: Catenula,

Suborder 2. Opisthandropora

  • You can choose to use freshwater or marine water.
  • The excretory system is made up of paired nephridia.
  • Penis equipped with a stylet, compact testes
  • There is no yolk gland.
  • A chain of zooids allows for sexual reproduction.
  • Examples: Macrostomum, Microstomum.

Suborder 3. Lecithopora

  • You can choose from freshwater, marine, or terrestrial forms.
  • Bulbose pharynx.
  • The excretory system is made up of paired nephridia.
  • Separate the yolk glands and ovaries.
  • Reproduction is strictly sexual.
  • Most are free-living, but some may be parasitic or comensal.
  • Examples: Anoplodium, Mesostoma.

Suborder 4. Temnocephalida

  • Freshwater ectocommensals form.
  • An anterior end of the body is equipped with 2-12 tentacles.
  • 1-1/2 adhesive discs are provided at the posterior end of your body.
  • Dolii form pharynx.
  • Simple gonopore.
  • Examples: Temnocephala, Monodiscus.

Order 3- Alloecoela

  • Moderate-sized between 1-10mm
  • Most common form of freshwater, marine and brackish.
  • Pharynx simple or Bulbose; intestine straight (short diverticula);
  • The excretory system is composed of paired protonephridia with 2 or 3 main branches, and nephridiopores.
  • Nervous system with three or four pairs of longitudinal nerve chords, each with transverse connectives.
  • There are many testes in the reproductive system.
  • Penis papilla can be found most of the time.
  • Some people are ectoparasitic, or ectocommensals.
  • Examples: Prorhynchus, Plagiostomum, Geocentrophora.

Suborder 1. Archophora

  • Marine form.
  • Plicate pharynx.
  • Primitive female reproductive system, no female ducts.
  • Simple opening in the posterior of the male copulatory apparatus
  • Examples: Proporoplana (only some examples).

Suborder 2. Lecithoepitheliata

  • You can choose to be marine, freshwater, or terrestrial.
  • Bulbose or simple pharynx.
  • Penis with the cuticular stylingt.
  • Simple or none female ducts.
  • There are no yolk glands.
  • Ovarian cells are surrounded by nutrients.
  • Examples: Prorhynchus, Geocentophora.

Suborder 3. Cumulata

  • You can choose to use freshwater or marine water.
  • Bulbose or the plicate pharynx.
  • Diverticula is a common problem in the intestine.
  • Unarmed penis.
  • germovitellaria or separate ovaries and yolk cells make up the female reproductive system.
  • Examples: Hypotrichina.

Suborder 4. Seriata

  • Most common in freshwater and marine forms.
  • Plicate pharynx.
  • Intestine often with lateral diverticula.
  • Separate yolk glands and ovaries make up the female reproductive system.
  • Statocyst can be found in large quantities.
  • Examples: Otoplana, Bothrioplana.

Order 4- Tricladida

  • Turbellarians of large size (2 to 60 cm in length)
  • You can choose from terrestrial, freshwater, or marine forms.
  • Mid-ventral Mouth
  • The pharynx plicate is usually directed backwards.
  • Three branches of the testes, each with multiple diverticula.
  • Eyes are usually present.
  • Protonephridia is a lateral network that includes many nephridiopores.
  • There are 2 to 3 testes in the male reproductive system.
  • The female reproductive organ is composed of two ovaries, yolk glands, and a copulatory bone.
  • Single gonopore.
  • Examples: Gunda, Dugesia, Bdelloura, Geoplana.

Suborder 1. Maricola

  • Only marine form.
  • There are two eyes and one auricular groove.
  • A stylet is sometimes used to arm the penis papilla.
  • Present round copulatory brusa
  • Only sexual reproduction is possible.
  • Examples: Bdelloura,

Suborder 2. Paludicola

  • It is mainly freshwater.
  • Eyes 2, 3, or none at all
  • Brusa is usually present prior to the penis.
  • Most often, asexual reproduction.
  • Examples: Planaria or Dugesia.

Suborder 3. Terricola

  • Forms of terrestrial, tropical, and subtropical life.
  • Most people have a longer body.
  • You can have 2 to 3 eyes.
  • Brusa is almost absent.
  • Usually, male and female antras are separated.
  • It is possible to have sex.
  • Examples: Bipalium, Geoplana.

