Ecology 340

Westminster College, Salt Lake City, Utah

Bridgette Jenne



Planarian. 1999-2000. Encyclopedia Britannica. http://www.britannica.com/seo/p/planarian/


What is a planarian?

What are the food sources?

Anatomy of the planarian

How do they move?

How do they reproduce?


These small, free-living flatworms are found under rocks in the fresh

water of Emigration Creek where it crosses the campus of Westminster

College. They are classified in the phylumPlatyhelminthes and in the

class Turbellaria. We have identified the Emigration Creek species as

Dugesia tigrina, a common NorthAmerican flat worm.


Planarians are generally zoophagous, feeding on small living

invertebrates and decaying organisms found in their ecosystem.

Emigration Creek planarians could eat segmented worms, leeches, snails,

and other small herbivores. These small herbivores, in turn, eat

the filamentous algae, Cladophora, and the diatoms growing in the 

"biofilm" of rocks.  Planarians may play an  role as a "detritivore"

organism.  Detritivore organisms gain their food energy with the help

 of decomposing  fungi and bacteria which, with the help of various

 invertebrates,  start the fragmentation process of materials that fall 

into the stream.   A major source of energy input for this detritivore 

food chain is the leaves of the local riparian trees.  The leaves are 

partially decomposed by bacteria and fungi which can then be eaten

 by Dugesia and other aquatic organisms.


The planarian eats by using its long, highly muscular pharynx. This

protrudes through the mouth in the presence of food. The tip is placed

against the food and soft or disintegrating particles of tissue are sucked

up into the main gastrovascular cavity by the muscles in the pharynx.

The cavity will be come full of fluid and small bits of tissue after about

30-80 mins of feeding. Individual particles are ingested by pseudopodial

action of large gastrodermal cells.


Planarians are usually between 3 to 15 mm. The colors seen in the North

American genus, Dugesia, are white, grey, brown, and black. The

triangular head contains two eye spots and sometimes tentacles for

gripping. The mouth, or feeding structure called a pharynx, is located on

the underside of the worm halfway down towards the tail.


These soft bodied, ciliated

flat worms move much like

slugs in an undulating

motion. The movie to the

right shows a Dugesia

tigrina from Emigration

Creek moving under a

dissecting microscope.



Dugesia tigrina reproduces both sexually and asexually.  Under the appropriate

 conditions of temperature and food, asexual individuals pinch in around 

mid-length of the worm's body.  The pinching continues until the two halves 

are complete.  Each half will then form a whole worm.  This aexual reproduction 

 occurs only above 

10degrees C with the frequency of division increasing to a maximum at 25 to 28

 degrees C.  Sex organs may develop during the winter and early spring in some

 strains of D. tigrina. Planarians are hermaphroditic in that organs for both sexes

develop on each worm. Egg capsules, or cocoons that are oval in

shape and about 5 mm long, are deposited in May and June. As the

water temperature rises the reproductive organs degenerate and in July,

August, and September reproduction may be entirely by fission. By late

autum the sex organs begin to develop again. Fully developed young

emerge and develop without metamorphosis. Planarians are also capable

of reproducing body parts if they have been damagedor removed. This

has made them very popular in laboratory experiments.



     1. McCafferty, Patrick W. 1981. Aquatic Entomology. (p.74). Arthur C.

Bartlett, London

     2. Pennak, Robert W. 1978. Fresh-water Invertebrates of the United

States: Second Edition. (p. 116-131). John Wiley & Sons, Inc., USA.

     3. Planarian. 1999-2000. Encyclopedia Britannica.


     4. Planarian. 2000. The Columbia Electronic Encyclopedia.