Phylum Platyhelminthes – The Classification And Characteristics Of Flatworms

Platyhelminthes is a phylum of free-living worms known as flatworms.

Flatworms are a group of soft-bodied bilaterian, and unsegmented invertebrates that don’t feature a body cavity.

Phylum Platyhelminthes - The Classification And Characteristics Of Flatworms

These Platyhelminthes are typically found in moist terrestrial or marine environments, including freshwater.

Here is the ultimate guide to the classification and characteristics of flatworms.

Classification Of Platyhelminthes

Interestingly, there are over 20,000 species of flatworm recognized, including tapeworms, hymenolepsis, girardia tigrina, polycladida, and turbellaria.

This makes the Platyhelminthes phylum one of the largest phyla in the world, after mollusks, arthropods, and chordates.

Flatworms are part of the animal kingdom (Kingdom Animalia), more specifically characterized by the multicellular eukaryotic organisms.

However, some classifications classify flatworms under Eumetazoa, which is the basal animal clade.

Interestingly, Platyhelminthes also fall under the category Bilateria in Eumetazoa, which includes animals with a head and a tail, as well as a stomach and dorsal area.

There are four classes of Platyhelminthes.

These include Monogenea, which are the ectoparasites of fish, Turbellaria, which are the free-living marine species, Cestoda, also known as tapeworms, and Trematoda, which are the internal parasites of species including humans.


Monogenea is a category of flatworms (defined as ectoparasitic) found on fish, including the gills, fins, and skin.

These flatworms don’t require a host as they have a direct lifecycle, plus the adults possess both female and male reproductive organs.

In terms of how they reproduce, some Monogeneans are viviparous while others are oviparous, meaning they will either produce offspring live or in egg form.

While viviparous Monogeneans release larvae (which then attach to the host, in this case a fish), oviparous Monogeneans will release eggs directly into the water.

When a fish becomes infected with flatworms, they exhibit symptoms of lethargy, such as swimming up to the surface of the water rather than in a straight line, and gulping for air.

The infected area is often rubbed against underwater surfaces, resulting in scale loss, swollen gills, and pink fluid.

Interestingly, Monogeneans share similar traits to trematodes, even though they are in a different class.

The key difference between them is that Monogeneans have a posterior organ called a haptor or opisthaptor.

This posterior organ is adhesive thanks to the presence of hooks, which helps the parasite attach to the host.

Monogeneans don’t all look the same – some species are cylindrical and small, while others are larger and flatter.

Some even resemble the shape of a leaf.

All Monogeneans possess raptors of varying sizes according to their overall size, which helps to adhere to the gills, fins, and even the oral cavity.

This is the best way to feed off the outer epidermal layer.

Despite the varying body shapes and sizes, Monogeneans can both shorten and elongate their bodies depending on their environment to help them move.

Interestingly, Monogeneans also don’t have a circulatory system, respiratory system, or an anal opening, meaning they rely on a protonephridial system to excrete waste.

However, they do possess a nervous system built up of nerves and a nerve ring.


Consisting of roughly 3,000 species across 10 orders, class Turbellaria includes free-living organisms and parasitic members that thrive in moist conditions, including freshwater, marine, and tropical terrestrial environments.

Temnosewellia minor is a good example of an opportunistic (free-living) species.

This species uses their strong suckers to attach themselves to their chosen host, specifically crayfish.

They attach themselves to the tail and then feed on the smaller organisms according to where the fish goes.

Free-living means the species doesn’t rely on the host to survive.

They don’t harm the host itself, they simply use the crayfish as a mode of transport to source environments with their chosen food sources.

Parasite species, such as Icthypophaga subcutanea, rely heavily on hosts for survival.

They live below the skin of the host and slowly eat away at the host to gain their nutrition. This is what leads to the rotting of scales, gills, and fins.

Interestingly, most of the species in class Turbellaria feed on smaller invertebrates. However, others are scavengers, herbivores, and ectoparasites.

