Characteristic planarians, reproduction, feeding, species

Charles McCarthy

The planarians or peatlands are a group of animals that belong to the phylum of flatworms. They are flat worms that can measure up to about 5 cm. This subphylum was first described in 1831 by the German zoologist Christian Ehrenberg.

Planarians are a group of animals that require abundant humidity conditions. This is why they live, either in bodies of water or in terrestrial environments where there is enough of this element. It covers a large number of species, approximately 3000 and many of them are characterized by the color patterns they present.

Planaria. Source: Jean-Lou Justine, Leigh Winsor, Delphine Gey, Pierre Gros, Jessica Thévenot / CC BY-SA (

Article index

  • 1 General characteristics
  • 2 Taxonomy
  • 3 Morphology
    • 3.1 External anatomy
    • 3.2 Internal anatomy
  • 4 Habitat and distribution
  • 5 Playback
    • 5.1 - Asexual reproduction
    • 5.2 - Sexual reproduction
  • 6 Food
    • 6.1 Digestion
  • 7 Examples of species
    • 7.1 Pseudoceros dimidiatus
    • 7.2 Pseudoceros bedfordi
    • 7.3 Pseudoceros gloriosus
    • 7.4 Catenula lemnae
  • 8 References

General characteristics

Planarians are multicellular eukaryotic organisms, which means that they have a structure called the cell nucleus, within which the DNA is found, forming the chromosomes. Likewise, they are made up of different types of cells, each one specialized in a specific function..

These animals are triblastic because during their embryonic development they present the three germ layers: ectoderm, endoderm and mesoderm. From these layers the different organs and structures that will make up the adult organism are formed..

They are also cellophane, because they lack the internal cavity known as a coelom. They have bilateral symmetry, since they are made up of two exactly equal halves, separated by an imaginary line on the longitudinal axis..

They are hermaphrodites, that is, they have both female and male reproductive organs. Its reproduction is asexual and sexual. With regard to the latter, fertilization is internal and development in most species is direct. Only a few have an indirect development with larval stages.

This is a group of animals found in both aquatic and terrestrial ecosystems. Some are adapted to living in fresh water environments and others, the majority, in brackish water environments. In the following video you can see a planarian swimming:


The taxonomic classification of planarians is as follows:

  • Domain: Eukarya
  • Animalia Kingdom
  • Phylum: Platyhelminthes
  • Subphylum: Turbellaria


External anatomy

Planarians do not have the typical shape of a worm, as their body is flattened dorsoventrally. Its size is varied; there are species that measure as little as 1 cm, even others that can exceed 5 cm.

Some species show evident cephalization. In some, the differentiated cephalic region of the body is appreciated, since it has a characteristic triangular shape. In this region you can distinguish small processes called atria..

Also in the cephalic region there are small spots that are known as ocelli and that function as organs of vision..

Exemplary of planaria. Source: Nhobgood / CC BY-SA (

In the ventral region of the peat bogs, several orifices can be seen: the first corresponds to the mouth, through which the pharynx can exit; the rest of the orifices, in variable number (between 1 and 3), correspond to genital orifices.

Internal anatomy

Body wall

The wall of the body of the planarians is made up of several layers:

  • Epithelium: it is the outermost layer and contains a wide variety of cells - glandular, epithelial, sensory and cells with rabdites.-.
  • Basement membrane: it is located immediately below the epithelium.
  • Muscle layers: Below the basement membrane are three muscle layers. The first one is made up of circular muscles, the middle one of longitudinal muscles and the last one of diagonal muscles..
  • Nerve plexus: a nerve network that is located between the muscle layer and the parenchyma.
  • Parenchyma: it is a type of tissue that is made up of cells, among which there are some spaces that are known as endolymphatic systems or spaces.

Digestive system

It is quite simple. It is made up of the mouth, which is on the ventral surface of the animal. Following the mouth is the pharynx, which can have different morphology (simple, bulbous, folded), depending on the species.

The pharynx empties into the intestine, which is blind and branched. There is no exact number of branches. The bogies do not have an anal orifice.

Nervous system

These animals have a cerebral ganglion, from which two lateral nerve cords arise. Both are connected by nerve fibers that go from one to the other.

In addition to this, planarians present some sensory organs such as ocelli (visual) and statocysts (balance). They also have cells that function as receptors, allowing them to perceive external stimuli. These are chemoreceptors, tangoreceptors, and reoreceptors..

Excretory system

The excretory system of the planarians is made up of a system of structures known as protonephridiums. These are blind tubules that open to the outside on the surface of the animal's body through an opening called a nephrostoma..

Respiratory system

They do not have a proper respiratory system, the respiration of the planarians is cutaneous. This means that gas exchange occurs through the skin..

Habitat and distribution

From the point of view of distribution, planarians are animals that are widely distributed throughout all regions of the world..

However, due to their anatomical and physiological characteristics, as well as their requirements, planarians must live in humid places, where there is ample availability of water..

There are planarians that are clearly aquatic, while there are others that can be found in terrestrial habitats..

Regarding those that live in aquatic environments, there are some that have managed to colonize brackish water ecosystems, which is why they are generally found as part of the biodiversity of coral reefs.

On the contrary, there are others that have adapted to living in freshwater environments. Due to this, it is common to find them in fresh water bodies that have little flow..