Order 5 Polycladida

  • Moderate-sized turbellarians (2-20 mm).
  • Many bottom dwellers and littoral zones in the marine environment.
  • Plicate pharynx is a highly branched intestine.
  • Numerous nerve cords are arranged in a radially ordered manner within the nervous system.
  • Numerous eyes.
  • The gonopores of male and female are distinct.
  • There are no yolk glands.
  • There are many and scattered testes and ovaries.
  • Examples: Leptoplana, Notoplana, Cestoplana, Planocera, Thysanozoon.

Suborder 1. Acotylea

  • Vertical curtain-like pharynx.
  • The gonopore is free of suckers.
  • Tentacles of the Nuchal type
  • Anterior margin eye clusters are never a common sighting.
  • Examples: Euplana, Leptoplana, etc.

Suborder 2. Cotylea

  • Tubular pharynx
  • The female pore is surrounded by sucker.
  • An anterior margin pair of tentacles with eyes, or a group of eyes on the marginal tentacles.
  • Examples: Thysanozoon, Yungia.

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

  • Ectoparasitic and Endoparasitic are commonly referred to
  • Body flattened leaflike.
  • Teguments are thick, but they lack cilia or rhabdites.
  • The body should be undivided and covered in cuticle.
  • Hooks and suckers are sometimes present.
  • An incomplete digestive tract consists of the anterior mouth, simple pharynx, and two forked intestines or many branches. Anus is absent.
  • Three pairs of the longitudinal nerve chord.
  • Flame cells are part of the Protonephridial excretory systems.
  • Mostly hermaphrodites(monoecious).
  • One ovary, two to many testes
  • Development direct (in ectoparasites), or indirect (in the endoparasites), with alternation in hosts.

Order 1. Monogenea

  • Most ectoparasites are found in cold-blooded aquatic vertebrates.
  • Oral suckers can be either weak or absent.
  • Anterior end is provided with two adhesive structures.
  • An adhesive disc is usually attached to the posterior end, often with hooks.
  • Anteriorly, on the dorsal side, are excretory pores.
  • The gonopores of male and female are usually distinct.
  • Vagina one to two. Uterus is small and has a few shelled egg.
  • Only one host is allowed in this life cycle.
  • Ciled larvae that swim freely in the wild called
  • Examples: Diplozon, Polystoma, Gyrodactylus, Dactylogyrus.

Order 2. Digenea

  • Endoparasites invertebrates and vertebrates.
  • Two suckers with no hooks: the oral sucker around your mouth and the ventral sucker, or acetabulum.
  • One posterior excretory pore.
  • No vagina. The uterus is usually large with many shelled egg.
  • Complex life cycles involve many stages of the larval cycle.
  • There may be one or more intermediate hosts during the life cycle.
  • Before metamorphosis, larval forms reproduce sexually.
  • Examples: Fasciola, Bucephalus, Opisthorchis, Paragonimus, Schistosoma.

Order 3. Aspidocotylea (=Aspidogastraea)

  • No oral suckers
  • Large ventral suckers are subdivided into many suckers with hooks.
  • Only one testis is found in the male system.
  • Endoparasites found in the stomachs of reptiles and fishes.
  • Examples: Aspidogaster, Cotylapsis, Stichocotyle.

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

  • Endoparasitic in the intestinal tract of vertebrates
  • Tapeworm is also known as tapeworm.
  • The body is divided into multiple segments (proglottids), but it rarely undivided, elongated and flat ribbon-like.
  • Tegument with no microvilli
  • However, the body is covered with cuticle and not covered by epidermis or cilia.
  • The adhesive structures at the anterior end (scolex), are provided with hooks and suckers , except in cestodaria.
  • Totally absent are the mouth and digestive systems.
  • The excretory system is composed of protonephridia with typical terminal fire
  • The nervous system typically consists of two ganglia and two lateral longitudinal nerve chords.
  • Each mature proglottid or segment is monoecious and contains male and female organs.
  • The life cycle is often complicated by the involvement of 2 or more hosts.
  • Hooked Embryos

Subclass 1. Cestodaria

  • Endoparasitic in the intestine or coelom of vertebrates
  • Unsegmented body, leaf-like with no scolex and strobila (monozoic).
  • There is no alimentary canal.
  • Only one monoecious reproduction system is available.
  • Larva lycophore has 10 hooks.

Order 1. Amphilinidea

  • Endoparasitic forms of coelom fishes
  • Flattened, oval, or elongated body.
  • No sucker.
  • Scolex absent.
  • Protrusible pharynx.
  • Frontal glands are located at the anterior end.
  • The posterior location of male and vaginal pores.
  • Near the anterior end, the uterus has a tightly coiled opening.
  • Examples: Amphilina.