Turbellaria species have long, soft bodies that are tapered at both ends and a compressed middle section, making the surface area to volume ratio high.

In terms of appearance, terrestrial species are dull, while marine species are more colorful, exhibiting red coloration with yellow spots.

The pigment cells are located in their eyespots along with photoreceptors, so they don’t form images with their eyes.

Most species of this class rely on coordinated cilia to move around, which are located on the mucus trail.

Other species, however, rely on their muscles to contract for locomotion.

All species in this class don’t possess an anal opening, meaning they take in food through the pharynx and eject waste through their mouths.

Some species of Turbellarians have a peripheral nervous system that feature a complicated web of interlacing nerves, which are responsible for muscle movement.

In terms of reproduction, there are two key methods – reproduction through simultaneous hermaphrodites and asexual reproduction through transverse fission.

The sexual reproduction forms bundles of cocoons of eggs, which are either born into the water or on a host as larvae.



Composed of roughly over 4,000 species, Cestoda is a class of species commonly referred to as tapeworms.

These are internal worms that rely on multiple hosts to survive their life cycles.

These species are also characterized by their extremely long bodies, which can grow up to 18 meters long.

Tapeworms have notoriously complex life cycles.

They are hermaphrodites as they feature both male and female reproductive structures in proglottids, allowing them to reproduce thousands of eggs during their lifetime.

Their young also produce up to a thousand proglottids, allowing them to reproduce in other hosts.

Hosts ingest the eggs, making them the intermediate host.

The larvae then develop in a second host, known as the definitive host, before maturing and reproducing again.

Unlike Turbellarians and Monogeneans, species in the Cestoda class don’t have a digestive system.

Their bodies are covered by microvillus-like projections that absorb nutrients on the surface of their bodies.

This is why most tapeworms make their way to the host’s small intestine, as this is the easiest place to obtain nutrition.

Some Cestoda species include Taenia saginata, Diphyllobothrium latum, Taenia solium, Dipylidium caninum, and Hymenolepis nana.

There are two subclasses in Cestoda: Subclass Cestodaria and Subclass Eucestoda.

Subclass Cestodaria

There are about 15 species in this subclass, all of which are found in the intestine of primitive fish.

They are characterized as unsegmented and feature a single set of reproductive organs (meaning they aren’t hermaphrodites), no digestive system, parenchymal muscle cells, and suckers known as haptors.

Amphilinidea, Gyrocotylidae, and Caryophyllidae are the orders in Subclass Cestodaria.

Subclass Eucestoda

Most Cestodes belong to the Subclass Eucestoda. These species are known as true tapeworms.

Each tapeworm species features a body divided into a scolex including suckers to attach to the host, a neck, and strobila.

The strobila is made up of proglottids, which are essential for reproduction. Proglottids are a self-sufficient reproduction unit.

There are several orders that make up this subclass, including Diphyllidea, Nippotaeniidea, Tetreaphyllidea, Lecanicephalidea, Trypanorhyncha, and some others.


Trematoda is a class consisting of roughly 20,000 identified species, all of which are parasitic and known as flukes.

These species have separate species, meaning they aren’t self-producing hermaphrodites.

Interestingly, flukes are commonly found in the circulatory system or liver of the host, and they have a fantastic muscular system for locomotion and good excretory and alimentary systems.

They also feature ventral suckers, known as haptors, to attach themselves to the host.

There are two subclasses of Trematoda: Subclass Digenea and Subclass Aspidogastrea.

Subclass Digenea

Subclass Digenea consists of over 18,000 nominal species that belong to around 150 families.

These species have a complex life cycle, meaning they rely on intermediate hosts as well as definitive hosts, such as mollusks (intermediate) and vertebrates (definitive).

The species can be found in all vertebrate classes, resulting in the spread of disease in both animals and humans.

