Flatworms - Phylum Platyhelminthes

** Classification/Characteristics

Definition - What are Flatworms?

Flatworms (Platyhelminthes) are a group of bilaterally symmetrical, acoelomate, soft-bodied invertebrate animals found in marine, freshwater as well as moist terrestrial environments.


Flatworm species include:

  • Turbellaria
  • Tapeworms
  • Polycladida
  • Hymenolepis
  • Girardia tigrina



With more than 20,000 species currently identified, the phylum Platyhelminthes makes up one of the largest phyla after chordates, mollusks, and arthropods. On the other hand, they make up the largest phylum among acoelomates. 


Kingdom: Animalia - Flatworms fall under the animal kingdom that is characterized by multicellular eukaryotic organisms. In some classifications, they are also classified under the basal animal clade Eumetazoa since they are metazoans that fall under the animal kingdom (Kingdom Animalia).


Bilateria- Flatworms also fall under Bilateria in Eumetazoa. This classification includes animals with bilateral symmetry consisting of a head and tail (as well as a dorsal part and belly).


Protostomia- As members of this clade, flatworms comprise three germ layers. As such, they are also often referred to as protostomes.


Apart from these higher classifications, the phylum is further divided into the following classes:


  • Turbellaria
  • Monogenea
  • Cestoda
  • Trematoda

Class Turbellaria

Class Turbellaria is composed of about 3,000 species of organisms spread across at least 10 orders. While a majority of these species live in marine environments, there are many others that can be found in freshwater environments as well as tropical terrestrial and moist temperate environments. As such, they require at least moist conditions to survive. 

Depending on the species, members of class Turbellaria either exist as free-living organisms or parasites. For instance, members of the order Temnocephalida exist as either entirely commensals or parasites. 

One of the best examples of a free-living (opportunistic) species in this order is the Temnosewellia minor. In their environment, emnosewellia minor use their strong suckers to attach onto crayfish (at the tail section) from where they feed on various smaller organisms like protozoa wherever the fish goes. As such, they do not cause harm to the crayfish and only depend on the crayfish for transport and stirring up environments with food sources. 

Ichthyophaga subcutanea, on the other hand, has been shown to be a true parasite of such marine fish as Hexagrammus decagrammus. In such hosts, the parasite lives below the skin where it obtains its nutrition.

Other characteristics of Class Turbellaria


  • As worms, they have an elongated boy that is relatively soft. The body is also have tapered at both ends with reduced thickness compared to the central part of the body.
  • With a compressed dorso-ventral body section, Turbellaria species have a high surface area to volume ratio.
  • Whereas the marine species are very colorful (yellow spots, reddish in color etc.) those found in terrestrial and interstitial environments are largely dull in appearance.
  • For a majority of the species, locomotion is achieved through the use of well-coordinated cilia that repeatedly flap in one direction. These cilia are located on the secreted mucus trail and therefore conveniently located for movement purposes. For some of the species, however, movement is achieved through the rhythmic contraction of muscles.
  • They are not segmented
  • They are acoelomate. As such, Turbellaria species lack a coelom (a body cavity located between the body wall and intestinal canal in most animals).
  • They have sub-epidermal rhabdites in their ciliated epidermis that distinguish this class from other flatworms.
  • They lack an anal opening. As a result, food material is taken in through the pharynx and ejected through the mouth.
  • While a majority of species in this class are predators of smaller invertebrates, others live as herbivores, scavengers, and ectoparasites.
  • Pigment cells and photoreceptors present in their eyespots is used in place of image-forming eyes.
  • Depending on the species, the peripheral nervous system of Turbellarians ranges from very simple to complex interlacing nerve webs that control muscle movement.
  • Reproduction is achieved through a number of methods that include sexual reproduction (simultaneous hermaphrodites) and asexual reproduction (transverse fission). In sexual reproduction, eggs are produced and bundled into cocoons from which juveniles hatch and develop. Asexually, some of the species split into two halves that the regenerate to form the missing half thus developing into a whole organism.



