LIFE IN A DROP OF WATER

Transcription

1 LIFE IN A DROP OF WATER Educational Video 24 Minutes Copyright, MMI Rainbow Educational Media, Inc. Distributed by: United Learning 1560 Sherman Ave., Suite 100 Evanston, IL

2 PRINCIPLE CREDITS Producer, Writer & Director: Consultants: Videography, Animation & Editing: Peter Matulavich Ira Jones, Ph.D. Dept. of Biological Sciences, California State University, Long Beach Michael Worosz, M.A. Curriculum Consultant Peter Matulavich Special Thanks To: Steve Barlow, Ph. D. Electron Microscope Center, San Diego State University Dan McKinney Swift Instruments Jones Biomedicals & Laboratory Pinecrest School, Woodland Hills, CA Video and Teacher's Guide produced for Rainbow Educational Media by Peter Matulavich Productions San Dimas, California

3 TABLE OF CONTENTS Introduction 2 Objectives 3 Summary 4 Review Questions 7 Discussion Questions 10 Activities 12 Glossary 14 Bibliography 18 Websites 20 Script 22

4 INTRODUCTION This video is designed to introduce students to the wide variety of microscopic organisms that inhabit fresh water. There are tens of thousands of different types and they can be found in every lake, pond, stream and puddle. Hundreds, even thousands, can be found in a single drop of water. The video discusses the proper methods of obtaining water samples at a pond, and how to prepare specimens for study under a microscope. Students learn that the organisms found in pond water share much in common with humans, in that they are all composed of cells. A cell is the basic unit of which all living things are composed and consist, at the very least, of a nucleus which controls cell activity, cytoplasm, a liquid in which many cell reactions occur, and the membrane, which holds the cell together. Much of the video is devoted to featuring the many fascinating organisms found in pond water, including bacteria, microscopic algae, protozoa, and various multicellular organisms. These organisms are important because all of them form the bottom of the food chain, some produce oxygen, and many are used in scientific studies. The video concludes with a discussion of how some of these organisms are carriers of disease and that millions of people die each year due to diseases carried by some types of bacteria and protists. After viewing this video, students will be left with a greater appreciation for these fascinating organisms, and that while they may be small, they are an essential part of the world around us.

5 OBJECTIVES After viewing this video, students should know: microscopic organisms can be found in every body of water, including lakes, ponds, streams, and puddles hundreds, even thousands of pond organisms, can be found in a single drop of water microscopic pond organisms share much in common with humans, in that they are composed of cells organisms that inhabit pond water include bacteria, algae, protozoa, and multicellular organisms common types of protozoa include Paramecium, Blepharisma, Spirostomum, Dileptus, Stentor, Vorticella, Bursaria, Didinium, and amoebas common multicellular microscopic organisms include rotifers, Daphnia, Cyclops, and hydras microscopic pond organisms form the bottom of the food chain algae are responsible for producing most of the air that we breath protists are often used in the study of genetics some bacteria and protists are carriers of disease

6 SUMMARY The video opens with a fast-paced sequence featuring a variety of fascinating pond organisms. The narrator then states that these organisms can be found in every lake, pond, stream and puddle. Mostly microscopic in size, hundreds, even thousands, can be found in a single drop of water. Students are then featured at a pond collecting pond water to study later at school. They start by collecting pond scum which is usually filled with micro-organisms. Then they add water and cap their jar. Back in the classroom, the students use an eyedropper to deposit a drop of water onto a glass slide. The slide is then covered with a coverslip and placed on a microscope stage. The narrator explains that while the organisms found in a drop of water may look strange, they share much in common with us, because they are composed of cells just as we are. To emphasize this point, a student is shown scraping the inside of his cheek with a toothpick and then dabbing this on a glass slide for examination under the microscope. The student discovers that the inside of his mouth is made up of tiny uniform structures called cells that are not that much different from the ones that make up the organisms found in a drop of water. Animation is then used to show how an animal cell is similar to an amoeba, a single-celled organism found in pond water. Both cells contain a nucleus, cytoplasm, and a membrane. It is explained that the nucleus is the control center of the cell, that cytoplasm is a fluid in which many cell reactions occur, and that the cell membrane controls what goes into and comes out of a cell.

