Definition, characteristics, and body structure of Platyhelminthes

Platyhelminthes (flatworms) comes from Greek, platys means flat and helminthes “Worms” (Ehlers & Sopott-Ehlers 1995: 1). Categorized as a flat shape, because it has a flattened body dorsoventrally (between the dorsal surface “upper” and ventral “lower”). Platyhelminthes live in moist terrestrial, freshwater and marine habitats with a total of 20,000 species. Platyhelminthes are partially free-living with nearly microscopic body sizes and live alongside other organisms as parasites with body sizes some up to 20 m (Raven et al. 2017: 662; Urry et al. 2017: 692).

Platyhelminthes are classified as animals with bilateral symmetrical bodies and are composed of triploblastic acoelomates. Animals with bilateral symmetry are animals that have a similar right and left sides. The triploblastic arrangement of the platyhelminthes consists of the ectoderm (outer layer) covering the body and nervous system, the mesoderm layer (between the outer and inner layers) forming muscles, and the endoderm (inner layer) forming the digestive tract. Platyhelminthes bodies do not have a body cavity so they are called acoelomates (Urry et al. 2017: 677-678). The shape of the layers and body cavities of the platyhelminthes can be seen in Figure 1, while the body structure of the platyhelminthes can be seen in Figure 2. See also other materials:
Annelidalining and body cavity of the platyhelminthes

Figure 1. Layers and Body Cavities of Flatworms
Image source: Urry et al. 2017: 678platyhelminthes body structure

Figure 2. Body Structure of Platyhelminthes
Image source: Raven et al. 2017: 662

Digestive system

Platyhelminthes have a gastrovascular digestive system with a single opening (disposal and intake of food by mouth). Gastrovascular functions as a process of digestion and distribution of nutrients from the intestine which extends throughout the body without the help of the blood circulation system. Platyhelminthes take food by swallowing and shredding it into small pieces using the pharyngeal muscles and digested by the intestines. The undigested food is excreted by mouth. Tapeworms (Cestoda) which are parasitic have a mouth on the front of the body and do not have a digestive cavity, also take food by absorbing nutrients from the host body (Campbell et al. 2012: 919-920; Raven et al. 2017 : 663).

Excretion and Osmoregulation System

The excretory system of the platyhelminthes is protonephridia (plural; protonephridium: single). Protonephridia is a network of tubules that branch throughout the body. In the tubules, there are hoods called fire cells. The fire cell has many cilia strands that stick out into the tubule. The pulsating motion of the cilia during filtration attracts water and solutes to the tubules. Movement of cilia similar to flames so that they are called fire cells. The resulting filtrate from the tubules is then discarded outside the body by diffusion on the body surface or removed from the mouth. Protonephridia in platyhelminthes can also regulate the body’s osmotic environment called osmoregulation (Urry et al. 2017: 982).

Nervous System and Sensory Organs

The platyhelminthes nervous system is composed of the anterior cerebral ganglion and nerve cords that branch out throughout the body in the form of a ladder. The sensory system in free-living platyhelminthes has vision in the form of sunken eye spots (ossicles). These ossicles have cells that are sensitive to light and are connected to the nervous system so that they can differentiate between dark and light areas. Most of the platyhelminthes stay away from bright areas. The advantage of this behavior can avoid the threat of predators (Urry et al. 2017: 1084, 1115; Raven et al. 2017: 663).

Platyhelminthes Reproductive System

The reproductive system of the platyhelminthes is mostly classified as hermaphrodite. Hermaphrodite individuals have both male and female reproductive systems. For copulation, most of the platyhelminthes are carried out by two individuals to produce genetic recombination with fertilization carried out internally. Sperm are injected into the copulatory tract and travel to the egg. In addition to sexual reproduction, platyhelminthes can perform axexual reproduction to regenerate new body parts when cut (Urry et al. 2017: 1018; Raven et al. 2017: 663).