Order 2. Gyrocotylidea

  • Endoparasitic forms are found in the intestines of fishes.
  • Flatter and more elongated body.
  • There is an anterior sucker and a posterior-rosette-shaped adhesive or.
  • Anterior end bears eversible proboscis.
  • The anterior half of your body contains the vaginal, male, and uterine pores.
  • Uterus runs straight to the pores.
  • Examples: Gyrocotyle.

Subclass 2. Eucestoda

  • Endoparasitic form found in the intestines of fishes.
  • The body is long and ribbon-like.
  • The body is divided into scolex and neck. There are many proglottids in the strobila ( polyzoic).
  • Scolex expanded bearing adhesives
  • Most often with multiple sets of monoecious reproduction organs.
  • Larva has 6 hooks

Order 1. Tetraphyllidea

  • Endoparasitic forms found only in the intestines of elasmobranchs.
  • Scolex with 4 leaf-like bothria (sessile suckers).often supplied with
  • The testes are the first to the ovaries.
  • Vitelline glands scattered.
  • Cirrus is armed with hooks and spines.
  • Common genital atrium marginal.
  • Examples: Phyllobothrium, Myzophyllobothrium.

Order2. Diphyllidea

  • Parasitic in the intestinal tract of elasmobranch Fishes
  • Scolex with 2 bothrias and a spiny head stalk.
  • Strobila is made up of no more than 20 proglottids.
  • Examples: Echinobothrium.

Order 3. Trypanorhyncha

  • Parasitic in the spiral gate of the digestive tract of elasmobranch salmon fishes.
  • Medium-sized body.
  • Scolex with 4 bothrias and 4 protruding spiny proboscides.
  • Vitellaria placed in continuous layers in the cortical parenchyma.
  • Laterally, the testes extend behind an ovary.
  • Ventrally open uterus; lateral gonopores
  • Examples: Haplobothrium, Tetrarhynchus.

Order 4. Pseudophyllidea

  • Parasitic in the intestinal tract of terrestrial vertebrates and teleost fishes.
  • Segmentation of the body into strobila and unsegmented.
  • Scolex with 2-6 shallow bothria (Suckers), rarely without adhesive organs.
  • Bilobed ovary, tests numerous, follicular, and scattered in mesenchyma proglottids.
  • Vitellaria follicular, numerous.
  • Midventral gonopores.
  • Examples: Bothriocephalus, Dibothriocephalus.

Order 5. Taenioidea or Cyclophyllidea

  • Parasitic in the intestinal tract of reptiles and birds.
  • Large-sized tapeworm.
  • Scolex bears four larges of cupped suckers (acetabula). Often with an apical rosetum armed with hooks.
  • Ovary with two or more lobed ovaries; uterine opening absent.
  • Gonopores at one or both of the margins.
  • The excretory system is made up of four longitudinal vessels.
  • Vitellaria (yolk-gum) is single and compact.
  • Examples: Taenia, Echinococcus, Hymenolepis, Moniezia.

Examples of Platyhelminthes

These are some examples of organisms that belong to the phylum Platyhelminthes:

Dugesia (Planaria)

They can be found in slow streams or freshwater ponds. They possess cilia, which can regenerate the part that has been lost. Two lateral lobes and a pair of eyes are found on the head.


It can be found in the blood vessels of the mesenteric and hepatic portal systems of human beings and is known as blood fluke. It shows well-marked sexual dimorphism.

Schistosoma is a cause of Schistosomiasis, which can spread through contaminated water. Anemia, diarrhoea, pain, fever, liver, spleen, and liver enlargements can all be experienced by the patient.


Because it lives in sheep and goat livers, it is sometimes called liver fluke. Although it is a hermaphrodite, cross-fertilization occurs.

Fasciolasis is a condition that can affect animals. This causes the animal’s liver to enlarge and the bile drains to become blocked. Muscle pain can result from the infection. It could also be fatal.

Taenia solium

It is also known by the name pork tapeworm. They are parasites that live in the small intestines of humans and the larvae they produce in the muscles of pigs. It is a hermaphrodite, and self-fertilizes.

Taenia Solium can cause taeniasis, where the patient feels abdominal pain, anaemia and restlessness.

Other organisms include Taenia.saginata which is transmitted through beef in the human intestinales and Echinococcus.granulosus which lives in the intestines of cats and dogs.

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5 Best Microbiology Books For B.Sc 1st Year Students What is a digital colony counter? Why do Laboratory incubators need CO2? What is Karyotyping? What are the scope of Microbiology? What is DNA Library? What is Simple Staining? What is Negative Staining? What is Western Blot? What are Transgenic Plants?
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