Likewise, the planarians that are found in terrestrial ecosystems are located mainly in places of high humidity and to which the sunlight does not reach directly. Among these places can be mentioned cracks, tree trunks or they can be found on the substrate, covered by remains of dead leaves..


In the bog, the two types of reproduction that exist are observed: asexual and sexual.

- Asexual reproduction

This type of reproduction does not involve the fusion of sexual gametes. Therefore, the descendants obtained will be exactly the same as the parent that originated them..

Planarians can reproduce asexually through two processes:


It is the most frequent type of asexual reproduction among bogs. It consists of the development of an adult individual from small fragments of another animal. This can occur if the planarian suffers some trauma that causes it to lose a piece of its body.

Reproduction by fragmentation is possible thanks to the totipotency of the cells that make up the planarians.


It is a type of reproduction that consists of the development of an individual from unfertilized ovules of virgin females. Parthenogenesis is generally present when different populations go through periods of stress, such as the absence of individuals of the opposite sex.

- Sexual reproduction

Sexual reproduction involves the union or fusion of female gametes (ovules) and male gametes (sperm).


Fertilization in peatlands is internal, as it occurs inside the body. Although it is known that these are hermaphroditic animals, there is no self-fertilization in them. Instead, fertilization can be of two types: cross-fertilization and hypodermic impregnation..

In the case of cross-fertilization, two individuals mate and copulation occurs. Here there is an exchange of sperm between both specimens. Sperm is stored in a structure called the copulatory bag..

On the other hand, hypodermic impregnation consists of mutual perforation of the body wall to introduce sperm. Here the mating between two planarians is observed:


Once fertilization occurs, the egg or zygote is formed. Depending on the species, two types of eggs are observed:

  • Ectolecyte: the yolk (nutritive substances that nourish the embryo) are found in the so-called vital cells.
  • Endocito: the yolk is inside the egg.

Embryonic development involves a segmentation process, in which cells divide through mitosis, expanding the number of cells that the embryo contains, so that they can then begin to specialize.

In the embryo of peatlands, the type of segmentation is spiral and, in most cases, the development is direct. This means that an individual emerges from the egg with the characteristics of an adult individual. On the contrary, there is a small proportion of species that present larval stages.


The peat bogs belong to the group of animals considered carnivores. This means that they eat other animals.

The main prey of peatlands are small invertebrates such as crustaceans, insects, mollusks and other worms.


The way of feeding is as follows: through different mechanisms, depending on the species of bog, it catches its prey and puts it in its mouth. There are some species that envelop the prey in a substance of mucous consistency, making it impossible to move, just as there are others that directly inoculate digestive enzymes..

The mouth is continued with a pharynx that is quite resistant and with great capacity, so it can assimilate prey of considerable size, compared to the size of the bog.

Immediately after the pharynx is the intestine, which is blind and branched. The number of branches depends on the species. This is where the process of digestion and absorption of nutrients takes place..

Now, it is important to note that digestion can be intracellular or extracellular. In the first case, it occurs thanks to the presence of a digestive vacuole, which secretes digestive enzymes (exopeptidases, lipases).

On the other hand, extracellular digestion occurs by the action of some enzymes secreted at the level of the pharynx, as well as thanks to specialized endopectidases.

In this video you can see how a planarian catches a snail:

Examples of species

Pseudoceros dimidiatus

Pseudoceros dimidiatus. Source: Hectonichus / CC BY-SA (

This species belongs to the Pseudocerotidae family. It is a planarian adapted to living in freshwater environments, which is why it is found mainly in the Indian Ocean, specifically in the area that goes from the Red Sea to the coasts of Australia..

This planarian is characterized by the bright colors that adorn its body, which allows it to be easily identified in coral reefs. On the anterior margin of their body they have very small extensions, which are known as pesudotentacles..

Pseudoceros bedfordi

It is also known as "Persian carpet flatworm". It is found exclusively in the Pacific Ocean, specifically off the coasts of Malaysia, Indonesia, Thailand, the Philippines, Australia, the Solomon Islands, and Myanmar, among a few other places.

Its physical appearance is quite characteristic, identifiable for any experienced diver. Its dorsal surface is black or brown, on which a pattern of pink lines is observed, as well as a large number of yellow dots. From there it derives its name.

On the front edge of its body it has very small extensions that resemble tentacles. They are your pseudo-tentacles. It moves through the middle thanks to undulating movements of its body.

Pseudoceros gloriosus

It is a beautiful planarian found in the area that ranges from the eastern coast of the African continent to the region known as Micronesia. This is why it is found in waters, both in the Indian Ocean and the Pacific Ocean..

The dorsal surface of this planarian is black, giving the illusion of a velvet appearance. It has a very particular colorful border, made up of orange, pink and burgundy. It can measure up to 8 cm.

Its diet is made up of some invertebrates belonging to the group of gastropods (snails) and crustaceans (crabs, shrimp, among others).

Catenula lemnae

Catenula lemnae. Source: Christopher Laumer from Somerville, PA, United States of America / CC BY (

This planaria is adapted to living in freshwater environments. Its body is made up of several elongated links. A complete adult planarian may form from each link.

It is found mainly in small bodies of fresh water such as ponds and lagoons. In these it is located at the bottom, under the remains of vegetation. It lacks eyes, but has a highly developed organ of balance that allows it to effectively orient itself in its movement through the environment.


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