Flatworms, or the Phylum Platyhelminthes, are a group of soft-bodied, bilateral, flattened from top to bottom invertebrates. They are found in marine, freshwater, and terrestrial settings.
Flatworms are frequently hermaphroditic and lack a coelom (body cavity) and circulatory system (possessing both male and female reproductive organs). They differ in size from microscopic to several metres in length, and their body forms and adaptations are diverse.
Under the phylum Platyhelminthes, there are four primary classes: Turbellaria (free-living flatworms), Monogenea (fish-parasitic flatworms), Trematoda (parasitic flatworms found in many species, including humans), and Cestoda (parasitic flatworms known as tapeworms, also found in various animals, including humans).
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Characteristics of Phylum Platyhelminthes (flatworms)
- These organisms are either free-living, commensal, or parasitic.
- Triploblastic worms are bilaterally symmetrical and dorsoventrally flattened.
- Bilaterally symmetrical with head and tail ends having distinct polarity.
- The body of a triploblastic organism is composed of three embryonic germ layers: ectoderm, mesoderm, and endoderm.
- Dorsoventrally fatty, with a well-developed ventral surface including a mouth and gonopore.
- The morphology of their bodies ranges from somewhat elongated and flattened to lengthy ribbon-like and leaf-like.
- They range in size from tiny to enormously elongated forms measuring up to 10 to 15 metres in length.
- Only members of the class Cestoda have segmented bodies.
- The bulk of them are white, colourless, and obtain their colour from the food they consume, but the free-living forms are grey, brown-black, or vividly colourful.
- The anterior portion of their bodies is differentiated into a head.
- In turbellarians, the mouth and genital apertures on the ventral surface are well-defined, whereas in cestodes and trematodes, they are less distinct.
- Their parasitic form is characterised by sticky features such as hooks, spines, and suckers, as well as adhesive secretions.
- The body is coated with cellular or syncytial, often ciliated epidermis, whereas trematodes cestodes lack epidermis and have a cuticle-covered body.
- As both the exo- and endoskeleton are lacking, the body is typically soft. There are cuticle, spines, thorns, hooks, and teeth on the hard section.
- They are acoelomate, which means they lack a body cavity.
- Special mesodermal tissues, the mesenchyme, and parenchyma fill the interstitial spaces between organs.
- Their digestive system is branching and incomplete in the absence of an anus, which is lacking in acoela and cestode.
- There are no skeletal, respiratory, or circulatory systems present.
- The excretory system consists of a lateral canal and one or two protonephridia containing flame cells or bulbs. In some basic form, absent.
- Their nervous system is rudimentary and resembles a ladder. The primary nervous system comprises of a pair of ganglia or the brain, one to three pairs of longitudinal nerve cords, and transverse nerves.
- Their sensory organs are rudimentary. A prevalent occurrence in tubellaria that is drastically diminished in parasitic form. Chemoreceptors and tangoreceptors are often ciliated pits and grooves.
- They are primarily monogamous (hermaphrodite).
- The majority of their reproductive systems are extremely advanced or sophisticated.
- In many freshwater turbellaria, asexual reproduction occurs by fission.
- In their most common form, eggs lack yolk. They are formed independently by the egg yolk and the vitelline glands.
- Cross-fertilization occurs in trematodes, whereas self-fertilization occurs in cestodes.
- Their complex life cycle requires one or more hosts.
- Trematodes and tapeworms exhibit parthenogenesis and polyembryony often.
- Some tapeworms reproduce through endogenous and extrinsic budding.
- Either the flatworm is free-living, ecto- or endocommensal, or parasitic.
Classification of Phylum Platyhelminthes (flatworms)
With a few alterations, the classification is based on Hyman, L.H. (1951) up to the suborder level only.
Class 1- Turbellaria (L., turbella= a little string)
- Certain ectocommensals and endocommensals, as well as parasites, are classified as terrestrial marine or freshwater organisms.
- Unsegmented and covered with ciliated cellular or syncytial epidermis, Mouth ventral has mucus-secreting cells and a rod-shaped body. Before the intestine is the muscular pharynx.
