Nervous System and Behavior Lab BACKGROUND
Name _______________________________
Period _____ Date ____________________
Learning about the creatures to be used in a lab will help you better understand how to handle them and
how to understand their behavior.
Paramecium
Paramecium are single-celled protozoa and are extremely fragile; therefore all manipulations
must be performed gently. Pipetting should be done slowly and cultures should not be handled
roughly.
Keep loosened caps on all culture-containing jars when not in use and be sure to keep out of
direct sunlight. Most protozoans do best at a temperature of 20° C, however the temperature may
be anywhere between 18° C and 22° C. Paramecium can be maintained in a 1 liter flask capped
with cotton, with hay for nourishment. New subcultures should be established every 2 to 4
weeks.
Paramecium Behavior
Being a single-celled organism, paramecium do not have nerve cells. They are, however,
responsive to a variety of stimuli, including very subtle changes in temperature and chemistry that
we can detect only by finely calibrated instruments. These responses are fixed (stereotyped).
As a paramecium swims, the strongly beating cilia around the oral groove create currents that
constantly bring a sample of water to the buccal cavity, which seems to be a testing area.
Paramecium continuously explores the environment that lies ahead, shifting and sampling.
In general, paramecium responds to changes in its environment by avoidance reactions. If it
receives a negative stimulus, it turns away – and always in the same direction, to its left. It will
then continue in the new direction indefinitely unless it receives other stimuli.
If the negative stimulus is powerful, such as a poisonous chemical, the paramecium will stop,
back up, then turn to its left 30°, and then start forward again. If necessary, it will repeat this
performance. Under a strong negative stimulus, it will turn a full 360° and will continue to turn
until an avenue of escape is found. It can find its way around solid objects in the same way.
The end result of this is about the same as if the organism were “attracted” to a favorable
situation. By turning away from the more negative area, the effect is the same.
Hydra
Hydra are freshwater coelenterates. Their simple structure consists of a polyp – a slender stalk
with a row of tentacles surrounding the mouth at the top, and a pedal disc at the base, by which
the hydra attach themselves to the substrate. Voracious predators, hydra’s tentacles are armed
with stinging cells (nematocysts) that “harpoon” their prey (usually small crustaceans).
Hydra usually reproduce asexually, by budding. This process last two to four days, ending with
the daughter hydra detaching from the parent to become a separate organism. Hydra can also
reproduce sexually.
Hydra are best kept in an aerated aquarium or tub. Keep the temperature relatively low (18 - 21°
C). Use only pond water or a mixture of pond water and deionized water. Hydra may be fed the
larvae of brine shrimp, or if the hydra is large enough, daphnia.
Hydra Behavior
The behavior of the multicellular hydra is far more varied and complex than the paramecium.
Hydra has a network of nerve cells extending though the entire animal, called a nerve net. It is
unlikely that there is a specialized controlling group of nerve cells (a brain). The nerve cells
communicate with one another by sending chemicals across gaps (synapses) between neighboring
nerve cells, similar to “higher” animals, but hydra nerve cells send signals in all directions at
once. This is an inefficient kind of nervous system and impulses travel very much more slowly
than in higher types.
The whole body of the hydra responds to a local stimulus applied to only a part of one tentacle,
which is not efficient, but apparently is beneficial enough for hydra.
The nerve net not only transmits impulses but also co-ordinates hydra’s activities. When a small
animal touches one tentacle, the other tentacles will come to grasp the prey and will work
together to cram it into the mouth. The importance of the coordinated activity is emphasized by
what happens when it fails. Sometimes the hydra swallow its food so rapidly that it takes in one
of more of its own tentacles (but it will usually eventually release it).
The simplest method of locomotion in a hydra is a gliding by the basal cells. Sometimes, though,
the animal somersaults: it bends over, attaches tentacles to the bottom, loosens its base, swings to
the base over the mouth, and attaches to the bottom. It then loosens the tentacles and repeats.
Hydras react to a variety of stimuli, usually by a kind of trial-and-error procedure. A hydra will
move away from a region where the temperature rises above 25° C. Many species react to light
and tend to move toward the lighted side of their container.
Planaria
Planaria are free-living aquatic flatworms. They have a simple nervous system (but more
complex that paramecium or hydra) with a head and “eyes.” the eyes have no lenses and so
cannot form an image, but they are sensitive to light. (To get a sense of how planaria “see,” close
your eyes and face a bright light.) Planaria are carnivorous, eating living or dead animal matter.
Planaria reproduce both sexually and asexually. There are two methods of asexual reproduction:
fragmentation and spontaneous “dropping tails.” Fragmentation begins with a constriction just
behing the pharynx, which increases until the two parts separate. The head grows a new tail, and
the tail grows a new head. In stagnant water, planaria will frequently “drop” their tails and the
tails regenerate to form complete, but stunted, animals.
Keep planaria in a covered shallow enamel pan or dish (or in an aquarium). Use pond water.
Change the water every day or at least every other day. Keep water at 21° C or lower. Feed the
planaria beef liver three times a week. They will also eat earthworm fragments and chopped mealworms.
Planaria Behavior
In flatworms we see the first appearance of a central nervous system, the kind possessed by all
“higher” animals. The head contains a concentration of nervous tissue called a brain. From the
brain two nerve cords run backward through the body. Side nerves branch off from these central
nerves.
Stimuli are received by sensory cells distributed all over the body surface, but are concentrated in
the head to form sense organs that are especially sensitive to touch, to water currents, and to food
and other chemicals.
The planaria shows a more varied behavior and much more rapid responses than does the hydra.
Planarias avoid light. If placed in a dish exposed to light, they immediately turn and move toward
the darkest part of the dish. They are sensitive to touch. In addition, they react quickly to the
presence of food by turning and moving directly toward it.
Nervous System and Behavior PRELAB Assignment
Read the background information about paramecium, hydra and planaria. Then complete the chart below.
Paramecium
Nervous System
structural
characteristics
Behavioral
characteristics.
Discuss responses
to various stimuli
and their adaptive
values.
Hydra
Planaria
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