BACKGROUND:
Nature for some reason likes symmetry. Many organisms
display a beautiful pattern in their group. Early philosophers would write
about why nature is so consistent. For instance, to the left is a picture of
fuchsia, which grows in multiples of four. So even a plant, knows its time
tables! We call this 4-part symmetry.
Other organisms have bilateral symmetry, which is a mirror image of one side.
Bilateral symmetry is characteristic of many animals, including insects,
fishes, amphibians, reptiles, birds, mammals, and many crustaceans.
Some organisms have a 5-part or pentagonal symmetry.
Echinoderms including sand dollars, sea urchins, and sea stars have this
"star" symmetry.
Other organisms have no symmetry or asymmetrical. Spiral symmetry, like a
snail, is asymmetrical. A circle has radial symmetry, which is no matter how
you cut it through the center, one side will look like the other.
PROCEDURE:
- Pass out the
student worksheets. Ask the students to look at the shells and then describe
the differences orally. Many will find it difficult. Record
their responses on the board.
- Instruct students to
begin working on the exercise. Give students guidelines to help them compare
objects. First ask students to compare: color, length, width,
lines, and shape.
- Color is dependant on
the real clam or snail you give them. You may want to use real objects
for the entire exercise. Their answers will differ depending on the color
of their sample.
- The shape of these two
objects can be divided into two groups: bilateral and spiral. Bilateral
objects can be split in two leaving two objects that mirror each other.
A spiral object cannot be divided into any parts that look like the other
side (asymmetrical).
- Review their charts
when finished. Use the following chart as a guideline. Discuss the
similarities and differences.
|
CLAM
|
SNAIL
|
| length |
5.5 cm |
7.5 cm |
| width |
6.3 cm |
4 cm |
| lines |
across, up and down |
up and down |
| color |
depends on sample |
depends on sample |
| shape |
bilateral |
spiral |