- Many adhesive organs (suckers) are present.
- Sense organs, such as Tango, chemo, and photoreceptors, are typical of free-living organisms.
- The flame cells comprise the protonephridia of the excretory system.
- Reproduction is predominantly sexual, asexual, and regenerative.
- Life cycle simple.
Order 1- Acoela
- Tiny, marine-only, and smaller than 2 mm in length.
- No muscular pharynx and no gut; a ventral mouth.
- The excretory system is lacking entirely.
- There are no flame cells, gonads, gonoducts, and yolk glands are present.
- Some reside in the intestines of sea-urchins and sea-cucumbers. Usually free-living, found under stones or bottom mud, algae, and in the digestive tracts of sea-urchins and sea-cucumbers.
- Several brown or coloured by symbiotic algae.
- Examples: Convoluta, Ectocotyle, Afronta.
Order 2- Rhabdocoela
- Tiny freshwater, marine, and terrestrial organisms (less than 3 mm).
- Pharynx and intestinal sacs devoid of diverticula.
- The nervous system has two principal longitudinal trunks.
- Excretivum of Protonephridia
- Eye is typically present.
- Few compact gonads, gonoducts, and a cuticularized structure replace the penis papilla in the reproductive system. Yolk gland present or absent.
- Freshwater, marine, or terrestrial. Forms that are free-living, commensal or parasitic
- Examples: Catenula, Microstomum, Macrostomum, Mesostoma.
Suborder 1. Notandropora
- Uniquely freshwater organisms
- Uncomplicated pharynx
- The excretory system consists of a single protonephridia in the median position.
- The testes are a single, compact mass, and the penis is unarmed.
- No yolk gland.
- In conjunction with the creation of the chain of zooids, asexual fission occurs.
- Examples: Catenula,
Suborder 2. Opisthandropora
- Forms in freshwater or marine environments.
- The excretory system is composed of two nephridia.
- Testes are small and the penis has a stylet.
- No yolk gland.
- Reproduction asexually through a chain of zooids.
- Examples: Macrostomum, Microstomum.
Suborder 3. Lecithopora
- Freshwater, marine, and terrestrial species exist.
- Pharynx with an abundance of glandular tissue.
- The excretory system is composed of two nephridia.
- Disconnect the ovary from the yolk glands.
- Reproduction is sexually exclusive.
- Mainly free-living, with a few commensal and parasitic species.
- Examples: Anoplodium, Mesostoma.
Suborder 4. Temnocephalida
- Forming are freshwater ectocommensals.
- The anterior end of the body is furnished with two to twelve tentacles.
- The posterior end of the body is equipped with one to two sticky discs.
- Dolii create the pharynx.
- Simple gonopore.
- Examples: Temnocephala, Monodiscus.
Order 3- Alloecoela
- Moderate-sized between 1 and 10mm.
- Typically, marine, fresh, and brackish water form.
- Pharynx basic, Bulbose or plicate; straight or branching intestine (short diverticula).
- The excretory system is comprised of paired protonephridia with two or three primary branches and nephridiopores.
- Nerve system with three or four pairs of longitudinal nerve cords with transverse connectives.
- Several testes and a pair of ovaries constitute the reproductive system.
- Penis papilla is predominant.
- Some are habitually ectoparasitic or ectocommensal.
- Prorhynchus, Plagiostomum, and Geocentrophora are among examples.
Suborder 1. Archophora
- The marine form
- Expand the pharynx.
- Primitive female reproductive system, no female ducts.
- The male copulatory apparatus has a simple posterior entrance.
- Examples: Proporoplana (only examples).
Suborder 2. Lecithoepitheliata
- Forms may be marine, freshwater, or terrestrial.
- Pharynx with a simple or bulbous structure.
- Penis containing a cuticular stylet.
- Simple or none female ducts.
- No yolk glands.
- Ovoe are surrounded by nutritive cells.
- Examples include Prorhynchus and Geocentophora.
Suborder 3. Cumulata
- Forms in freshwater or marine environments.