Common orders belonging to class Turbellaria include:


  • Order Acoela - About 600 species found in marine environments
  • Order Neorhabdocoela - About 600 species characterized by a saclike intestine, two longitudinal trunks, and a nervous system
  • Order Catenulida - Consists of about 70 species found in freshwater
  • Order Macrostomida - 200 species often found between sand grains


Some of the other orders include:


  • Tricladida
  • Proseriata
  • Polycladida
  • Lecithoepitheliata
  • Kalyptorhynchia 

Class Monogenea

This is one of the largest groups of flatworms whose members as almost exclusively parasites of aquatic vertebrates (ectoparasites). 

Though they are grouped in a different class to trematodes, class Monogenea has been shown to have many similar traits to trematodes. However, they can be easily differentiated from trematodes and cestodes by the fact that they possess a posterior organ known as a haptor. 

Monogeneans vary in size and shape. For instance, whereas larger species may appear flattened and shaped like a leaf (leaf-shaped) the smaller ones are more cylindrical. Rather than an oral sucker, monogeneans have a large posterior adhesive disk known as opisthaptor that makes it possible to attach to the host. This structure consists of hooks that enhance attachment.

Using these structures, monogeneans are well able to attach to such animal parts as fins, gills as well as the oral cavity from where they can feed off the outer epidermal layer of the host.

The head of monogeneans at the anterior region may contain eyespots with pigments. This region of the body also has an oval shaped pharynx that is poorly developed.

Characteristics of Class Monogenea 

Some of the other characteristics of monogeneans include:


  • They are hermaphrodites - All members of class Monogenea are hermaphrodites. As such, they possess both male and female reproductive organs (testis and ovary) that are located at the central part of the body. Fertilized eggs are directly released in water where they hatch to produce free-swimming larva. To complete their life cycle, the larva infects a host as they develop to continue the cycle; Monogeneans do not have intermediate hosts in their life cycle.
  • Use adhesive organs known as haptors to attach to the host. This structure also consists of suckers, clamps or anchors.
  • Although they have specific body shapes depending on the species, they have been shown to be capable of elongating/shortening their bodies as they move in their environment
  • Do not have an anal opening and therefore use protonephridial system for the purposes of excreting waste products
  • Lack a respiratory and circulatory system but have a nervous system consisting of a nerve ring and nerves that extend to the posterior and anterior part of the body
  • As parasites, monogeneans often feed on the skin cells mucus as well as the blood of the host which causes damage to the mucous membrane and skin that protects the animal (fish). These infections can also affect other organs of the host including the gill tissue

Class Cestoda

Class Cestoda is composed of over 4,000 species commonly known as tapeworms. Typically, cestodes are internal worms (endoparasites) that require more than one host for their complex life cycles. 

Compared to the other types of flatworms, cestodes are characterized by their long flat bodies that can grow up to 18 meters long consisting of many reproductive units (proglottid).

Class Cestoda is further divided into two other subclasses that include:


Subclass Cestodaria - Members of this subclass (about 15 species) are unsegmented and can be found in the intestine (or body cavity in some cases) of primitive fish. Cestodaria species are characterized by a single set of the reproductive organ (either male or female), suckers, they lack a digestive system as well as parenchymal muscle cells.

This subclass consists of the following orders; Amphilinidea, Caryophyllidea, and Gyrocotylidea.


Subclass Eucestoda - The majority of cestodes belong to this subclass and are referred to as true tapeworms. The body of these organisms is divided into scolex (containing suckers and/or hooks), a neck as well as a strobila that consists of a series of units known as proglottids. For members of this subclass, the proglottids play an important role in reproduction. 

Subclass Eucestoda is composed of a number of orders that include: Tetraphyllidea, Lecanicephalidea, Diphyllidea, Trypanorhyncha, and Nippotaeniidea among a few others. With the subclass consisting of over 3,000 species, species in the different orders are differentiated by the number of suckers, presence or absence of sex ducts as well as a structure of the scolex among other features.

Characteristics of Class Cestoda

A complex life cycle - The body of true tapeworms consists of many segments known as proglottids. Each of the proglottids contains both a male and female reproductive structures (as hermaphrodites) that are capable of reproducing independently.

