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Dichotomous Key 2
Examples of Student Work
Contributed by: New York State Alternative Assessment in Science Project (NYSED)

Student #1

Task: With the millions of living organisms in the world, scientists need a method of identifying an unknown organism. To do this, scientists use a dichotomous key. At this station, you will be using a dichotomous key to identify unknown organisms.


Use the dichotomous key below to identify any three (3) species of the genus Triangulum in the accompanying Species Sheet. Record the choices made, and the resulting scientific name, on the answer sheet provided.

Dichotomous Key

1. A. Three (3) sided with straight lines   Go to 2
  B. Three (3) sided with wavy lines   Go to 10
2. A. Has no eyes   Go to 3
  B. Has eyes   Go to 5
3. A. Has flagella for movement   Go to 4
  B. Has cilia for movement   Go to 5
4. A. The three sides are of equal length   Triangulum equalius
  B. The three sides are not of equal length   Go to 12
5. A. Has crossed eyes   Go to 6
  B. Eyes not crossed   Go to 9
6. A. Has a single flagellum for movement   Triangulum monoflagelleum
  B. Has two or more flagellum for movement   Triangulum polyflagelleum
7. A. Total number of cilia for movement are odd   Triangulum oddcilius
  B. Total number of cilia fo rmovement are even   Go to 8
8. A. Has a pointed nose   Triangulum pointiatus
  B. Has a rounded nose   Triangulum roundiatus
9. A. Has two cilia on each side for movement   Triangulum biciliatus
  B. Has more than two cilia on each side   Triangulum Polycilius
10. A. Has crossed-eyes   Go to 11
  B. Eyes not crossed   Triangulum waveus (samplest correctus)
11. A. Lower half of the body has a dot pattern   Triangulum lowdotteus
  B. Upper half of the body has a dot pattern   Triangulum upperdotteus
12. A. Has a pointed nose   Triangulum pointiflagelleum
  B. Has a rounded nose   Triangulum roundiflagelleum

Answer Sheet

  1. Choose any three of the species of Triangulum from the species Sheet and key them to their scientific names. Be sure to write in the numbers of the species that you are trying to identify. In the proper spaces below write in the number and letter for each of the choices you made as you identified the species (see example with species #101). When you are sure of the species identification, write in the scientific name in the space provided.



    Scientific name of Example # 101 = Samplest correctus


  2. The following Triangulum organism was created by a student. It is not one of the thirteen organisms shown on the Species Sheet. Can you determine its species? If so, list the steps, as you did before, in making this determination.


  3. Draw another example of the organism in question #2 that would "key out" as being the same species.


  4. A new species of Triangulum was discovered by a lab student in a sample of stagnant pond water. The dichotomous key must be modified to identify the new organism shown below.



    1. Between which steps in the key should the new trait be added?

    2. Write descriptions of this trait, for this step, based on the drawing of the newly discovered organism.

    3. Write the scientific name for this new organism.


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