- Pharynx with a bulbous or plicate appearance.
- Intestine typically devoid of diverticula.
- Unarmed genitalia
- Germovitellaria, or distinct ovaries and yolk glands, constitute the female reproductive system.
- Hypotrichina are some examples.
Suborder 4. Seriata
- Mostly marine and freshwater species.
- Expand the pharynx.
- Typically, the intestine has lateral diverticula.
- Separate ovaries and yolk glands make up the female reproductive system.
- Statocyst is predominant.
- Examples include Otoplana and Bothrioplana.
Order 4- Tricladida
- Large-sized turbellarians (2 to 60cm long).
- Forms may be marine, freshwater, or terrestrial.
- The mouth is mid-ventral.
- Normally, the plicate of the pharynx faces backward.
- Each branch of the three-branched intestine has several diverticula.
- Usually, eyes are present.
- Several nephridiopores are seen in the lateral networks of protonephridia.
- The male reproductive system includes two or more testes and a penis papilla.
- The female reproductive apparatus is made up of two ovaries with yolk glands and a copulatory brusa.
- Solitary gonopore.
- Examples: Gunda, Dugesia, Bdelloura, Geoplana.
Suborder 1. Maricola
- Entirely marine in nature
- Eyes and auricular grooves are present.
- Normal penis papilla is occasionally equipped with a stylet.
- There are rounded copulatory brusa.
- There is only sexual reproduction.
- Examples: Bdelloura,
Suborder 2. Paludicola
- Forming predominantly freshwater, brackish water is rare.
- Eyes are either in excess or lacking.
- Brusa is typically located anterior to the penis.
- Asexual reproduction predominates.
- Examples: Planaria or Dugesia.
Suborder 3. Terricola
- Forms terrestrial, tropical, and subtropical
- Mostly elongate body shape.
- 2 excess eyes
- Brusa is scarcely present.
- Male and female antra are often distinct.
- Asexual reproduction is also a possibility.
- Examples: Bipalium, Geoplana.
Order 5- Polycladida
- Moderate -sized turbellarians (2 to 20 mm).
- Many bottom dwellers or littoral zones are marine.
- Very branching intestine and throat.
- Many nerve cords are organised radially throughout the nervous system.
- Many eyes.
- Male and female gonopore separate.
- No yolk glands.
- Many and dispersed testes and ovaries are observed.
- Examples: Leptoplana, Notoplana, Cestoplana, Planocera, Thysanozoon.
Suborder 1. Acotylea
- Typically vertical pharynx curtain.
- Absent suckers behind the gonopore.
- Tentacles with a nuchal configuration.
- On the anterior edge, eyes never appear as a pair of clusters.
- Examples: Euplana, Leptoplana, etc.
Suborder 2. Cotylea
- Tubular pharynx.
- The presence of a sucker behind the female pore.
- At the anterior margin, a pair of tentacles bearing eyes or a cluster of eyeballs.
- Examples: Thysanozoon, Yungia.
Class 2- Trematoda (Gr., trematodes= having pore)
- Ectoparasitic or endoparasitic, sometimes known as
- Body unsegmented dorsoventrally flattened leaf-like.
- Dense epidermis devoid of cilia and rhabdites.
- Undivided body with cuticle covering.
- Suckers and occasionally hooks are found.
- An incomplete digestive tract consists of an anterior mouth, a simple throat, and two forked or several branches of intestine; the anus is lacking.
- There are three pairs of longitudinal nerves.
- The flame cells make up the protonephridial excretory system.
- Most of hermaphrodites (monoecious).
- One ovary and more than two testes.
- Alternation of hosts leads to direct (in ectoparasites) or indirect (in endoparasites) development.
Order 1. Monogenea
- Mostly ectoparasites of aquatic cold-blooded animals.
- Oral suckers are either missing or weak.
- Provided with a pair of adhesive structures at the front end.
- Posterior end with a sticky disc and typically hooks.
- A pair of excretory holes located anteriorly on the dorsal side.
- Male and female gonopores are often distinct.
