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Planaria Definition

Last updated date: 19th Mar 2023
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The phylum Platyhelminthes is a major phylum of the kingdom Animalia. It constitutes flatworms that are segmented and bilaterally symmetrical. There are several parasitic varieties of flatworms, some of which are quite harmful to human populations. Planaria is among the many examples of flatworms in Platyhelminthes. Planarians have been studied extensively over the last century for their regenerative properties and more recently as a model for stem cell biology.

What is Planaria?

Planaria are flatworms belonging to the phylum Platyhelminthes that have incredible regeneration ability, earning them the nickname "immortal under the knife." Freshwater, marine, and terrestrial habitats all have a diverse range of organisms. The majority of planarians live in freshwater and can be found in huge groups; some species are marine, while others are terrestrial. Some species are parasitic, meaning they feed on other living creatures’ bodies.

Scientific Classification and Diagram of Planaria

Planaria Classification

Kingdom: Animalia

Phylum: Platyhelminthes

Class: Rhabditophora

Clade: Adiaphanida

Order: Tricladida

Family: Planariidae

Genus: Planaria

Planaria Diagram

Structure of Planaria

Structure of Planaria

Anatomy of Planaria

When extended, the body is soft, leaf-shaped, and ciliated. Two eyes and tentacles can be found on the spade-shaped head. The tail is angular. The mouth is located on the ventral (lower) side of the animal, frequently more than halfway to the tail. There is no body cavity or coelom. The pharynx, which may extend from the mouth, leads to a normally blind gut. The length ranges from 3 to 15 mm (0.1 to 0.6 inches), with some reaching more than 30 cm (approximately 1 foot) in length. Species from the tropics are frequently brilliantly coloured. Dugesia is a black, grey, or brown genus found in North America.

Planarians swim in an undulating pattern or move along the ground like slugs. The majority of them are carnivorous night feeders. Protozoans, small snails, and worms are among their favourite foods. All are simultaneous hermaphrodites, meaning they have both male and female reproductive organs in the same person. Early in the autumn, the reproductive organs begin to mature. In the spring, cocoons holding fertilised eggs are laid. In most species, fully developed young emerge and develop without metamorphosis (radical change). However, in a few marine species, free-living, ciliated larvae are discharged. In some species, the organism in the cocoon divides into two halves, each of which grows into a complete individual. Buds form at the end of others of the genus Microstomum and can stay attached to the parent for a long time; chains of three or four buds have been seen. Planarians are frequently employed in experiments to explore the process of regeneration because of their exceptional capacity to restore missing portions.

The Nervous System of Planaria

  • Flatworms were the first invertebrates to acquire a central nervous system with a brain, as well as bilateral symmetry.

  • A brain, longitudinal nerve cords, and peripheral nerve plexuses (interlacing networks of peripheral nerves) make up the neurological system of a free-living flatworm-like Planaria.

  • The brain, which is placed in the front of the animal, is made up of two cephalic ganglia that are connected by a commissure.

  • Longitudinal nerve cords, usually three to five pairs, extend posteriorly from the brain; transverse commissures connect them, and smaller, lateral nerves emerge from the cords.

  • The peripheral nerve plexuses are formed by the lateral nerves. The submuscular nerve plexus, which consists of sensory cells, ganglion cells, and their processes, is located beneath the subepidermal musculature in loose tissue (mesenchyme). Above the muscle layer, another subepidermal plexus is found at the bases of epithelial cells.

  • Planaria nervous system has a plethora of sensory receptors.

  • The nerve plexuses include single sensory cells that are extensively dispersed throughout the organism.

  • Ciliated pits and grooves, auricles, the frontal organ, statocyst, and eyeballs are among the sensory organs present.

  • Chemical receptors, or chemoreceptors, are found in the ciliated pits and grooves, allowing the animal to detect food. The statocyst is in charge of balance and reflexes like rising to the water's surface or sinking.

How does Planaria reproduce?

Regeneration in Planaria

Regeneration in Planaria

Planaria have both male and female sex organs, making them hermaphroditic. The ovaries are found near the eyespots in a rostral direction. Of course, this is not the same as the human ovaries, which are located in the lower abdominal cavity! A single egg cell is produced by the ovaries of planarians. Testes and seminal ducts, which together make and carry sperm, are positioned laterally along the sides of the body. A penis is also present in planarian worms, which are contained in a genital chamber. Nutrients are delivered to eggs via specialised yolk glands.

Sexual Reproduction: Planarian worms have both testes and ovaries, as well as the ability to generate both sperm and eggs. Sexual reproduction, on the other hand, occurs when one flatworm transports sperm to the seminal receptacle of the recipient flatworm via the penile duct. In the seminal receptacle, the recipient's egg is fertilized and transmitted outside the body into the planarian worm's liquid environment. A thin mucous filament is usually used to bind the egg to a surface in the liquid.

Asexual Reproduction: Planaria have the power to create new persons from the components of their bodies. This is referred to as regeneration. Planaria's body can be cut or broken into several pieces, each of which grows into a whole organism. Regeneration varies from reproduction in that regeneration can occur from any part of the organism, such as a lizard's damaged tail. Reproduction, on the other hand, is the process through which a new organism is created from specialised cells known as gametes. Furthermore, regeneration does not always result in the emergence of a new person, whereas reproduction always results in the emergence of new individuals from the parent organism.


In many regions of the world, planaria are widespread. In freshwater ponds and rivers, planaria typically dwell beneath rocks or aquatic plants. Salinity is the home of several marine animals. In humid environments, several terrestrial species can be found on or in the soil, under logs, and on plants. Polluted water is intolerable to planaria. They are frequently researched as bio-indicators in an aquatic habitat as a result.

The most striking feature of planaria is its ability to regenerate itself when cut with a knife.

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FAQs on Planaria

1. What are the characteristics of the digestive system in planaria?

Planaria have a mouth, pharynx, and gastrovascular cavity with a single aperture. The planarian's mouth does not remain near its eyes. Instead, its mouth is positioned in the centre of the underside of the body (occasionally even closer to the tail). The pharynx is a tube that connects the mouth to the gastrovascular cavity, which has branches all over the body. The digestive tract of a Planarian has only one opening. As a result, it uses its mouth as an anus to eliminate undigested food detritus.

2. How exactly does a planarian move?

Planaria use cilia located on the ventral (which translates to "belly") portion of their bodies to travel across surfaces. They can produce a layer of mucus that has the consistency of a gel, which acts as a lubricant. They can swim with an undulating motion or creep like slugs by contracting their muscles, which is another way that they move. The nerve cords are responsible for the control and coordination of these muscles.

3. What kind of food does a planarian consume?

Dietary options for planaria can be split into two categories. Planaria gorge themselves on trash. Plants and animals remain, as well as faeces, are all good diets for these planaria. Some planaria, on the other hand, are effective predators that search for worms, snails, and small arthropods like daphnia, isopods, and baby shrimp.

Planarians consume by sucking food (living or tiny dead animals) into their muscular mouths. Food travels from the mouth down the pharynx and into the intestines, where digestive enzymes break it down. The nutrients then diffuse to the rest of the planarian via the gastrovascular cavity's branches.

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