Planarian Anatomy (AWC)

Jasmine Lau

Classification/Diagnostic Characteristics

Kingdom: Animalia

Subkingdom: Eumetazoa

Superphylum: Platyzoa

Phylum: Platyhelminthes

Class: Turbellaria

Order: Seriata

Suborder: Tricladida

Family: Planariidae

Genus: Dugesia (example)

Species: Dugesia polychroa (example)
(4) (SJ)

Planaria are flatworms that are considered to be part of the turbellarian class. Although most flatworms are parasites, planaria are non-parasitic and do not feed off of hosts.

Planaria have a soft, flat, wedge shaped-body that can be black, brown, gray, or white. They are usually half an inch long (1.3 cm). Their blunt, triangular head has two ocelli, eye spots, which are pigmented areas sensitive to light. they have two auricles, ear like projections, at the base of the head which is sensitive to touch and certain chemicals. The mouth is located on the middle of the underside of the body. [1] (SM)

Relationship to Humans
Parasitic flatworms absorb digested food from the guts of their hosts, and can cause serious human diseases. One particular type of disease is schistosomiasis.

Flatworms live in healthy streams ponds and lakes so if a human is wondering if a body of water is safe to drink from, check if flatworms are in the water. If they are the water is healthy[2] (CC)

Habitat and Niche
Planaria tend to live in cold water environments. They can be found beneath rocks, logs, or dead leaves in springs, spring brooks, ditches, wetlands, streams, and lakes. (LC) 2

Planaria are typically found in freshwater. They are very sensitive to pollution in water, and do not live in polluted water. Because of this planaria are a sign of good water quality. (MDS)[3]

Predator Avoidance
Planaria have anti-predator avoidance behaviors in response to chemical signaling. When a planaria is harmed by a predator, that planaria releases a chemical into the surrounding water that signals to other planaria in the area that there is danger nearby. Because of this the warned planaria can respond by swimming away from the area where the predator is present avoiding being eaten. (8) (BH)

Nutrient Acquisition

As mentioned previously, parasitic flatworms attain their nutrients by absorbing digested food from the guts of their hosts. They lack
specialized organs for transporting oxygen to their internal tissues, therefore each cell has to be near a body surface. Flatworms with a digestive track have mouths opening into a blind sac, which is often highly branched and forms patterns to increase the surface area for absorption of nutrients.
Through its long, muscular pharynx, which protrudes the mouth in the presence of food, Planaria eat. The tip of the pharynx is placed against food particles of tissue which are sucked up into the main gastrovascular cavity by the muscles of the pharynx. The cavity is filled with fluid and tissue are 30-80 minutes of feeding. (9) (MC)

Video: Planaria basic anatomy and feeding strategy (10)(AA)

Reproduction and Life Cycle
Flatworms living in freshwater snails and mammals have complex life cycles.

The most common species studied in the lab is the brown planaria, Dugesia. All dugesia are hermaphrodites, and they can reproduce sexually and asexually. For sexual reproduction, two dugesia pair up and fertilize one another's eggs. Then, the eggs are released in a cacoon. For asexual reproduction, one can reproduce through a process called transverse fission. Transverse fission is when the planaria cuts itself in half, and the tail creates a new head, and then the head portion regenerates a new tail. (3) (PS)


A picture of the anatomy of a planaria, including specific anatomical structures especially involved in reproduction. (5) (E.S.S.)

Planaria Having Sex
Planaria Having Sex
([4] ) (BS)

external image fragmentation-in-planaria.jpeg

Growth and Development

A type of planaria is the Dugesia , which has very basic nervous system. The ganglia, which is just a structure made out of cell bodies on the nerve,is on the front part of the brain. They began to develop cephalization, where the neural and sensory organs are in the front of the organism's body.
Because the Dugesia has two horns sticking out of its head, it is bilaterally symmetrical to its eyes. (11)(NC)


The epidermal layer (the outer most layer of an organism) is covered in tiny cilia, hair like projections that help the planaria move, and gland cells, secretary cells. Rod-shaped bodies along the epidermis called rhabdites secrete mucus to form a mucous layer around the worm for protection. [12](AY)

Free-living flatworms are powered by broad bands of cilia, which help them glide over surfaces.

Planaria also use a sort of "inching" or "ambeiod" movement when they come into contact with a surface. This is accomplished by using small muscle contractions moving down the body. In moving water they normally move with the current or attemot to stay still. (13) They also appear to be sensitive to light and attempt to avoid it which has played a role in many conditioning experiments in the past. (14) (DA)

They release a sticky mucus which helps the cilia glide over surfaces. This helps them climb up plants, glide upside down on surfaces, or glide on top of the surface of water[5] (CC)

Video of planarian movement under a microscope (15) (WSS)

Sensing the Environment
Small free-living flatworms have heads with chemoreceptor organs, two simple eyes, and a tiny brain made of anterior thickenings of longitudinal nerve cords.
The eyespots of a planaria can detect changes in light in the planarian's environment. Shining a flashlight on the planarian, will cause it to attempt to move out of the light. (3)(KG)
They have a simple nervous system that includes a ganglia located in its anterior region to serve as a brain. The dugesia exhibits the trait of cephalization, where the majority of its sense organs are located in the anterior region.[6] (TM)

Gas Exchange
Flatworms do not have specialized organs for transporting oxygen to their internal tissues, so each cell needs to be near a body surface.

Though they do not have specialized organs, they can still use excretion and internal transport methods for gas exchange. Most all of this can be done through a simple process of diffusion. (17)(ES)

Waste Removal
Planaria remove waste through specialized cells called flame cells that are located on the edges of the planarian. Cilia on the flame cell forces interstitial fluid into tubules. Tubules from different flame cells merge, and the fluid is excreted into the environment through pores called nephridiopores on the dorsal side of the planaria. While the mechanism of how flame cells and tubules filter waste products from the interstitial fluid is unknown, the flame cell system is known to filter certain salts out of the fluid before the fluid containing undesirable salts and nitrogenous wastes is pumped out of the body. [7] (FZ)

Environmental Physiology

Internal Circulation
The planaria lacks a cirulatory system due to its simple body structure not needing it. (3)(KG)

Chemical Control
The brain of the planaria has been recently studied and shown to be more complex than previously thought. The brain sends chemical signals as neuropeptides, small chains of amino acids that derive from longer prohormones, important in vertebrates. These have been shown to be heavily responsible for sexual reproduction in planaria, since when they are inhibited, the planaria's sexual organs shut down and it can no longer reproduce. (16) (DM)


  1. ^

  2. ^

  3. ^
  4. ^;topic=13358.0;attach=74766;image
  5. ^

  6. ^
  7. ^

1. Hillis, David M. Principles of Life. Sunderland, MA: Sinauer Associates, 2012. Print

Review Questions
1. True or false: although planaria acquire nutrients from the digested food of their hosts, they never cause damage or disease to the humans whose guts they occupy. (JLev)
2. Describe the processes of both asexual and sexual reproduction in the planaria. (BB-V)