- Vaginal number one or two. The uterus is tiny and contains a few eggshells.
- There is only one host during the life cycle.
- Examples of free-swimming, ciliated larvae are Diplozon, Polystoma, Gyrodactylus, and Dactylogyrus.
Order 2. Digenea
- Vertebrate and invertebrate endoparasites.
- Oral sucker surrounding the mouth and ventral sucker or acetabulum.
- A posterior excretory pore that is unique.
- No vagina. The uterus is typically lengthy and contains many shelled eggs.
- The complex life cycle includes numerous larval stages.
- There may be one or more intermediate hosts during the life cycle.
- Asexual reproduction occurs in larval stages prior to metamorphosis.
- Fasciola, Bucephalus, Opisthorchis, Paragonimus, and Schistosoma are some examples.
Order 3. Aspidocotylea (=Aspidogastraea)
- Lack of oral suckers
- Huge ventral suckers that have been partitioned into many suckers without hooks.
- One testis is present in the male system.
- Endoparasites in fishes and reptiles’ digestive tracts.
- Aspidogaster, Cotylapsis, and Stichocotyle are some examples.
Class 3- Cestoda (Gr., ketos, gridle+ eidos, form)
- Endoparasites of vertebrate intestines.
- Usually known as tapeworms.
- Body frequently segmented (proglottids), but infrequently undivided, elongated, flat, and ribbon-like.
- Skin lacking microvilli.
- Without epidermis or cilia, the cuticle covers the body.
- Except for cestodaria, the anterior end (scolex) is equipped with adhesive features (hooks, suckers).
- Lacking a mouth and digestive system entirely.
- The excretory system is composed of protonephridia with characteristic terminal flame
- Typically, the nervous system consists of two ganglia and two lateral longitudinal nerve cords.
- Each mature segment or proglottid has both male and female reproductive organs.
- Often, the life cycle of a parasite involves two or more hosts.
- Embryos sporting hooks.
Subclass 1. Cestodaria
- Coelom or intestinal endoparasites of vertebrates.
- Unsegmented, leaf-like body lacking scolex and strobila (monozoic).
- No alimentary canal.
- There is only one pair of reproductive organs in a monoecious species.
- The larval lycophore has ten hooks.
Order 1. Amphilinidea
- There are endoparasitic organisms in the coelom of fishes.
- Flattened, oval, or elongated body
- No fools.
- Scolex missing.
- Pharynx protrusible
- The anterior region contains frontal glands.
- Both male and female pores are located posteriorly.
- The uterus is tightly coiled and opens near its anterior end.
- Examples: Amphilina.
Order 2. Gyrocotylidea
- Endoparasitic organisms inhabit the intestinal tract of fish.
- Body lengthened and compressed.
- There is an anterior sucking organ and a posterior rosette-shaped sticky organ.
- The anterior end has a retractable proboscis.
- The uterine, male, and vaginal pores are all located in the frontal region of the body.
- Short, straight uterus leads directly to pores.
- Examples: Gyrocotyle.
Subclass 2. Eucestoda
- Endoparasites inhabit the intestinal tract of fish.
- Body long, ribbon-like.
- Many proglottids are located in the scolex, neck, and strobila of the body (polyzoic).
- Scolex expanded sticky bearing structures.
- Often possessing many pairs of monoecious reproductive organs.
- Larva having six hooks.
Order 1. Tetraphyllidea
- Endoparasitic forms found exclusively in the intestinal tract of elasmobranch fishes.
- Scolex with four bothria resembling leaves (sessile suckers) is frequently supplied.
- The testes precede the ovaries.
- Vitelline glands dispersed.
- Armed with spines and hooks, Cirrus.
- Common genital atrium marginal.
- Phyllobothrium and Myzophyllobothrium are two examples.
Order 2. Diphyllidea
- Intestinal parasites of elasmobranch fishes.
- Scolex with two bothria and a head stalk with spines.
- Strobila contains less than twenty proglottids.
- Instances: Echinobothrium
Order 3. Trypanorhyncha
- Parasitic in the spiral valve of the elasmobranch digestive tract.