7 The narrator explains that the smallest organisms that can be seen with an ordinary microscope are bacteria. Most bacteria are so small it would take about 10,000 of them laid end to end to equal one centimeter. While they may be small, they grow in great numbers and are an important food for other microscopic organisms which feed on them practically non-stop. Another important food source for many pond organisms are the algae, which are plantlike organisms. Algae belong to a larger group called protists, which include organisms that are neither plant nor animal. Most algae are filled with chlorophyll, a green substance that can be compared to blood in animals, and which permits photosynthesis. Several types of algae are then featured, including some that have a thin, barely visible tail called a flagellum which helps propel them through water. Students learn that protists that possess flagella are called flagellates. Other featured algae include Volvox and Spirogyra. Also featured are euglenas and diatoms, which are closely related to algae. The video then introduces the protozoa, which are protists that are more animallike than plantlike. A well-known protozoan is the paramecium, which resembles a tiny slipper as it swims about looking for food. Paramecium is referred to as a ciliate because it is covered by rows of short hairlike strands called cilia. Most protozoa are ciliates. Like most protozoa, para-mecia use structures called contractile vacuoles to maintain the proper amount of water within their bodies. Paramecia reproduce themselves during a process called fission in which it appears that two paramecia are joined at the ends. Sometimes confused with fission, is conjugation, during which two paramecia are joined at the sides in order to exchange cell material.

8 Protozoa featured in the video are Blepharisma, Spirostomum, Dileptus, Lacrymaria Olor, Stentor, Vorticella, Bursaria, Didinium, and amoeba. Microscopic multicellular organisms featured are rotifers, Daphnia, Cyclops, seed shrimp, and hydras. The video concludes with a discussion that while these organisms may be small, they are very important. All of them form the bottom of the food chain, some are responsible for producing most of the air that we breath, many are used in scientific studies, and some are carriers of disease. So as small as these organisms are, they are an essential part of the world around us.

9 REVIEW QUESTIONS 1. In what bodies of water can you find freshwater microscopic organisms? They can be found in every lake, pond, stream and puddle. 2. How many different kinds of pond organisms are there? Tens of thousands. 3. What do we share in common with pond organisms? We are all composed of cells. 4. What do you call a cell's control center? The nucleus. 5. What is the name of the fluid that surrounds the nucleus? Cytoplasm. 6. What gives a cell its shape? The membrane. 7. What are the smallest organisms that can be seen with an ordinary microscope? Bacteria.

10 8. Why are bacteria important? They are an essential food for other micro organisms. 9. What is another important food source for many pond organisms? Algae. 10. How do algae obtain their food? Through photosynthesis. 11. What is the green substance found in most algae? Chlorophyll. 12. What are clusters of algae called? Colonies. 13. What are protists that possess flagella called? Flagellates. 14. Why is a euglena's eyespot important? It enables it to find areas of light. 15. What does protozoa mean? First animals. 16. What are protozoa which possess cilia called? Ciliates.

11 17. What structures in protozoa allow them to take in and expel water? Contractile vacuoles. 18. How do most protozoa reproduce? Through fission. 19. What is the name of the process during which protozoa exchange cell material? Conjugation. 20. What is the name of the enclosure a protozoan forms when it enters a state of suspended animation? A cyst. 21. How does an amoeba move? By sending out streams of cytoplasm called pseudopods. 22. What is the name of the bubbles which protozoa form to surround food? Food vacuoles. 23. In what ways are protists important? All of them form the bottom of the food chain, some are responsible for producing most of the air that we breath, many are used in scientific studies, and some are carriers of disease.

12 DISCUSSION QUESTIONS 1. The video mentions that hundreds, even thousands of microscopic organisms can be found in a drop of water. Is it possible to find even more? Yes. Some protozoa are just a few microns (a few thousandth of a millimeter) long. Tens of thousands of these could easily fit into a drop of water. Some bacteria are even smaller. Millions, even billions, could be found in a drop of water. How large are the largest freshwater protozoa? The largest include some amoeba, which can range from one to five millimeters in length. 2. Only freshwater protozoa are featured in this video. Are protozoa found elsewhere? They are also found in saltwater, in damp soil, and within the bodies of other organisms. 3. The video mentions how an animal cell shares similarities with the single cell of a protozoa. How are these cells different? A protozoan cell is a complete life form capable of living independently of other cells. An animal cell is highly specialized and is dependent on other types of cells to live. 10

