Name _____________________________________
Planaria
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.
QUESTIONS
1. What type of symmetry does the dugesia have?
radial
bilateral
asymmetry
2. What term is used to describe how a planarian can regrow its body parts?
regeneration
fission
cephalization
3. Which of the following is associated with the planarian's nervous system?
flame cells
diffusion
ganglia
4. Asexual reproduction can occur through a process called:
transverse fission
binary fission
cocoooning
5. A hermaphrodite is an animal that:
can asexually reproduce
can regenerate
has both male and female parts
6. The dugesia belongs to the phylum:
platyhelminthes
planaria
turbellaria
7. How could a a person catch a dugesia?
using a net to siphon them from the surface of the water
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):
turbellarian
hermaphrodite
acoelomate
9. How does the dugesia obtain oxygen?
diffusion
through gills
with its flame cells
10. The dugesia will tend to stay in what areas?
dark
light
warm