- A body of average size.
- Scolex containing four bothria and four protruding spiny proboscides.
- Vitellaria in continuous layers of cortical parenchyma.
- The testes extend posteriorly behind the ovary.
- Lateral gonopores; ventral uterine opening.
- Haplobothrium and Tetrarhynchus are examples.
Order 4. Pseudophyllidea
- Intestinal parasites of teleost fishes and terrestrial vertebrates.
- Body separated into strobila or unsegmented.
- Scolex has 2 to 6 shallow bothria (Suckers) that are infrequently devoid of sticky organs.
- Many, follicular testes are dispersed throughout the mesenchyme of proglottids.
- Vitellaria follicular, many.
- Midventral gonopores.
- Bothriocephalus and dibothriocephalus are examples.
Order 5. Taenioidea or Cyclophyllidea
- Intestinal parasite of reptiles, birds, and mammals.
- Sizeable tapeworms
- Scolex often produces four larges in cupped suckers (acetabula) with a rostellum armed with hooks at its apex.
- Ovary with two or more lobes; absence of uterine entrance.
- On one or both edges, gonopores.
- This system consists of four longitudinal vessels.
- The vitellaria (egg-yolk gland) is solitary and compact.
- Examples: Taenia, Echinococcus, Hymenolepis, Moniezia.
Physiological Processes of Flatworms
- The species of free-living flatworms are predators or scavengers. Parasitic forms feed on their hosts’ tissues. The majority of flatworms, such as the planarian depicted in Figure 1, have a gastrovascular cavity as opposed to a complete digestive system.
- The “mouth” of these animals is also utilised to discharge waste from the digestive system. Several species have an anal entrance as well. The digestive tract might be a simple sac or greatly branching.
- Extracellular digestion occurs, with phagocytosis bringing digested components into the cells of the intestinal lining. The cestodes do not have a digestive system.
- The excretory system of flatworms consists of a network of tubules with openings to the environment and surrounding flame cells whose cilia beat to drive waste fluids accumulated in the tubules out of the body.
- The system is in charge of regulating dissolved salts and excreting nitrogenous wastes. The nervous system consists of a pair of nerve cords along the length of the body with connections between them and a big ganglion or concentration of nerves at the anterior end of the worm, where photosensory and chemosensory cells may also be concentrated.
- There is neither a circulatory nor a respiratory system; diffusion and cell-cell junctions are responsible for gas and nutrient exchange. This constrains the body thickness of these organisms, limiting them to being “flat” worms.
- Most species of flatworms are monoecious, and fertilisation normally occurs internally. Asexual reproduction is prevalent in certain populations.
Parasitic adaptations of Phylum Platyhelminthes
Taenia is an endoparasitic worm that inhabits human internal organs such as the intestine. To accommodate its parasitic lifestyle, it must overcome a number of unfavourable situations. And so, this tapeworm exhibits many modifications. The following are Taenia’s significant parasitic adaptations:
- Externally, the body is coated by tegument, which protects against the digestive activity of the host’s alkaline digestive fluids. This tegument is water and nutrient permeable.
- Within the worm, the osmotic pressure is greater than that of the surrounding host fluid. This facilitates the host’s rapid absorption of the digested meal.
- Adults and larvae lack cilia because neither need movement.
- A well-organized and developed scolex with suckers and spines is present. These suckers and spines aid in attachment, preventing the parasite from being expelled from the host’s colon as a result of peristaltic contractions.
- They inhabit places with a constant supply of pre-digested food material that is easily absorbed by the parasite. Thus, there is no need for a digestive tract. In order to improve the absorption surface area, the tegument is also changed into microvilli.
- There are no circulatory, respiratory, or sensory organs in these parasites.
- The nervous system is likewise underdeveloped since it is unnecessary.
- The reproductive system is the most developed system. An enormous quantity of eggs are produced to overcome dangers and difficulties.
- The zygotes and embryos are protected from unfavourable conditions by a protective shell.
- Hermaphroditism and proglottization ensure self-fertilization or cross-fertilization between proglottids of the same worm.