13 4. The nucleus is the control center of a cell? Do protozoa have only one nucleus? A few protozoa, including some types of amoebas, have more than one nucleus. Some ciliates have a macronucleus and a micronucleus, the latter controlling reproduction. 5. When photographed, many ciliates like Paramecium appear to have cilia lining only their outer edges. Are those the only places cilia are found? The bodies of Paramecium and many other ciliates are completely covered with cilia. When viewed through the backlight of a microscope, only the cilia lining an organism's edges are usually visi ble. 11

14 ACTIVITIES These activities are designed to encourage students to learn more about some of the things covered in the video. 1. The video features students observing a variety of pond organisms. Activity: Have students collect pond water to study later in class. 2. Students are shown drawing a pond organism as they observe it through a microscope. Activity: Have students do the same. They might use a protozoa reference book to assist them. 3. Some bacteria and protists are known to cause disease. Activity: Have students research and report on a plague or disease caused by bacteria or protists. 4. A variety of organisms are shown in the video. Activity: Have students research and report on any of these organisms, or any others they might be interested in. 5. The video features a boy examining his own cheek cells. Activity: Have students scrape the inside of their cheeks with toothpicks in order to observe their own cheek cells through a microscope. 12

15 6. The video discusses how euglena favor light. Activity: Obtain a thick euglena culture from a biological supply company and pour it into a petri dish. Cover it with a lid that is half covered with foil. Place this near a window. Have students examine the dish the next day. Students should discover that the euglena favor that part of the dish exposed to light and will congregate there, making that part of the dish darker green from the greater number of euglena. 7. Bacteria are featured in the video. Activity: Have students produce their own culture rich in bacteria by adding dried egg yolk, rice grains, or the broth obtained from boiling wheat kernels to a sample of spring water. Have them make periodic observations to see what changes occur over time. 8. Protozoa are easy to keep. Activity: Have students keep a protozoa culture several weeks, adding spring water when necessary. Have them make periodic observations to see what changes occur in the culture over time. 9. Cyclops and hydras are multicellular organisms named after characters from Greek mythology. Activity: Have students research these characters to see why their names were lent to the mentioned organisms. 13

16 GLOSSARY algae: plantlike protists that rely on photosynthesis amoeba: a single-celled pond organism which moves by sending out streams of cytoplasm bacteria: the smallest and simplest single-celled organisms; some cause disease Blepharisma: a protozoan with a large oral funnel lined with long cilia brood chamber: the enclosure in some organisms in which eggs are carried Bursaria: a very large protozoan with a gaping mouth used to swallow other protozoa whole cell: the basic unit of which all living things are composed chlorophyll: the green substance in plants and some protists that can be compared to blood in animals cilia: the hairlike structures found on the surface of some protists used for propulsion and food gathering ciliate: a protozoan with cilia colony: a group of organisms that live together for a common good compound eye: a multi-faceted eye conjugation: the process by which some simple organisms exchange cell material 14

17 contractile vacuole: a round structure found in protozoa which is used like a pump to take in and expel water coverslip: a thin piece of glass used to cover a specimen on a glass slide Cyclops: a multicellular pond organism with a single eye and long drooping antennae cyst: a protective enclosure in which protozoa protect themselves while dormant cytoplasm: the cell fluid in which many reactions occur Daphnia: a micro crustacean with large antennae related to shrimp, crabs, and lobsters diatom: a hard-shelled protist related to algae Didinium: a fast-moving protozoa which remains in constant motion searching out Paramecium, its only food Dileptus: a protozoan with a long trunk which it waves through the water euglena: a protist related to algae that relies on photosynthesis fission: the process by which some simple organisms reproduce themselves flagellate: a protist that possesses at least one flagel-lum flagellum: a thin, hairlike structure used by some protists for propulsion 15

18 food chain: a model that shows how energy moves in an ecosystem, based on large animals eating smaller animals which eat even smaller animals food vacuole: a bubble-like enclosure a protozoan forms to surround captured prey and food Giardia: a protist that can cause sickness in animals when swallowed glass slide: a rectangular piece of glass upon which a specimen is placed for microscopic study hydra: a multicellular pond organism with tentacles Lacrymaria Olor: a protozoan with a long neck and which resembles a swan membrane: the thin layer of tissue that surrounds cells micro-organism: a microscopic organism nucleus: the control center of a cell oral groove: a channel along the side of Paramecium which is used for directing food into its gullet Paramecium: a single-celled organism that resembles a slipper photosynthesis: a process in which sunlight is converted into food protist: an organism that is neither plant nor animal protozoa: microscopic, single-celled organisms pseudopod: a stream of cytoplasm sent out by an amoeba; false foot 16