They are considered syncytial animals, meaning they typically create respiratory issues in the host.

The way they attach themselves to a host is with a ventral sucker as well as a mouth sucker, allowing for extreme adhesion.

There are four orders in this subclass, including:

  • Plagiorchida, which is characterized by a slender probe called a stylet on the cercaria
  • Strigeiddida, which is characterized by a fork-tailed-shaped cercaria
  • Opisthorchiidae, which are species possessing one unarmed cercaria
  • Echinostomida, wherein the life cycle relies on three different hosts rather than two

Key Terms Explained

So, now we’ve looked at the official classifications of platyhelminthes, you’re probably a bit overwhelmed with all the complicated words.

Here’s a quick list of important keywords to refresh your mind.

  • Proglottid – This is a segment of the tapeworm that contains both female and male reproductive organs, making them self-reproducing hermaphrodites.
  • Acoelomate – This is when a species lacks a coelom, so they don’t possess a fluid-filled cavity.
  • Ectoparasite – This is when a parasite just lives on the surface of a host rather than inside the host.
  • Endoparasite – This is when a parasite lives beneath the surface of a host rather than on the surface.
  • Scolex – A structure towards the end of an adult tapeworm that consists of suckers and hooks used to attach the species to a host.

Phylum Platyhelminthes Characteristics

While each class in the phylum Platyhelminthes features species with their own characteristics, here are the key general characteristics of flatworms.

  • Flatworms are triploblastic organisms, featuring three embryonic germ layers. These layers include the outermost germ layer (the ectoderm), the innermost germ layer (endoderm), and the central germ layer (the mesoderm).
  • As they lack both an exoskeleton and endoskeleton, flatworms are considered invertebrates. The only hardened parts of their bodies are the hooks, teeth, and spines, meaning they are completely soft-bodied.
  • These species are bilaterally symmetrical organisms, meaning they can be divided into equal parts. The multicellular bodies display an unsegmented structure, meaning they don’t possess a body cavity or circulatory or respiratory systems. This leaves the flatworms flat, hence the name!
  • Flatworms adhere to hosts thanks to hooks and suckers, which are attachment organs. They also possess adhesive secretions from the eosinophilous and cyanophilous glands.
  • Not only do they lack a body cavity, but true flatworms also don’t have a digestive tract. The nervous system is also primitive and shaped like a ladder, with two nerve cords running down the whole body with nerve connections between them.
  • While they don’t have eyes, the nervous system features a concentration of nerves at the anterior end of the organism, which might have a concentration of photosensory cells.
  • As flatworms don’t possess a respiratory or circulatory system, the gas and nutrient exchange is completely dependent on cell-cell junctions and diffusion. This is another reason why flatworms are so flat.

How Do Flatworms Digest Nutrients?

How Do Flatworms Digest Nutrients?

So, if flatworms lack a digestive system, how do they digest the nutrients they consume from their hosts?

Instead of a digestive system, most flatworms possess a gastrovascular cavity.

This works to expel waste materials from the mouth opening, while it consumes nutrients through the pharynx.

Cestodes (tapeworms) are the key group of flatworms that lack a digestive system.

Instead of relying on anal openings to excrete waste, they have a network of tubules that allow for small openings to the environment.

This is accompanied by nearby flame cells, wherein the cilia help to direct the waste fluids, that have been concentrated by the tubules, out of the body.

This system also regulates dissolved salts as well as the excretion of nitrogenous wastes.

To put it simply, flatworms will take in nutrients through their mouths and then rely on either anal openings to excrete waste, or their mouths.

While most flatworms don’t typically posses anal openings, some species still do.

How Do Flatworms Reproduce?

The majority of flatworm species are hermaphroditic, meaning they have both male and female reproductive systems.

This means that flatworms can reproduce either sexually or asexually depending on the species.

In most cases, flatworms will reproduce through cross-fertilization.

