Phylum Platyhelminthes-Characteristics, Classification, Examples

Phylum Platyhelminthes (flatworms) definition

Phylum Platyhelminthes are acoelomate flatworms that are triploblastic, bilaterally symmetrical, dorsoventrally flattened, and lack a distinct circulatory, skeletal, or respiratory system, but do have a protonophridial excretory system and mesenchyme that fills the spaces between the body’s numerous organs.

Phylum Platyhelminthes (flatworms) characteristics

  • They can be parasitic, commensal, or free-living.
  • They are triploblastic worms that are bilaterally symmetrical and dorsoventrally flattened.
  • Clearly polarised head and tail ends, with bilateral symmetry.
  • Ectoderm, mesoderm, and endoderm are the three embryonic germ layers that make up a triploblastic body.
  • having a well-developed ventral surface with a mouth and gonopore, or dorsoventrally fattened.
  • Their bodies often have the shape of a worm, although they can also be relatively elongated and flattened, long and ribbon-like, or even leaf-like.
  • They range in size from tiny to exceedingly elongated, measuring up to 10-15 metres, and are modestly to moderately sized.
  • Except for class Cestoda, their bodies are not segmented.
  • Their free-living forms are grey, brown-black, or brilliantly coloured, although the bulk of them are white, colourless, and some get colour from ingested food.
  • Their heads are distinct from the anterior end of their bodies.
  • On the ventral surface, mouth and genital apertures are clearly visible in turbellarians, but less so in cestodes and trematodes.
  • In their parasitic stage, they produce adhesive secretions as well as adhesive features like hooks, spines, and suckers.
  • The cellular or syncytial, frequently ciliated epidermis covers the body, whereas trematodes cestodes lack an epidermis and have a cuticle-covered body.
  • Since there is no exo-or endoskeleton, the body is often soft. The cuticle, spines, thorns, hooks, and teeth make up the hard component.
  • They lack a bodily cavity since they are acoelomates.
  • The mesenchyme and parenchyma, as well as specific mesodermal tissues, fill the space between the organs.
  • Without an anus, their digestive systems are branching, imperfect, and completely lacking in acoela and cestode.
  • Their circulatory, respiratory, and skeletal systems are absent.
  • A lateral canal and one or two protonephridia with flame cells or bulbs are part of the excretory system. Rather primitively absent
  • Their basic nervous system resembles a ladder. One or three ganglia, or brain, one or three pairs of longitudinal nerve cords, and transverse nerves make up the body’s major nervous system.
  • Their sensory systems are basic. It is a frequent occurrence in tubellaria, but its parasitic form is far less prevalent. The most prevalent types of chemo-and tangoreceptors are ciliated pits and grooves.
  • Most of them are monoecious (hermaphrodite).
  • In the majority of types, their reproductive systems are highly developed or complex.
  • In many freshwater turbellaria, fission is the method of asexual reproduction.
  • In their most common form, eggs are yolkless. They are each made individually in the vitelline or yolk glands.
  • Internal fertilisation occurs, but trematodes and cestodes also cross-and self-fertilize.
  • Their complex life cycle includes one or more hosts.
  • Trematodes and tapeworms frequently experience parthenogenesis and polyembryony.
  • In some cases, tapeworms can reproduce by exogenous or endogenous budding.
  • The flatworm can be parasitic, ectoparasitic, endoparasitic, or free-living.

Phylum Platyhelminthes (flatworms) Classification

With some revisions, Hyman, L.H., (1951)’s classification is used up to the suborder level.

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

  • primarily free-living, but some parasitic species, including ectocommensals and endocommensals
  • freshwater or marine on land.
  • Unsegmented body with ciliated cellular or syncytial epidermis covering it, which contains cells that secrete mucus and a rod-shaped body known as
  • ventral mouth. intestine came before the pharynx’s muscles.
  • There are several adhesive organs (suckers) present.
  • Tango, chemo, and photoreceptors are common sense organs in free-living organisms.
  • The flame cells, or protonephridia, make up the excretory system.
  • primarily sexual, asexual, and through renewal.
  • basic life cycle

Order 1-Acoela

  • less than 2 mm, entirely marine.
  • No intestine; muscular pharynx; ventral mouth.
  • There is no excretory system at all.
  • Flame cells are absent; gonads, gonoducts, and yolk glands are clearly visible.
  • Algae are mostly free-living and can be found in the dirt or under rocks. Some algae also live in the intestines of sea cucumbers and urchins.
  • Some are made brown or tinted by symbiotic algae.
  • Convoluta, Ectocotyle, and Afronta, for instance.