19 rotifer: a multicellular organism with cilia on top of its head which resemble two spinning rotors seed shrimp: a multicellular pond organism enclosed in a seed-like enclosure Spirogyra: algae with spiral-shaped structures that contain chlorophyll and which form long strands sometimes referred to as pond scum Spirostomum: a large, worm-like protozoan stage: a platform on which a specimen is placed for study under a microscope Stentor: a large protozoan with a large mouth lined with long cilia tentacle: a long, slender part growing around the head or mouth of some animals Volvox: a ball-shaped colony of green algae Vorticella: a bell-shaped protozoan with a coiled stalk 17

20 BIBLIOGRAPHY Burgess, Jeremy and Marten, Michael and Taylor, Rosemary. Under the Microscope. Cambridge University Press, 1987 Dobell, Clifford. Antony Van Leeuwenhoek and His "Little Animals". Dover Publications, Inc. Ford, Brian J. Single Lens. Harper & Row, Publishers, 1985 Headstrom, Richard. Adventures with a Microscope. Dover Publications, Inc.. Adventures with Freshwater Animals. Dover Publications, Inc., 1964 Johnson, Gaylor and Bleifeld, Maurice. Hunting with the Microscope. Arco Publishing, Inc Kelley, Susan G. and Post, Frederick J. Basic Microbiology Techniques. Star Publishing Company, 1982 Kudo, Richard R. D.Sc. Protozoology. Charles C Thomas, Publisher, 1966 McCormick, J.B., M.D. and Turner, Gerard L'e, M.Sc. Antique Microscopes. Replica Rara Limited, 1975 Pennak, Robert. Fresh-Water Invertebrates of the United States, John Wiley & Sons, 1978 Rainis, Kenneth G. Exploring with a Magnifying Glass. Franklin Watts,

21 Reid, George K., Ph.D. Pond Life. Golden Press, 1967 Singleton, Paul and Sainsbury, Diana. Introduction to Bacteria. John Wiley & Sons, 1981 Stehli, Georg, Dr. The Microscope and How to Use it. Dover Publications, Inc., 1960 VanCleave, Janice. Microscopes and Magnifying Lenses. John Wiley & Sons, Inc., 1993 Worvill, Roy, M.Sc. The Telescope & Microscope. Ladybird Books Ltd.,

22 WEBSITES The following list includes some of the best websites devoted to pond organisms, protists and microscopy. Protist Image Data ( This award-winning site provides pictures and information on selected algae and protozoa. While the text is advanced, the picture gallery features top quality images. Each featured species has articles on appearance, reproduction, classification and much more. Steve Durr's Homepage ( This site is put together by the photographer of the Institute of Neurology in London. Steve's goal is to make a permanent record of the interesting plant and animal life found in freshwater in and around London. Steve's images are outstanding and so is his text. Society of Protozoologists ( Maintained by America's leading protozoology organization, this site features images and articles on a wide variety of protists. CellsAlive This site features a rich assortment of photographs, artwork and articles dealing with cells, including protists and bacteria. Light Microscopy Forum ( This site features images and articles on microscopes, pond organisms and polarized light. 20

23 Micscape ( This acclaimed site, devoted to the miniature world, includes a handy pulldown menu that leads to a variety of interesting topics, including articles and images on algae, protozoa, crystals, bat droppings, snow just about anything that can be placed under a microscope and examined. Molecular Expressions ( This outstanding site features an expansive photo gallery with hundreds of images, and articles on microscopes and microscopy. One of the highlights is a section featuring antique microscopes. NanoWorld ( oz.au/nanoworld/images_l.html) This site features a wide variety of fascinating specimens photographed with an electron microscope. Project Micro ( mepage.html) Sponsored by the Microscopy Society of America, this site offers suggestions on how to incorporate microscopy into the classroom and makes recommendations for books, videos and CD-ROMs related to microscopy. Note: Some servers have difficulty reaching every site address. If you encounter troubles, try reaching the site by entering its name in a search engine. 21

