Planaria (Platyhelminthes)



Planaria is a genus of flatworm. The phylum Platyhelminth contains all known flatworms. The three major types of flatworms are tapeworms, flukes, and turbellarians.

Kingdom: Animalia
Class: Tubellaria
Order: Tricladida
Family: Planariidae
Genus: Planaria
(SL 10)

The planarian has a soft, flat, wedge-shaped body that may be black, brown, gray, or white and is about a half inch (1.3 cm) long. The blunt, triangular head has two ocelli (eyespots), pigmented areas that are sensitive to light. There are two auricles (earlike projections) at the base of the head, which are sensitive to touch and the presence of certain chemicals. The mouth is located in the middle of the underside of the body, which is covered with cilia (hairlike projections). The nervous system consists of a simple brain from which two nerve cords extend the length of the body. Other nerves connect these cords, forming a ladderlike structure. There are no circulatory or respiratory systems; oxygen entering and carbon dioxide leaving the planarian's body diffuses through the body wall. (SM) [17]

Relationship to humans

They can act as internal parasites absorbing digested food from their host’s stomach. Flatworms can also transmit human diseases, such as schistosomiasis. Scientists study organisms like Planaria to learn about regeneration, and what it could mean for humans (SL 11). Planaria are and humans are in the same kingdom, Animalia, meaning that they are both eukaryotic heterotrophs. (SL 10)

Habitat and niche

Tapeworms and flukes are internal parasites that mainly live in the stomachs of vertebrates and absorb digested food from them. Turbellarians are external parasites of fishes and other aquatic vertebrates.

Turbellarians are extremely thin and require moist habitats. They live in ponds, lakes, streams, and springs, usually under rocks or in leaf litter. They can also live on land in moist soils. Turbellarians are sensitive to light and therefore prefer dark habitats. Freshwater planarians live across North America and throughout certain parts of Europe. (19) (20) (LK)
Tapeworms live on land and in water where vertebrates live and they act as parasites. Adult tapeworms usually live in the intestines of humans and other animals, such as dogs and cats. The larvae can live in places such as the brain, muscles, lungs, and eyes of vertebrates. (21) (LK)

Predator avoidance

Although planaria individually do not have very strong defenses against predators, they do have the ability to release chemical cues when injured by a predator in an effort to save other planaria in the area by warning them of nearby danger. Because planaria generally live in liquid environments, the injury-released chemical cues are sent through the water and provide a very good and accurate warning for other planaria nearby. While this mechanism is beneficial to the population as a whole, the individual planaria being attacked by a predator will most likely be unable to save itself. (8) (CM)

Nutrient acquisition

Tapeworms and flukes absorb food that is in the gut of their host. The digestive tract, if a flatworm has one, is a mouth opening into a blind sac that is branched to form patterns that increase the surface area and rate of nutrient absorption.

This short clip shows the planaria not only moving around in its environment and responding to it, but also feeding. It shows the protrusible muscular pharynx, which extends through the bottom to absorb food and sends it to the digestive tract, if present. (AG) (18)

Planaria secrete digestive enzymes in their mouths before beginning internal digestion. Planaria eat animals, both alive and dead, by sucking them up into their mouths, where digestive enzymes begin external digestion. Food then travels from the mouth to the pharynx, then to the gastrovascular cavity and the intestines, where nutrients are digested from it and diffuse throughout the body. (5) (YR)

(22) (SP)

Reproduction and life cycle

Most of the interior of their bodies is taken up by sex organs.
Planaria can reproduce both sexually and asexually. They are hermaphrodites meaning they have both male and female sex organs. I sexual reproduction, two planaria will line up next to each other and fertilize each others eggs. These eggs are then released into a cocoon that holds between 5 and 20 eggs. The eggs separate from each other, surrounding themselves with the yolk from the egg sack. As they mature, they eat themselves out of the yoke sack and flatten to become planaria. There is no larval stage. This cycle takes about 21 days. Planaria reproduce asexually by transverse fission. This process includes constricting part of the body until it cleaves in two. These two partial bodies can then regenerate and become two clones of the parent. Planaria live for about 3 years (4).
Planaria Reproduction.jpg
(SL) (9)

Growth and Development

One of the most interesting features of planaria is the fact that they are able to regenerate any part of their bodies-save the tip of the head- should it be damaged or destroyed. Neoblasts, stem cells can develop into any type of cell and are related to embryonic stem cells, are responsible for this ability. However, neoblasts can continue to replace cells even in adult planaria. When a planaria is wounded, rapidly dividing neoblasts around the site form a blastema, an area at the tip of regenerating tissue that consists of neoblasts that have yet to specialize their cell functions. In the instance of nutrient depletion in the environment, planaria have the ability to downsize their cell count in order to optimize resource distribution. (S.S-3)