Order 2-Rhabdocoela

  • freshwater, marine, and terrestrial forms that are small (less than 3mm).
  • simple pharynx and intestine that resembles a sac.
  • The nervous system has two primary longitudinal trunks in the brain.
  • excretory system of protonophidia.
  • generally has an eye.
  • The penis papilla is absent, and the reproductive system consists of a few compact gonads, gonoducts, and a cuticularized structure. whether the yolk gland is present.
  • terrestrial, freshwater, or marine. parasitic, commensal, or free-living form.
  • Catenula, Microstomum, Macrostomum, and Mesostoma are a few examples.

Suborder 1. Notandropora

  • only freshwater organisms.
  • basic pharynx
  • The median protonephridia make up the excretory system.
  • tests a single, compact mass with an unarmed penis.
  • zero yolk gland.
  • Asexual fission takes place with the creation of the chain of zooids.
  • For instance, Catenula,

Suborder 2: Opisthandropora

  • marine or freshwater forms.
  • There are two paired nephridia in the excretory system.
  • Compact tests with a stylet-armed penis.
  • zero yolk gland.
  • Zooids are used in an asexless chain of reproduction.
  • Macrostomum and Microstomum are examples.

Suborder 3: Lecithopora

  • terrestrial, marine, or freshwater forms.
  • Bulbous throat
  • There are two paired nephridia in the excretory system.
  • Keep the yolk glands and ovaries apart.
  • Reproduction is solely a sexual activity.
  • Most of them are free-living, while some are commensals or parasites.
  • Anoplodium and Mesostoma are examples.

Suborder 4. Temnocephalida

  • Ectocommensals form in freshwater.
  • There are 2–12 tentacles on the body’s anterior end.
  • One or two sticky discs are given on the body’s posterior end.
  • Dolii creates the pharynx.
  • basic gonopore
  • Temnocephala and Monodiscus, for instance.

Order 3- Alloecoela

  • medium-sized, ranging from 1 to 10 mm.
  • Most water forms are marine, freshwater, and brackish.
  • simple pharynx, Straight or branching intestine; bulbous or plicate (short diverticula).
  • Nephridiopores and paired protonephridia with two or three main branches make up the excretory system.
  • A nervous system containing three or four longitudinal pairs of nerve cords and transverse connectives.
  • There are several testes and two ovaries in the reproductive system.
  • Most of the penis papilla is present.
  • Some people have an ectoparasitic or ectocommensal habit.
  • Examples include Prorhynchus, Plagiostomum, and Geocentrophora.

Suborder 1: Archophora

  • Marine dress
  • fragile pharynx
  • No ducts; primitive female reproductive system.
  • single posterior entrance of the male copulatory apparatus.
  • Illustrations: Proporoplana (only examples).

Suborder 2: Lecithoepitheliata

  • terrestrial, freshwater, or marine forms.
  • Bulbose or simple pharynx.
  • Using the cuticular stylet, the penis
  • simple female ducts or none at all.
  • yolk glands absent.
  • Around the ova are nutrient cells.
  • Prorhnchus and Geocentophora are two examples.

Suborder 3: Cumulata

  • marine or freshwater forms.
  • Pharynx with plicae or bulbosity.
  • Diverticula are typically not present in the intestine.
  • Armless penis
  • Ovaries and yolk glands that are distinct, called germovitellaria, make up the female reproductive system.
  • Consider the hypotricina.

Suborder 4: Seriata

  • mostly freshwater and marine forms.
  • fragile pharynx
  • usually has lateral diverticula in the intestine.
  • The ovaries and yolk glands in the female reproductive system are distinct from one another.
  • Most of the statocysts are present.
  • Otoplana and Bothrioplana are examples.

Order 4: Tricladida

  • Turbellarians have a large size (2 to 60 cm long).
  • terrestrial, freshwater, or marine forms.
  • mouth is midventral.
  • Usually, the pharynx plicate faces backward.
  • Three branches of the intestine each contain many diverticula.
  • Usually, there are eyes.
  • Nephridiopore-rich lateral networks of protonephridia
  • Two or more testes and a penis papilla make up the male reproductive system.
  • The copulatory brusa and a pair of ovaries with yolk glands make up the female reproductive system.
  • only one gonopore.
  • Examples include Geoplana, Gunda, Dugesia, and Bdelloura.