24 SCRIPT You are looking at some of the most fascinating life forms on earth. Organisms like these inhabit every lake, pond, stream and puddle. There are tens of thousands of different types. Mostly microscopic in size, hundreds, even thousands, can be found in a single drop of water. Let's find out more about these fascinating organisms. The journey into a drop of water begins with a visit to your local pond. These students are collecting pond water to study later at school. They start by collecting some pond scum. This is usually filled with microorganisms. Then they add water. Finally, they cap their jar. There's enough air in the water to keep the organisms alive for a day or two. They'll also collect water from several spots. Back in the classroom, an eyedropper is used to deposit a drop of water onto a glass slide. The slide is then covered with a thin piece of glass called a cover-slip. The slide is then placed on a microscope stage beneath two clips. The organisms you see in a drop of pond water may look strange, but they share much in common with you and me. Here's how you can prove it. Scrape the inside of your cheek with a toothpick. Dab this on a glass slide and cover it with a coverslip. Then place this under your microscope. As you can see, the inside of your mouth is made up of tiny uniform structures called cells. A cell is the basic unit of which all living things are composed. 22

25 A person's entire body is made up of cells. Billions of cells, that are not that much different from the ones that make up the organisms you find in a drop of water. In this example, the model on the right is meant to represent an animal cell, and the one on the left, an amoeba, a single-celled organism you find in pond water. Both cells contain a nucleus. Some amoebas may contain more than one. The nucleus is the control center of the cell. Its job is to control the cell's activities including reproduction. This is the nucleus of a cheek cell. Surrounding the nucleus is a substance called cytoplasm. This is a fluid in which many cell reactions occur. In the amoeba, the cytoplasm is constantly flowing, carrying cell materials with it. Every cell is surrounded by a membrane. The membrane is something like a sack that holds the cell together and gives it its shape. The cell membrane controls what goes into and comes out of a cell. In this dying organism, the membrane has broken and the organism explodes, spewing out its cytoplasm and other cell material. All organisms on earth are made up of one or more cells. Many of the ones you find in a drop of water are made up of a single cell. Each one possesses everything it needs to live as separate organism. Each cell is a complete, independent life form. 23

26 The smallest organisms you can see with an ordinary microscope are bacteria. They're the tiny specks jittering about. Most bacteria are so small it would take about 10,000 of them laid end to end to equal one centimeter. While they may be small, they grow in great numbers and are an important food for other microscopic organisms which feed on them practically nonstop. In this scene, a tiny pond organism creates a current, directing countless bacteria into its mouth. Another important food source for many pond organisms are the algae. These are plantlike organisms which appear in a variety of forms. Some resemble plants you find on land. Algae belong to a larger group called protists, which include organisms that are neither plant nor animal. But like plants, most algae are filled with chlorophyll, a green substance that is similar to blood in animals, and which permits photosynthesis, a process in which sunlight is converted into food. Under a microscope, you can often see the movement of chlorophyll within algae. Some algae exist as single cells, but many others join together in clusters called colonies. Each of these cells has a thin, barely visible tail called a flagellum which helps propel it through water. Protists that possess flagella are called flagellates. This colony is called Volvox and can consist of thousands of cells. Volvox colonies appeared billions of years ago and are thought to be the ancestors of multi-cellular plants we find throughout the world today. The cells in this colony move by sliding back and forth. They resemble a carpenter's folding ruler. The cells in other colonies form long stands. This one is called Spirogyra. It gets its name from the spiral structures filled with chlorophyll. 24

27 While pretty to look at under the microscope, large masses of it form the green, smelly substance we refer to as pond scum. Closely related to algae are the euglenas. As with many algae, these protists are filled with green chlorophyll and rely on photosynthesis to obtain much of their food. They move by wriggling their bodies and by waving their flagella which are located at their front ends. Euglenas have a reddish eyespot which enables them to find areas of light which is important for their survival. Other organisms closely related to the algae are the diatoms. They have hard outer walls and are found in both fresh water and oceans. Some diatoms are quite beautiful and are sometimes referred to as jewels of the sea. Some companies sell microscope slides with diatoms arranged in beautiful snowflake patterns. Common one-celled organisms found in pond water are the protozoa, of which there are thousands of types. Like algae, they too belong to the protists, but protozoa are more animallike than plantlike. Protozoa means "first animals" and they may very well resemble some of the first single-celled organisms that lived on earth billions of years ago. Among the most common protozoa is the paramecium, of which there are many types. These organisms resemble tiny slippers as they swim about looking for food. Paramecia are covered by rows of short hairlike strands called cilia, which are visible here along their outer edges. A protozoan that has cilia on any part of its body is called a ciliate. Most of the protozoa you will encounter are ciliates. A paramecium moves by beating its cilia and spinning its body in a spiral motion. In open water it can reach 25