Examples of Platyhelminthes
- Dugesia (Planaria): They are found in ponds and slow-moving streams. Their body has cilia and the ability to regenerate the destroyed section. There are two eyes and two lateral lobes on the head.
- Schistosoma: It is known as blood fluke because it is discovered in the mesenteric blood vessels and hepatic portal system of humans. It exhibits considerable sexual dimorphism. Schistosoma causes Schistosomiasis, which spreads by water that has been contaminated. The patient has anaemia, discomfort, fever, enlarged liver and spleen, and diarrhoea.
- Fasciola: It is sometimes referred to as a liver fluke since it inhabits the liver and bile duct of sheep and goats. It is hermaphrodite, yet there is cross-fertilization. It causes animals to develop fascioliasis. With this condition, the animal’s liver enlarges and the bile ducts become obstructed. The infection weakens the animals’ muscles, resulting in muscular pain that may have deadly effects for the animals.
- Taenia solium: It is common in all nations where pork is consumed and is also known as the pork tapeworm. They reside as parasites in the human small intestine, and their larvae are detected in the pigs’ muscles. It is hermaphrodite and self-fertilization occurs. Taenia solium causes taeniasis, characterised by abdominal pain, anaemia, indigestion, restlessness, area, and false appetite. Other organisms such as Taenia saginata and Echinococcus granulosus are transmitted through beef in the human intestines and reside in the intestines of cats and dogs, respectively.
Significance of flatworms
- Decomposition: Flatworms serve a crucial role in decomposing organic debris, such as dead plants and animals, into compounds that can be taken by other creatures in the environment. This makes them crucial decomposers in numerous habitats.
- Food chain: Flatworms are a part of the food chain in numerous environments. They are consumed by numerous predators, including fish, birds, and insects, which facilitates the movement of energy and nutrients up the food chain.
- Research: Due of their regenerating qualities, flatworms are frequently employed in scientific studies. They are investigated in order to gain a deeper understanding of tissue regeneration and stem cell biology, which may have implications for future medicinal treatments.
- Parasitism: Although parasitic flatworms can cause harm to people and other animals, they also perform a crucial role in the ecosystem by reducing populations of other organisms that may be considered pests, such as snails.
- Bioindicators: Flatworms, which are sensitive to environmental changes such as pollution and habitat loss, can function as bioindicators of ecosystem health. Monitoring flatworm populations can provide insight into an ecosystem’s overall health.
What is Phylum Platyhelminthes?
Phylum Platyhelminthes, commonly known as flatworms, is a group of soft-bodied, bilateral animals that are flattened from top to bottom. They are found in both marine and freshwater environments, as well as on land.
How many species of flatworms are there?
There are over 20,000 known species of flatworms, but it is estimated that there may be as many as 100,000 species that have not yet been discovered.
Are flatworms parasitic?
Not all flatworms are parasitic, but some species within the phylum Platyhelminthes are parasitic and can cause diseases in humans and other animals.
Do flatworms have a circulatory system?
Flatworms lack a true circulatory system and instead rely on diffusion to transport oxygen and nutrients throughout their bodies.
What is the body shape of a flatworm?
Flatworms are flattened from top to bottom, giving them a ribbon-like appearance. They exhibit a wide range of body shapes and adaptations.
Are flatworms hermaphroditic?
Most flatworms are hermaphroditic, meaning that they possess both male and female reproductive organs.
How do flatworms reproduce?
Flatworms reproduce sexually by exchanging sperm with their partners or asexually by regeneration.
What is the significance of flatworms in the ecosystem?
Flatworms play important roles in the ecosystem as both predators and prey. Some flatworms are scavengers, while others help to break down dead organic matter.
Can flatworms regenerate lost body parts?
Yes, many flatworms have the ability to regenerate lost body parts, including their heads, tails, and internal organs.
Are flatworms harmful to humans?
Some flatworms, particularly those in the class Trematoda and Cestoda, can be harmful to humans and cause diseases such as schistosomiasis and tapeworm infections. However, not all flatworms are harmful to humans.
- 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.