Cnidarian Lab: Response of Hydras to External Stimuli
How do hydras
respond to light? Hydras, a type of Cnidarian, are some of the simplest known
animals to have a nervous system. Cnidarians gather information from their
environment using specialized sensory cells. Both polyps and medusas have a
nerve net, shown in the figure below. A nerve net is a loosely organized
network of nerve cells that together allow Cnidarians to detect stimuli such as
the touch of a foreign object. The nerve net is usually distributed uniformly
throughout the body, although in some species it is concentrated around the
mouth or in rings around the body. Cnidarians also have statocysts,
which are groups of sensory cells that help determine the direction of gravity.
Ocelli (oh-sel-eye; singular: ocellus) are
eyespots made of cells that detect light. Many hydras, such as the green
hydras, have a symbiotic relationship with algae. The alga has a place to live
and provides nutrients to the hydra by performing photosynthesis. If one were
to compare how green hydra and brown hydra react to light, then a better under-standing
of why green hydra are attached to light will be achieved.
Green Hydras – pre-ordered
Brown Hydras – pre-ordered
Slide w/ Well (cover slip)
(6) Test Tubes with Screw Caps
- Using an eye dropper, carefully
remove (1-3) hydra(s) from their container with minimal amount of water
and place it/them on a clean slide; do not use a cover slip.
- Observe the hydra under the
microscope at different levels of power; adjust the lighting if necessary.
Make a mental note about size, shape, color, movements …
- Remove the slide (clean, dry and
return) and turn off the microscope.
- Using a pencil, number each of the
test tubes (1-6).
- Fill each test tube with pond or
spring water to within about 2 cm from the top.
- In test tubes
(1-3) use an eye dropper to gently place three (3) brown hydras into each
- In test tubes
(4-6) use an eye dropper to gently place three (3) green hydras into each
- Wrap the bottom half of each test
tube in aluminum foil.
- Tightly cap all 6 test tubes.
- Place test tubes 1 and 4 right
side up in the test tube rack.
- Place test tubes 2 and 5 upside
down in the test tube rack.
- Tape test tubes 3 and 6 together.
Place tape only on the foil or caps.
- Place your test tube rack in the
designated light source place.
- Lay test tubes 3 and 6 on their sides
next to or on the test tube rack.
- Label your test tube rack with a
small piece of paper and/or tape.
- Make sure that all test tubes are
equally well exposed to the light source.
- Ask yourself (confer with your
partner), what would you hypothesis might happen based on your knowledge
of hydras and photosynthetic relationships.
- Clean and put away all materials.
If you have not done so, see step #17 above before retrieving/observing your
- Carefully (do NOT invert or set
upright any test tubes) return your hydras back to your lab table.
- Using a magnifying glass and desk
lamp, count the number of hydras in the light and in the dark in
each test tube one at a time. Note: remove foil if necessary to count
those in the dark part of the tube.
- Record these observations in the
data table for your group.
- Totals may be figured for the class when
all groups are done observing/recording.
- Answer questions proposed at the end
of this lab. Clean and put away all materials.