28 remarkable speeds. It is one of the fastest protozoa. In tight spaces, a paramecium is able to bend its body back on itself in order to make a sharp turn. They can distort their bodies to squeeze into narrow channels. They can contort their bodies so much, it can be difficult to tell what they are. A paramecium eats by swimming back and forth through pond debris, feeding mostly on bacteria. A paramecium doesn't have a mouth. Instead, it has an oral groove. This is a channel along the side of its body which directs food into its gullet or stomach. You can see the oral groove more clearly as paramecia rotate their bodies. When you examine the interior of a paramecium, you will notice several round structures that open and close. These are called contractile vacuoles and work like little pumps, as a protozoan takes in and expels water. The vacuoles maintain the proper amount of water within a protozoan. You can learn a lot more about an organism by drawing it as you observe it. Your pictures don't have to win any awards. They're merely meant to help you become a better observer. Sometimes you will come across what appears to be two paramecia connected at the ends. This is actually a single paramecium reproducing itself in a process called fission. A single paramecium may divide up to three times in a single day. Other times you may encounter two paramecia joined 26

29 at the sides. This is called conjugation. It is sometimes confused with fission, but it is not. During fission, a single paramecium reproduces itself lengthwise. During conjugation two paramecia join at the sides in order to exchange cell material. Conjugation enables organisms to live longer and to better adapt to their environment. As you examine organisms under a microscope, you may wonder how small they really are. This may give you an idea. These are paramecia swimming about within an eye of a needle. As you can see, they are very small indeed. Another common protozoan you find in pond water is Blepharisma. Like paramecia, it swims along feeding mostly on bacteria. Blepharisma has a large oral funnel, or mouth, lined with long cilia which it uses to direct food into the funnel. As with paramecia and many other protozoa, Blepharisma reproduces itself through fission. These look like worms, but they're not. They're cili-ates belonging to the group Spirostomum. They are just about the longest protozoa you'll find in a drop of water. They're gentle giants and feed mostly on bacteria. Dileptus is an unusual looking protozoan because of its long neck, or trunk. It waves this through the water, directing food toward its mouth, located here at the base of the trunk. Dileptus feeds mostly on bacteria and tiny protozoa. Another protozoan with a long neck is Lacrymaria Olor, sometimes referred to as the "swan" because of rts resemblance to that bird. It whips it's neck in and ut of pond debris searching for food. 27

30 This protozoan, called Stentor, likes to anchor itself to pond debris as it sways back and forth through the water, feeding on bacteria and small protozoa. Its large mouth is lined with long cilia which it uses to create a current that helps direct food into its stomach. When it is disturbed, it will suddenly contract, disappearing into pond debris. Stentor takes on a different appearance when it detaches itself to swim freely through the water. The cilia lining its mouth are then used to help propel it. The delicate vorticella is another protozoan that likes to anchor itself to pond debris as it feeds. The cilia lining its mouth create twin currents, pulling in bacteria. Larger organisms like these green algae get caught in the current as well, but are not consumed. Vorticella have coiled stalks which they can extend and contract quickly. They often appear in a cluster and seem to take turns extending and contracting. One of the largest protozoa is Bursaria. It is a giant in this microscopic world. With a large gaping mouth, it can swallow other protozoa whole. This unlucky paramecium is stuck in this bursaria's stomach and will soon be digested. When it comes to hunting paramecia, none are better at it than Didinium. These are the sharks of the microscopic world. They hunt by staying in constant motion as they search out paramecia, their only food. Once a didinium comes in contact with a paramecium, it stabs it with a needle-like organ that paralyzes the paramecium. The paramecium may struggle, but seldom is it able to escape. The didinium then sucks the paramecium into its stomach. Just a few didinia can wipe out a large group of paramecia in a very short time. When there are no more paramecia, didinia will enter into a state of suspended animation in which they resemble spheres called cysts. They will remain 28