Given that a single tapeworm can produce as many as a thousand proglottids; this allows tapeworms to continue thriving. For instance, a single proglottid is capable of producing thousands of eggs, their lifecycle can continue in another host when the eggs are ingested.

Here, the host that ingests the eggs is known as the intermediate host given that it is in this particular host that the eggs hatch to produce a larvae (coracidium). The larvae, however, continues to develop in the second host (definitive host) and mature in the adult stage.


They lack a digestive system - Compared to the two other classes of flatworms, tapeworms lack a digestive system. Instead, the surface of their bodies are covered by small microvillus-like projections similar to those found in the small intestine of many vertebrates.

Through these structures, tapeworms effectively absorb nutrients through their outer covering (tegument). For this reason, a majority of tapeworms can be found in the small intestine of many of their hosts where they can easily obtain nutrition.

They have well-developed muscle.

Modified cilia on their surface are used as sensory endings.

The nervous system is made up of a pair of lateral nerve cords.


Examples of cestodes species include:


  • Taenia solium
  • Dipylidium caninum
  • Taenia saginata
  • Hymenolepis nana
  • Diphyllobothrium latum

Class Trematoda

Commonly known as flukes, members of class Trematoda are all parasitic in nature. Currently, about 20,000 species of class Trematoda have been identified. 

These are spread across two subclasses of Trematoda including:


Subclass Aspidogastrea


Subclass Aspidogastrea consists of about 80 species that exist as parasites of both marine and freshwater mollusks and vertebrates such as fishes. 

Some of the other characteristics of Aspidogastreans include:


  • A complex nervous system
  • Varying sensory receptors
  • Host specificity is low
  • They can survive for long periods of time outside the host
  • They are considered archaic trematodes


Subclass Digenea


Compared to subclass Aspidogastrea, subclass Digenea is a significantly large group consisting of well over 18,000 nominal species spread across approximately 150 families. They have a more complex life cycle that requires one or more intermediate hosts (mollusk) as well as a definitive host (vertebrates). 

Species of subclass Digenea can be found in virtually all vertebrate classes and cause a range of animal and human diseases (schistosomiasis etc). 

Some of the primary characteristics of this subclass include:


  • Mouth part with a sucker as well as a ventral sucker
  • They are regarded as syncytial animals


Subclass Digenea is divided into several orders that include:


  • Strigeidida - characterized by a fork-tailed cercaria
  • Echinostomida - life cycle is dependent on three different hosts
  • Plagiorchida - characterized by a Cercaria that has a stylet
  • Opisthorchiida - possess a simple, unarmed cercaria


Characteristics of Trematodes


  • Separate sexes that can live in blood for a number of years
  • Have oral suckers as well as ventral suckers that make it possible for the organisms to attach on to the host. This makes feeding easier for the organisms.
  • Adult flukes can be found in the liver or the circulatory system of the host
  • Well developed alimentary canal and excretory system,
  • Well developed muscular system

General Characteristics of Flatworms (Platyhelminthes)

  • Flatworms are bilaterally symmetrical organisms with multicellular bodies that display organ grade organization
  • Triploblastic organisms with three embryonic germ layers that include the ectoderm (outermost germ layer) mesoderm (central germ layer) and endoderm (innermost germ layer)
  • Bodies are typically soft
  • Lack both an endo and exoskeleton. Instead, only some parts such as the hooks, spines and teeth and hardened
  • Apart from attachment organs such as hooks and suckers, flatworms also secrete adhesive secretions from two main glands namely; the eosinophilous and cyanophilous gland
  • Lack a true body cavity
  • Digestive tract is completely absent
  • Nervous system, for the most part, is primitive (ladder-like)

See also:  Class RhabditophoraWorm under a MicroscopeParasites under the Microscope

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Agarwal V.K. (2011). Zoology for Degree Students B.Sc. First Year eBook.

Cannon, L.R., and Lester, R.J. (1988). Two turbellarians parasitic in fish . 

Teresa Adell , José M. Martín-Durán , Emili Saló, and Francesc Cebrià. (2015). Platyhelminthes.

Marta Álvarez-Presas and Marta Riutort. (2014). Planarian (Platyhelminthes, Tricladida) Diversity and Molecular Markers: A New View of an Old Group.






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