Platyhelminthes classification

1. Turbellaria – Free Living Flatworms

Most of the Turbellaria group live in marine habitats. The Turbellaria group that lives in the famous freshwater belongs to the genus Dugesia, namely planaria. Planarians live in small rivers or ponds that are not polluted by preying on small animals or eating animal carcasses. Movement of planarians uses cilia on the ventral surface and lubricates the surface of the substrate with mucus and can swim with pulsed motion. At the anterior end of the planaria is a pair of eye spots that are light sensitive to distinguish dark and light areas. The reproductive system of planarians can be done asexually by regenerating new body parts to form new and sexual individuals (Urry et al. 2017: 693-694).

2. Monogenea

The Monogenea group is a platyhelminthes group that lives on other animals as parasites in fresh or marine waters. Most of them ride on the outer surface of the host’s body and are called ectoparasites. Monogenea parasites its host throughout life without the need for an intermediate host (intermediate hosts), in contrast to the Trematoda group. To attach to its host, Monogenea has an operator structure posteriorly and a prohaptor anteriorly (CABI 2021: 1).

3. Trematodes

The Trematoda group is a group of platyhelminthes which become parasites in the host’s body so that it is called endoparasites. The trematode host is an intermediate host (intermediate hosts), namely snails and end up being parasites in humans. Examples of well-known Trematode species are Schistosoma mansoni. Egg S. mansoni hatch into miracidium. Miracidium infects snails and develops into sporocysts. The sporocyst exits the snail and develops into larvae of cercaria (from asexual reproduction) which have a tail for swimming. Cercaria larvae that infect humans and lose their tails during penetration into the skin become schistosomulae. Schistosomulae enter the blood circulation system and end in venous flow during the mesenteric phase. Mesenteric (adult worms) move to the rectum to lay eggs (from sexual reproduction) in feces (Urry et al. 2017: 694).

4. Cestoidea

The Cestoidea group or tapeworms are classified as a parasitic group. Cestoidea hosts are mostly vertebrates, especially humans. The parasitic support structure in Cestoidea is a scolex at the anterior end which has hooks and suction. Feeding in the Cestoide a group by absorption of nutrients from the intestine of the host. At the posterior end of the Cestoidea there are proglottids as a pocket of sex organs and store many eggs. An example of the Cestoidea group is Taenia Saginata.

Egg T. Saginata which sticks to grass can parasitize cattle or pigs when eaten. The eggs hatch into the oncosphere (oncospheres) penetrate the walls of the digestive tract of the host and migrate to muscle tissue. The onscoffer of muscle tissue becomes cysticerci. Beef or pork infected with cysticerci when eaten by humans develops T. Saginata adult. Cows and pigs are intermediate hosts, while humans are the actual hosts T. Saginata faeces (Urry et al. 2017: 694-695).

The role of Platyhelminthes

  1. In food webs, play the role of scavengers, like the Turbellaria group;
  2. Parasitic in livestock and humans;
  3. The nature of regeneration in planaria is a subject of research that is being developed.


CABI. 2021. Monogenean infections of fish: overview. Accessed on 26 February 2021, 16.00 WIB.
CDC. 2013. Parasites – Taeniasis: Biology. Accessed on 26 February 2021, 16.35 WIB.
CDC. 2019. Parasites – Schistosomiasis: Biology. Accessed on 26 February 2021, 17.30 WIB
Ehlers, U., & B. Sopott-Ehlers. 1995. Plathelminthes or Platyhelminthes ?. Hydrobiologia 305: 1—2.
Raven, PH, GB Johnson, KA Mason, JB Losos, & SR Singer. 2017. Biology 11th ed McGraw-Hill Education, New York: 1410 p.
Urry, LA, ML Cain, SA Wasserman, PV Minorsky, & JB Reece. 2017. Biology 11th ed. Pearson Education, Inc, New York: 1490 p.


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