What is a Planarian?

planarian cut

Planaria are free-living flatworms that live in quiet ponds or bodies of water. In some areas you can even catch a few planarian by attaching a piece of liver to a fish hook and a sinker and dropping it into the water. Wait a few minutes and pull the liver out and you may find tiny little black worms feasting on the meat. Like all flatworms, planaria belong to the Kingdom Animalia, and the Phylum Platyhelminthes. This phylum also contains parasitic flatworms, like the tapeworm and the liver fluke.

Free-living flatworms like the planaria are grouped into the Class Turbellaria. The most common species studied in the lab is the brown planaria, Dugesia. The animal has an acoelomate body (no internal cavity to hold organs), no anus and lacks a circulatory system. Most are scavengers and will eat other animals that have sank to the bottom of their ponds, hence why you can use liver to capture them.

The Dugesia does have a simple nervous system that includes a ganglia located in its anterior region to serve as a brain. As such, the dugesia exhibits the trait of cephalization, where the majority of its sense organs are located in the anterior region. It has a triangular head with two prominent eyespots. Upon closer inspection of the eyes, you can see that they have a curious cross-eyed expression to them. The presence of the two eyes and lateral horns on the head indicate that the planarian has bilateral symmetry.

The planarian will swim in a shallow petri dish by undulating its body across the surface of the dish. Most will stay close to the bottom or the edge of the dish. If given a choice, the planarian will actively seek an area of the dish that is dark or has some kind of cover. The eyespots can in fact, detect changes in light in the planarian's environment. If you shine a flashlight on the planarian, it will attempt to move out of the light.

The planarian does not have gills or lungs, it obtains its oxygen by simple diffusion over its flat body. The dugesia cannot survive outside of the water, so biologists studying it must make sure that the specimen has plenty of water that is aerated. The dugesia does have an excretory system to remove wastes. Tiny cells, called flame cells, line the lateral edge of the organism and function to remove waste.

The dugesia can reproduce sexually, and all dugesia are hermaphrodites. Two dugesia will pair up and fertilize each other's eggs. Those eggs are then released in a cocoon. If there is not another dugesia present, one can reproduce asexually through a process called transverse fission. The organism will pull itself in half and the tail portion will regenerate a new head, and the head portion will regenerate a new tail. This process can be replicated in the lab by using a razor blade or scalpel to cut the dugesia in half. In a couple of weeks, you should have two dugesia swimming around in your petri dish.


___1. What type of symmetry does the dugesia have? a) radial b) bilateral c) asymmetry

___2. What term is used to describe how a planarian can regrow its body parts? a) regeneration b) fission

___3. Which of the following is associated with the planarian's nervous system? a) flame cells b) ganglia

___4. Asexual reproduction can occur through a process called: a) transverse fission b) binary fission

___5. A hermaphrodite is an animal that:
     a) can asexually reproduce b) can regenerate c) has both male and female parts

___6. The dugesia belongs to the phylum: a) platyhelminthes b) planaria c) turbellaria

___7. How could a person catch a dugesia?
     a) using a net to siphon them from the surface of the water     b) using a hook and raw liver to attract them

___8. An animal that has no internal body cavity to hold organs is called a(n): a) hermaphrodite b) acoelomate

___9. How does the dugesia obtain oxygen? a) diffusion b) through gills c) with its flame cells

___10. The dugesia will tend to stay in what areas? a) dark b) light c) warm