Suborder 1. Maricola

  • only in marine form.
  • There are two eyes and auricular grooves.
  • a typical penis papilla that occasionally carries a stylet.
  • present is a rounded copulatory brusa.
  • There is only sexual reproduction.
  • For instance, Bdelloura

Suborder 2. Paludicola

  • Freshwater predominates; seldom does brackish water develop.
  • eyes that are either too numerous or not at all.
  • Brusa typically appears in front of the penis.
  • Asexual reproduction predominates.
  • Planaria or Dugesia are two examples.

Suborder 3. Terricola

  • forms that are terrestrial, tropical, and subtropical.
  • primarily a lengthy body
  • two eyes too many.
  • Most of Brusa is missing.
  • Normally, male and female antra segregate.
  • Additionally, asexual reproduction is possible.
  • Geoplana and Bipalium, for instance.

Order 5-Polycladida

  • Small to medium-sized turbellarians (2 to 20 mm).
  • numerous bottom dwellers or littoral zones in the ocean.
  • highly branching gut and a delicate pharynx
  • Numerous nerve cords are distributed in a radial fashion throughout the neurological system.
  • many eyes.
  • Female and male gonopores are distinct.
  • yolk glands absent.
  • The ovaries and testes are dispersed and abundant.
  • Leptoplana, Notoplana, Cestoplana, Planocera, and Thysanozoon are a few examples.

Suborder 1: Acotylea

  • pharynx, which is often vertical and draped.
  • Absent are the suckers behind the gonopore.
  • tendrils of the nuchal kind.
  • The anterior border never has two clusters of eyes.
  • Examples include Euplana and Leptoplana.

Suborder 2: Cotylea

  • pharynx with tubules.
  • Present behind the vaginal pore is a sucker.
  • a group of eyes near the anterior margin, or a pair of marginal tentacles with eyes.
  • Examples include Yungia and Thysanozoon.

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

  • Endoparasitic or ectoparasitic is another term for
  • Unsegmented body with a leaf-like dorsoventral flattening.
  • Rhabdites and thick teguments without cilia
  • undivided and cuticle-covered body
  • Present are suckers and occasionally hooks.
  • The front mouth, simple throat, and two forked or numerous branches of the intestine make up the incomplete digestive system; the anus is lacking.
  • three longitudinal nerve cord pairs.
  • flame-cell-based protonephridial excretory system.
  • Hermaphrodites predominate (monoecious).
  • It has just one ovary and too many testicles.
  • Direct or indirect development with a change in hosts is seen in ectoparasites (endoparasites).

Order 1. Monogenea

  • mostly ectoparasites in aquatic cold-blooded vertebrates.
  • Oral suckers are either nonexistent or weak.
  • It is supplied with two sticky devices on the front end.
  • A hook-equipped sticky disc is normally attached to the posterior end.
  • On the dorsal side, anteriorly, are two pairs of excretory holes.
  • Normally, the male and female gonopore are distinct.
  • One or two vaginas A few shelled eggs are present in a tiny uterus.
  • The life cycle only involves one host.
  • termed a free-swimming ciliated larva
  • Diplozon, Polystoma, Gyrodactylus, and Dactylogyrus are a few examples.

Order 2. Digenea

  • Vertebrate and invertebrate endoparasites
  • 2 suckers—the ventral sucker, often known as the acetabulum, and the oral sucker surrounding the mouth.
  • only one excretory pore at the back.
  • Zero vagina It is usually lengthy and filled with several shelled eggs.
  • The complex life cycle has several larval stages.
  • During the life cycle of one or more intermediary hosts
  • Before transformation, larval forms reproduce in an asexual manner.
  • Examples include Schistosoma, Opisthorchis, Paragonimus, Fasciola, and Bucephalus.

Order 3. Aspidocotylea (=Aspidogastraea)

  • Avoid oral suckers.
  • Numerous hookless large ventral suckers were separated into smaller suckers.
  • In the male body, there is just one testis.
  • Fish and reptiles’ guts contain endoparasites.
  • Aspidogaster, Cotylapsis, and Stichocotyle are among examples.