The integument of a planarian is composed of one layer of epithelial which is covered by cilia, flagella, and still tactile hairs. Epithelium is a type of animal tissue which forms many glands for secretion, selective absorption, protection, and sensation of the extracellular environment. The epidermic cells are flat and columnar and differ in function and structure. Indifferent ciliated cells, cells containing structures, gland cells, and glutinous cells make up the integument of a planarian. The glutinous cells of the integument appear on the ventral surface and the hind end of the body and secrete sticky fluids by which the planarian can attach to nearby surfaces. (AC) (3)


Flatworms that live freely use broad bands of cilia for propulsion.

A planaria’s primary method of propulsion consists of utilizing motile cilia, or short hair-like extension of a cell. A Planaria contains a type of cilia called motile cilia which is used for movement rather than sensory functions like its non-motile counterpart. The planaria moves by beating the cilia’s against a smooth surface and using the generated force to propel the organism. To make this process easier, the planaria will generally secrete a minute film of mucus to slide on. (SF)(17)

Sensing the environment

Flatworms have eye cups. These eye cups cannot produce detailed images, but the photoreceptors on the two sides of them are unequally stimulated due to the arrangement of pigments and photoreceptors. The only time that the photoreceptors on both sides are equally stimulated is if the flatworm is facing directly toward or away from a source of light. The information from the eye cups is used by the flatworm to move away from light.

Planarians have relatively advanced nervous systems when compared to similar flatworms. Rather than employing a scattered cluster of neurons, Planarians have a ladder-like nervous system, a network within the body which transmits messages throughout the body, with structures containing ganglions, a cluster of nerve cells, on each end. Two structures at the base of the organism called auricles are responsible for most sensory perception while the two eyespots that are sensitive to light are responsible for limited sight (as described in the former paragraph).
(2) (SR)

Gas exchange

They lack specialized organs to transport oxygen to their internal tissues. Each of their cells thus has to be near a body surface, so their bodies are flattened out.

Planarian exchange gas through simple diffusion. Their flat body surface allows for a high surface area to volume ratio, so the most amounts of nutrients can be transferred. Their are cells that line the intestine, which absorbs nutrients for the planarian and then diffuse the nutrients through the rest of the organism. (22) (RS)
Simple Diffusion (23) (RS)

Waste removal

They have a gut that opens to the exterior and serves as mouth and anus.
Planarians and most other flatworms have gastrovascular cavities that exchange materials through a single opening. The shape of the body and the branching of the gastrovascular cavity throughout planaria allows all cells to be "bathed by a suitable medium". Because of the short distance materials have to travel to the external environment and the surrounding fluid is suitable, diffusion is how wastes exit planarians (21). (SP)

Environmental physiology

Planaria are freshwater animals, so they must pump out water and expend more energy to pump the ions back into their bodies. Planarians use the flame cell system as their excretory system, which also regulates the interstitial fluid contents directly. In order to regulate water and salt, the planaria must excrete very dilute fluid. Some scientists believe that the lining of the tubules is a transport epithelium that specializes in the re-absorption of certain salts (19). Planaria use nephridia, which are tubular structures that filter body fluids other than blood. The planaria's cilia draw the fluids into this system, and then tubules then reabsorb useful substances like glucose and return them to the tissues. The extra ions, water, and metabolic wastes are expelled from the body by through nephridiopores in the body wall (20). (SM)

Internal circulation

Branches penetrate to all parts of the Planarian. It has a gastrovascular cavity that exchanges materials with the environment through a single opening. Because diffusion distances for nutrients, gases, and wastes make circulation difficult, body movement helps distribute materials to various parts of the body (6). This system causes the animal to maintain a suitable cell medium, a relatively small size or to shape, and low diffusion distances. (7)

Chemical control

Planaria have a collection of neurons in the head known as the cerebral ganglion which serve as a sort of a rudimentary brain. There are two long nerve cords that run the length of the body with crosses nerves connecting them together. Thus, the planarian's nervous system is called a ladder-like nervous system, and allows the organism to move in a coordinated fashion. (24) (NU)
Planaria do not have circulatory nor respiratory systems; oxygen enters and carbon dioxide leaves the planarian's body by diffusing through the skin. (25) (NU)


1. Explain how Planaria are able to go through process of regeneration (


1. (SF)
2. (SR)

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