This sexual reproduction is where two individuals fertilize each other, allowing for both to reproduce.

The eggs are then stored inside each flatworm, and one of two things happens.

The eggs are either laid into the marine life after being retained inside the parent, or they are laid onto the host in the form of larvae.

In some cases, flatworms will reproduce over winter, wherein the eggs are almost always retained inside the parent until spring.

This is to heighten the chances of successful reproduction as the environments get warmer.

Asexual reproduction of flatworms can be completed with two methods known as budding and transverse fission.

This type of reproduction is ideal for flatworms who reside in stressful environments, or when they can’t find a suitable mating partner.

Budding is where buds grow along the body of the parent.

These buds eventually form chains before turning into a fully formed fetus, wherein they then separate into new flatworms.

Transverse fission is when the posterior half of the flatworm is attached to a substrate, and the anterior half moves away from the posterior.

The two halves then pull apart to regenerate into two complete worms.

There are even a few species of flatworms that can split themselves into more than one piece to regenerate into multiple worms.

The lifespan of a flatworm is largely unknown, but the rough estimate from captive members is approximately 65 to 140 days per worm.

What Do Flatworms Eat?

Depending on the species, flatworms can be scavengers, carnivores, or herbivores.

Scavengers and carnivorous flatworms mostly feed on animal matter including protozoans, bacteria, and small invertebrates.

As the name suggests, scavengers will eat just about anything that comes their way.

Herbivorous flatworms are pretty rare, but the few flatworm species that are herbivores have a diet consisting of eating only plant matter, which is mostly microalgae.

They will reside on the surface of hosts to gather food in different environments rather than eat the bacteria on the host itself.

Carnivorous flatworms will also utilize their flat bodies to slip into the shells of clams and mollusks before they eat the animal.

However, most herbivorous species will switch from their plant matter diet to a carnivorous diet as they get older.

The reason for this is uncertain, but it could be to support their bodies. For the majority of flatworm species, flatworms rely on their mouths to swallow food.

They will either swallow the food whole, bite it into chunks, or they will simply suck the fluids out of the host.

For endoparasites, most will reside inside the small intestine of the host, as this is the best place to get nutrients.

Flatworms And Humans

Tapeworms and flukes are the two main types of flatworms that come into contact with humans, as they are predominantly terrestrial species.

When residing on a human or animal host, they cause diseases such as schistosomiasis, which is the second-most deadly disease caused by parasites behind malaria.

While the disease has a low mortality rate, schistosomiasis damages internal organs, resulting in chronic illnesses.

This disease is most often caused by several flukes of the namesake genus Schistosoma, and they travel through water.

Livestock and humans who work in organic farming are at the highest risk of becoming the host to fluke and tapeworms.

This is because of the exposure to natural fertilizers like sewage sludge and manure, as well as eating seafood and uncooked meat from high-risk areas.

Developed countries that are highly polluted are the most common places to spread parasitic flatworms.

However, some have been introduced to the United States for intentional infection by people who want to lose weight quickly.

What Do Flatworms Look Like?

Flatworms vary greatly in size and appearance, but the one similarity is that – as the name suggests – all flatworms are distinctly flat.

However, with over 13,000 species of flatworm, it’s hard to say what they all generally look like.

Tapeworms are the most well-known species of flatworm known for growing to incredible lengths of up to 18 meters.

Some species are more large than long, with some resembling the shape of a leaf.

Others exhibit interesting colors and patterns, like the oyster flatworm that is cream with short side tentacles and small eyespots on its head.

In most cases, terrestrial flatworms are dull. There is no clear reason for this other than the bacteria they eat.

Marine and freshwater flatworms typically have more color pigmentation, leaving them reddish with orange or yellow spots. Some can even be purple.


So, there you have it!

Hopefully, this guide to the classification and characterizations of the Platyhelminthes phylum has taught you everything there is to know about flatworms.

Jennifer Dawkins

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