SNAIL RACES
BACKGROUND
Much can be learned from the study of the common garden snail. For example, many of us have used the term
"slow as a snail." But what do we really mean? Just how slow is a snail? Today we will have an opportunity
to find out by using some of the procedures involved in scientific investigation. We will also attempt to become
more familiar with the metric system.
WORKING WITH THE METRIC SYSTEM
In a world where most people use the metric system rather that the English system of measures, it is
important for Americans to know both systems, as the following report illustrates:
MARS PROBE LOST TO SIMPLE MATH ERROR
1 October 1999: NASA lost its $125-million Mars Climate Orbiter because spacecraft engineers failed
to convert from English to metric measurements when exchanging vital data before the craft was
launched, space agency officials said. A navigation team at the Jet Propulsion Laboratory used the
metric system of millimeters and meters in its calculations, while Lockheed Martin Aeronautics in
Denver, which designed and built the spacecraft, provided crucial acceleration data in the English
system of inches, feet and pounds. As a result, JPL engineers mistook acceleration readings measured
in English units of pound-seconds for a metric measure of force called newton-seconds. "That is so
dumb," said John Logsdon, director of George Washington University's space policy institute. Over
the course of the journey, the miscalculations were enough to throw the spacecraft so far off track that
it flew too deeply into the Martian atmosphere and was destroyed when it entered its initial orbit
around Mars. John Pike, space policy director at the Federation of American Scientists, said that it
was embarrassing to lose a spacecraft to such a simple math error. "I can't think of another example
of this kind of large loss due to English-versus-metric confusion ... it is going to be the cautionary tale
until the end of time.
PROCEDURES
We will record some preliminary data. Name your snail. Be careful with your choice as snails are
hermaphrodites; each is both male and female. Using the small rulers at your desk, measure the length
and width of your snail's shell, first in inches and then in centimeters. The fleshy portion of your snail
varies depending on its “mood," so measures of the hard shell alone are more objective. Both scales
(inch and metric) are printed on the rulers. The numbered divisions on the metric side are centimeters
(the rulers are about 15 cm in length). Record these measurements in the table provided on the
worksheet at the end of this lab.
Now determine the weight and volume of your snail in metric measures and record the results in the
DATA section of the lab worksheet. Your instructor will show you how to use an electronic balance and
graduated cylinder for this purpose.
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Science has been defined as "organized common sense." Its purpose is to increase our knowledge of
nature. Any question that can be tested may be answered using scientific methodology. A possible
answer to a testable question is known as a hypothesis. Some hypotheses that are exceptionally well
supported by a large number of observations get advanced to theories. Theories are often broad in scope
and are supported by a wide body of evidence that it is scientifically accepted as a factual framework.
about some natural
phenomenon, generate testable Hypotheses (possible answers to scientific questions that explain the
phenomenon), generate Predictions for each hypothesis (if an hypothesis is true, then one should expect
(predict] a certain result of the experiment), Test the predictions (the test should determine which set of
predictions do not occur), and Conclusion (which hypothesis is supported by the evidence and which
should be rejected). The scientific method allows testing of multiple hypotheses to eliminate the
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incorrect ones.
The basic steps of the scientific method are: make Observations, Ask Questi
In most hypothesis-testing experiments, there are two main variables; the independent variable and the
dependent variable. An independent variable is the one that is changed or controlled by the
experimenter to test the effects on the dependent variable (usually represented on the x-axis). The
dependent variable is the one being tested and measured in the experiment (usually represented on the
y-axis). Its value only changes in response to the independent variable. What is the relationship between
weight and speed in snails? Are heavy snails faster than light snails, or are light snails faster than heavy
snails, or.....? Now propose a hypothesis, a tentative explanation for your question. What is your
hypothesis regarding the relationship between weight and speed in snails? Write your hypothesis in
the space provided on the lab worksheet. We can attempt to test your hypothesis with an experiment
that is known as the Bio 3 "snail race.
WARM SNAIL RACES
Place your snail on the surface of the table. Hold the snail in place until the instructor indicates the start
of a race. Each race will last 60 seconds. At the end of 60 seconds, determine how far your snail has
traveled. Snails seldom move in a straight line. Their curved path can be measured after the race by
laying a string over their curved slime trail and then stretching the string out along a meter stick.
The first race will be repeated five times with the snail's body temperature at room temperature. Record
the results of each "warm” race in the DATA section of the lab worksheet. After the fifth race, determine
the average distance your snail traveled under warm conditions. Add all five warm race distances and
divide this figure by five. The result is your snail's average speed at room temperature in centimeters
per minute.
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COLD SNAIL RACES
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Now we'll propose a second hypothesis and test it. What is the relationship between temperature and
che speed in snails? We've termed the first races "warm" because room temperature is relatively warm for
snails. We can now compare these results to those obtained from "cold" snails under the same
conditions. What is your hypothesis for this experiment? How fast will cold snails travel compared to
warm snails?
For this experiment, chill your snail down by placing it in a small watch glass containing crushed ice.
The snail must be chilled for one minute prior to the first "cold race," and again before each of the cold
races. Record the results of each "cold” race in the DATA section of the lab worksheet and calculate
your snail's average cold racing speed.
Contribute your snail's data to the table your instructor has provided on the whiteboard/spreadsheet.
Copy the entire class's data onto the chart provided in the DATA section of the worksheet.
Now that we have compiled everyone's snail data, it is time to process them. This will involve some
simple addition and division. Since you will use pocket calculators, which provide what appear to be
fantastically "accurate” calculations, take a moment to consider the following points regarding
significant figures and rounding off.