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

  • Vertebrate intestines are endoparasitic.
  • is referred to as a tapeworm.
  • The body is elongated, flat, and ribbon-like, separated into several segmental proglottids but seldom undivided.
  • Without microvilli, the tegument
  • Body coated with cuticle but lacking an epidermis and cilia.
  • Except in cestodaria, the front end (scolex) is equipped with sticky features (hooks, suckers).
  • No mouth or digestive system at all.
  • The protonephridia in the excretory system have the usual terminal flame.
  • Two lateral longitudinal nerve cords and a pair of ganglia typically make up the nervous system.
  • Each fully developed segment, or proglottid, has both male and female reproductive organs.
  • Complications in the life cycle often involve two or more hosts.
  • eggs that have hooks.

Subclass 1. Cestodaria

invertebrate coel or intestinal endoparasitic.

Unsegmented body with a leaf-like shape and no scolex or strobila (monozoic).

There is no digestive system.

The monoecious reproductive system only has one pair.

lycophore of a larva with ten hooks

Order 1. Amphilinidea

forms seen in fish coelom that are endoparasitic.

body that has become round, long, or flat.

No gullible.

Absent is Scolex.

pharynx that protrudes

It contains frontal glands on the anterior end.

Vaginal and male pores are located posteriorly.

The front end of the uterus has an extremely coiled opening.

Among them is Amphilina.

Order 2. Gyrocotylidea

Fish intestines contain endoparasitic organisms.

Body lengthened and became flatter.

There is a posterior rosette-shaped sticky organ and an anterior sucker.

Eversible proboscis is present on the anterior end.

The anterior portion of the body is where the uterine, male, and vaginal pores are all located.

Short, direct pathways from the uterus to the pores

Consider the gyrocotyle.

Subclass 2. Eucestoda

  • The endoparasitic type seen in fish intestines is
  • Long, ribbon-like body.
  • With several proglottids, the body is separated into the scolex, neck, and strobila (polyzoic).
  • Expanded bearing adhesive structures from Scolex.
  • Having many sets of monoecious reproductive organs is in the majority.
  • six-hooked larva 

Order 1. Tetraphyllidea

  • only found in the gut of elasmobranch fishes; endoparasitic types.
  • Scolex that frequently has four sessile suckers with the appearance of leaves.
  • Ovaries are in front of the testes.
  • Dispersed vitelline glands.
  • Cirrus carries hooks and spines.
  • marginal common genital atrium.
  • Phyllobothrium and Myzophyllobothrium are two examples.

Order 2: Diphyllidea

  • parasitic in elasmobranch fishes’ intestines.
  • Scolex with a spiky head stalk and two bothria.
  • There are no more than 20 proglottids in Strobila.
  • Among them is Echinobothrium.

Order 3. Trypanorhyncha

  • parasitic in elasmobranch fishes’ spiral valves of their digestive tracts.
  • Medium-sized body.
  • 4 bothria and 4 protruding spiny proboscides on a scolex.
  • Vitellaria are arranged in a continuous layer in the cortical parenchyma.
  • The testes extend posteriorly behind the ovary.
  • Lateral gonopores and an open uterus from the back.
  • For instance, there is Haplobothrium and Tetrarhynchus.

Order 4. Pseudophyllidea

  • parasitic in terrestrial vertebrates’ and teleost fishes’ intestines.
  • Unsegmented or segmented into strobila.
  • Rarely lacking sticky organs, Scolex has 2 to 6 shallow suckers.
  • Testes are abundant, follicular, and dispersed in the proglottids’ mesenchyma and have a bilobed ovary.
  • Numerous and follicular vitellaria.
  • gonopores in the midventrum.
  • Bothriocephalus and Dibothriocephalus are examples.

Order 5. Taenioidea or Cyclophyllidea

  • parasitic in mammals, birds, and reptiles’ and amphibians’ intestines.
  • a large tapeworm.
  • Scolex produces four enormous eggs in cup-shaped suckers (acetabula), frequently with a hook-tipped apical rostellum.
  • Ovary with two or more lobed chambers; no uterine entrance.
  • On one or both edges, gonopores
  • There are four longitudinal veins that make up the excretory system.
  • Vitellaria, a solitary, compact yolk gland.
  • Taenia, Echinococcus, Hymenolepis, and Moniezia are other examples.


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

Leave a Comment

Your email address will not be published. Required fields are marked *