31 inactive like this until there are more paramecia on which to feed. Many other protozoa form cysts as well, enabling them to survive unfavorable conditions such as drought. Among the best known protozoa are the amoebas, of which there are many types. This one is called Pelomyxa and is among the largest known amoebas. The stream of cytoplasm an amoeba sends out is called a pseudopod, which means false foot. Pseudopods not only enable an amoeba to move, but they are also used in obtaining food, circling and trapping unsuspecting organisms. Some organisms, like this tiny protozoan, appear to be attracted to amoebas, hoping to feed on their contents, but frequently end up being swallowed themselves. If you examine an amoeba's cytoplasm, you will often find still-living organisms, struggling within it. An amoeba forms a bubble, called a food vacuole, around its food. The food, in this case a paramecium, is then digested within the vacuole. This organism resembles a protist but is actually a rotifer, a multicellular animal. It gets its name from the cilia on top of its head which resemble two spinning rotors, and which draw in small organisms. Here, you can also see its tiny, rapidly beating heart. A rotifer will stay anchored in one area as long as conditions are favorable, spinning its cilia, and stretching its body in one direction, then another. When it's time to move on, it'll release its foot, and squiggle its way to a different spot. The daphnia is another multicellular animal. It is considered a micro crustacean, and is distantly related to shrimp, crabs, and lobsters. It moves by flicking its 29

32 pair of large antennae. It has a single compound eye. While the eye is quite large, the daphnia's vision is probably poor. It is near its back where you find its heart, which beats at three hundred times a minute. Females are sometimes seen carrying as many as forty eggs in its brood chamber. Another interesting micro crustacean is the cyclops, named after the mythical giant that possessed one eye. Cyclops are easy to identify by their long drooping antennae, which they use as sensory organs and for swimming. One of the cyclop's favorite foods is paramecia, which it snaps up so quickly, it is difficult to see. Seed shrimp are unusual because they are nearly entirely enclosed in a shell. Only their antennae and legs are exposed. They're not really shrimp, but they are related. The dark spot you see near the top of this one's shell is its eye. When a seed shrimp dies, and its shell opens, it sometimes resembles a delicate butter-fly. The carnivorous hydra is named after the mythical monster with nine heads. It is normally seen attached to plants, stones, or debris, with its tentacles extended high above it. The tentacles are lined with sharp barbs that attach themselves to the hydra's prey. This daph-nia, though, is too large, and breaks away from the hydra's grasp. This small cyclops is much more to the hydra's liking, and the hydra is able to stuff the cyclops into its mouth at the base of its tentacles. You have seen just a few of the fascinating organisms that you can find in a drop of water. While they may appear small, they are very important. 30

33 Together, they form the bottom of the food chain and are an important food source for a variety of larger organisms, including fish. The algae are responsible for producing most of the air that we breath, which is a byproduct of photosynthesis. Because they reproduce so quickly, protists are often used in the study of genetics. And some of these organisms are carriers of disease. This protist, called Giardia, can infect people when they drink contaminated water. And when these organisms are swallowed, they release bodies that can invade people's intestines. Millions of people die each year due to diseases carried by some types of bacteria and protists. Those are some of the reasons why scientists study these important organisms, so that they can learn more about them. Dr. Barlow Even though they're very small, they are an important part of the world we live in. Organisms eat smaller organisms which eat the smallest organisms which are the protists. Protists provide a lot of the oxygen that we breath. Protists can be responsible for disease. Because they're easy to grow, scientists can use them to study a number of different scientific principles. So, although small, they're very important. In this program, we have seen how the strange organisms found in a drop of water share much in common with you and me and consist one or more cells. A cell is the basic unit of which all living things are 31

34 composed and consist, at the very least, of a nucleus which controls cell activity; cytoplasm, a liquid in which many cell reactions occur; and the membrane, which holds the cell together. A single cell can be a complete organism in itself or it may be part of multicellular organism. Single-celled organisms include bacteria, some algae, and a wide variety of protozoa. Multicellular organisms include rotifers, daphnia, hydras, and many others. These organisms are important because all of them form the bottom of the food chain, some produce oxygen, many are used in scientific studies, and a few of them cause disease. So as small as these organisms are, they are an essential part of the world around us. 32