SIGNIFICANT FIGURES AND ROUNDING OFF
Your calculator might indicate that your snail's calculated average speed was, for example, 15.32666 cm per
minute. This impressive figure suggests you were able to measure your snail's travel distance to the nearest
hundred-thousandth of a centimeter! How can you ensure that your reported values are within your means of
accuracy?...by noting the accuracy of your measuring instrument and following the rules for significant figures.
The rulers you used to measure snails could measure only to an accuracy of 0.1 cm, the smallest marks on the
ruler (note that other measuring instruments you have used may have different levels of accuracy). This means
that all calculations from these measurements can be no more accurate than 0.1 cm. For example, if you divide
48.7 by 5, your calculator will give you 9.74 for an average speed. The 4 (0.04) in this figure is not a significant
figure because it is smaller than 0.1 cm. The 7 that precedes the 4 is the last significant figure you can report. So
what to do with the 4? Disregard it? In this case, yes. Because 4 is less than 5, 9.74 gets "rounded down" to 9.7.
If 9.74 had been 9.76, however, it would be “rounded up” to 9.8 because 6 is greater than 5. When it comes to
rounding off 5s, alternatively round up and then down as they are encountered.
ANALYSIS
1. Determine the average weight of snails in each of the three size classes. Your instructor will establish
which snails are considered large, medium, and small. Do this by separately adding up the weights
of all LARGE snails first, then for all MEDIUM snails, and finally for all SMALL snails. Divide
each of these sums by the number of snails in the respective size class. Record these averages in the
spaces provided below the table of everyone's snail measurements.
2. Determine the average speeds of large, medium, and small snails when raced at room temperature.
Do this by separately adding up warm speeds for all LARGE snails first, then for all MEDIUM snails,
and finally for all SMALL snails. Divide each of these sums by the number of snails in the respective
size class.
3. As in number 2 above, determine the average speeds of large, medium, and small "cold" snails.
4. Plot the average warm and cold speeds of snails on the graph provided in the lab worksheet. Set the
minimum and maximum speeds on the y-axis of this graph according to the minimum and maximum
3
speeds you actually observed. Plot three dots representing the average warm speeds. Link these dots
together with a solid line. Then plot three dots representing the average cold speeds and link them
together with a dotted line (independent of the warm race line). Different color inks can also be used
rather than solid versus dotted lines. If you look at the table of data on the whiteboard and then the
graph that you have created from the data, you will readily understand how effectively graphs can
convey the results of a study!
it to do
OBSERVATION
Louis Agassiz (1807-73) said, "Study nature, not books." Objective observation of real organisms and
events is one of the most important aspects of science. Observational records made by scientists must
be methodical, detailed, clear, and as free from bias as possible. Why? Scientists of the future must be
able to study their predecessors' records efficiently, and without any misunderstanding. In your lifetime,
many species will pass into extinction with little more known of them than a scientific name and a
narrative description. Observe and describe your snail as if it were the first and last specimen ever to be
known to science. Use a dissecting microscope to observe your snail in greater detail than your unaided
eyes are capable. Your instructor will review the proper use of this tool. More complete instructions for
microscope use can also be found in this manual in the chapter on Microscopic Life. Record your
observations in the worksheet at the end of this lab.
do not
Your description of your snail should be structured. Start by describing the overall organism: its general
shape, length, width, weight, color, surface texture, etc. Then describe each of the animals anatomical
regions. With organisms such as a snail that exhibit cephalization--anterior (head) and posterior (tail)
ends--you might logically begin with the anterior end and continue down the length of the animal. For
each region you should describe such aspects as relative size, shape, color, texture, etc. If you
know the biological name for some structures, try to use descriptive analogies, such as "pear-shaped”
or "blood red.” Actual physical measurements are preferred when it comes to recording the size of an
organism or its anatomical features, so make use of the metric rulers provided. Make a sketch of your
snail as you explore its anatomy. The act of sketching helps focus the mind on anatomical details
your
eye might otherwise overlook. Label the name of structures that
.
you know
An important way to remain objective in your observations is to try not to make assumptions! For
example, if you wrote that “the snail's eyes are on the tips of two small stalks on its head" ...you made
two mistakes. You assumed that those little black specks were eyes and guessed that the front part was
the head! A more objective statement might be the following: “Two small stalks on the top of the snail's
front end have tiny black specks at their tips which might be used for gathering sensory information.
And be careful about using terms that have very specific meanings in biology. For instance, only certain
arthropods (insects and crustaceans) have the segmented head appendages referred to as antennae. Your
snail, however, is a mollusk, and mollusks have fleshy head appendages referred to as tentacles.
After you have described your snail's anatomy, set it down and let it resume its normal activities.
Observe and describe its behavior. How does it move? How are the various body parts used in
locomotion and how are the movements of these parts coordinated? How does the organism respond to
different environmental stimuli such as different surfaces (e.g., wet versus dry) or contact with various
objects (e.g. ice, finger, paper, another snail, etc.). How does your snail react to light? To sound? If time
permits, you should also describe how the laboratory environment differs from the organism's normal
habitat and how this might have influenced your observations.
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ANALYSIS
Race Results (for all snails)
Make a line graph of average snail speeds. Establish a scale on the y-axis of the graph that is appropriate
for the minimum and maximum average snail speeds you calculated. Plot three data points for the warm
races and three for the cold races. Each of the points must correspond with a snail size on the x-axis
and an average speed on the y-axis. After you have plotted all points, connect the three data points for
the warm race with a solid line and the three points for the cold race with a dotted line.
ㅋ
Average Speed (cm/min)
Small
(9)
Medium
Large
(-9) (> -9)
Snail Size (g)
OBSERVATIONS
Sketch (label known structures)
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