Introduction:

In class you have learned how to use the compound microscope, how to estimate the sizes of objects in the field of view, and a little about types of cells. In this activity we will put these skills to use in an expedition into a drop of pond water. By utilizing all of your objective lenses, you will gain a perspective on the huge range of sizes in so-called "microscopic" organisms.

Materials:

• compound microscope, dissecting scope (for larger organisms) or digital microscope for Mac or Windows
• pond water
• clean microcope slide
• clean cover slip

Methods:

1) Take a drop of pond water from the stock culture and prepare a wet mount slide.

2) While the slide is over the lighted microscope stage, look to see if you can locate anything with your naked eye. With luck, you may be able to see some tiny white flecks swimming around in the water. These would be the largest protozoa.

3) Starting on low power, then medium, then high, scan around the field of view to locate as many living organisms as you can. You should spend at least five minutes at each magnification. (If your drop has little in it, you may take another sample if you wish.)

4) Locate at least one organism at each power, and draw it below:

5) See if you can identify what you have found.

6) EXTRA FOR EXPERTS: Try preparing a second wet mount with a little bit of methyl cellulose ("ProtoSlo") mixed into the water before you place the cover slip.

7) Search the Internet for images of things you found under the microscope...here's a great resource page:

Analysis:

The following are some of the organisms commonly found by students:

1. Oligochaet worms. These relatives of earthworms are about 1 or 2 mm in length and have tiny bristles along the body. They can compress and stretch their bodies like and accordion.
2. Nematode worms. These smooth tube-like worms whip from side to side. They are usually from 0.1 to 0.5 mm long.
3. Rotifers. These filter-feeders are from 0.1 to 0.5 mm long; they often work back and forth like someone vacuuming a carpet.
4. Paramecia. These ciliated protozoa are large and fast-moving. They tend to be from 0.02 to 0.1 mm long.
5. Hypotrichs. These ciliated protozoa have a few large spikes on the poseterior end. They are usually about 0.05 to 0.1 mm long.
6. Spirilli. These bacteria are shaped like corksrews and seem to "drill" their way through the water. They will only be visible with careful technique on the highest power. They are generally about 0.005 mm to 0.01 mm in length.
7. Bacilli. These tiny bacteria can be found clinging to the glass itself on highest power. They are about 0.001 to 0.005 mm long.
8. Euglenoids: Common flagellated photosynthetic single celled organisms.
9. Amoebae: A shapeless unicellular organism that consumes its prey by phagocytosis.
10. Hydra: A multi-armed predatory freshwater organism in the phylum Cnidaria, related to the corals and sea anemones.
11. Stentor: A funnel shaped filter feeding protozoan.
12. Volvox: a colonial green algae made up of a group of flagellated single celled organisms similar to Chlamydomonas.

Which of your "finds" were multicellular? Which had true cells with nuclei? Explain your reasoning...