Turbellaria are flatworms that mostly live in the ocean. Flatworms, like small other small organisms play an important role in the food chain. A flatworm that lives in the fresh waters of Nebraska lakes and rivers is planaria. One such specific phyla of planaria, is known as Dugesia. Most planarians have a spaded head with a tapered caudal tail. They are very rudimentary in their body systems and thus are a good point of study in the beginning comparative view of animals.
Imagine, you are swimming in a lake or pool. The water is cool. You dive head first to touch the bottom when all of a sudden you feel a sharp stabbing pain in your stomach. It pains you to open your eyes underwater, but you do so anyway. When you open your eyes you see a large flat worm that has stabbed a tube into your stomach. The tube begins to throb and pump your insides out of you and into the worm. You begin to realize that you are about to die and that this worm is pumping your insides out into it's intestines while you are still alive.
If planaria were the same size as us we might have to worry about a situation like this. Fortunately planaria and other such flatworms are very small. As a matter of fact most flatworms are so small it requires an electron microscope (a magnification of 50,000 times it's original size, or more) to see them.
Planaria feed themselves by jabbing a small, muscular tube called the pharynx, from the midpoint of it's ventral surface, into the prey. They then use muscular contractions of the pharynx to pump the inside contents of the prey directly into the intestine of the planaria. There are enzymes in the pharynx begin digesting the contents of the prey as it enters the intestine. The digested particles are absorbed directly from the intestine into cells. This means that there is no circulatory system to absorb and transport digested food particles.
Neural Control (nerves):
You will notice that planaria have eyes. These "eyes" are not really eyes at all. As a matter of fact they are called eyespots. The eyespots do not see like we do, they are able to detect the intensity and direction of light, but can not focus on us like we do them. They do not really have a brain. The have a cerebral ganglia. A ganglia is just a bunch of nerves in one central area. They use these nerves to interpret responses like touch and senses of light and respond accordingly.
Planarians move like other worms do. They swim in a wave-like motion of their body. Over solid surfaces they glide over a slime layer of mucus that they secrete, propelled by the cilia that cover their body. Cilia are small hair-like projections of the cell membrane that move back and forth to aid in the movement of the organism, much like the single flagellum moves in the bacteria cell studied in class a couple of weeks ago.
Planarians are free living and motile so they can encounter and mate with other planarians of the same species. Planarians are hermaphroditic organisms (have both female and male sex organs). When they reproduce sexually they simultaneously fertilize each other. Their eggs are laid in protective capsules that stick to rocks or debris and hatch in 2-3 weeks. Planarians also reproduce asexually. They do this by binary fission (you may also hear fission called budding). In this process, they constrict just behind the pharynx and gradually split into two separate organisms.
Planarians breath without the presents of lungs or a circulatory system. They simply absorb oxygen directly from the surrounding fluid medium. This type of breathing is not unusual. It is called cutaneous respiration, and may also be called skin breathing, because they actually breath from their skin by simple diffusion of oxygen from the water around them. This also means that they do not need a circulatory system. The presents of veins and arteries is not needed since breathing is done through the skin and absorption from the intestines is directly to surrounding cells.
Observations and Questions:
1. Obtain a slide from the slide box and observe the planaria.
- Draw what you see. Include in your drawing the eyespots and the pharynx.
2. If you had a live planaria and placed it under light, would it try and run away from the light (would it behave photo positive or photo negative)? Why? Explain your answer in regards to how this would be adventitious for the planaria.
3. Explain how a planaria can be placed on a table top, a couple of centimeters away from food but still get to it and eat.
4. Does a planaria see it's food? Explain how it can sense it's food is present if it cannot see it.
5. If you touched a planaria how would it react? Would it leach onto you or would it jump away? Explain based on what would be most beneficial to the planaria.
6. Is the planaria prokaryotic or eukaryotic?
Is it heterotrophic, autotrophic or saprophytic, bilaterally
or radially symmetrical?