Download Lab 2

BC 1501
INGESTION, DIGESTION, EXCRETION
OVERVIEW
The Big Picture: Last week you dissected an earthworm (Phylum Annelida) and a crayfish
(Phylum Arthropoda). This week you will re-examine the data you collected about body
length in crayfish and use statistics to determine if male and female crayfish differ in length.
Then, you will begin dissecting a fetal pig (Phylum Chordata), focusing on the digestive and
excretory systems. As you dissect your pig, make sure to compare the digestive and
excretory systems of Annelids, Arthropods, and Chordates.
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Before lab
During lab
Assignments (Due at the
beginning of next week’s lab)
Background Information:
o
Smith and Schenk (dissection
manual) Chapters 1, 4, and 7
o
Russell et al., CH 45
Today is the first graded weekly pre-lab quiz.
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Discuss answers to Arthropod and
Annelid comparisons
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Part A: Use statistics and pooled class
data from last week to determine if male
and female crayfish differ in length
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Part B: Begin dissecting a fetal pig
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Part C: Observe histological slides of the
esophagus and small intestine
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Worksheet 2A Statistical Analysis
(Group)
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Worksheet 2B Histology and
Comparative Anatomy (Individual)
Worksheet 2A, page 1 of 2
Thought and discussion questions:
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What is the phylum of the pig? How does the body plan of the pig differ from the body plans of
the earthworm and the crayfish? How is it similar?
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Why is the digestive system divided into so many different compartments? That is, why can’t
digestion be completed in a single structure?
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When you ingest a meal, what parts end up in feces and what parts end up in urine?
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What is the primary function of the excretory system?
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What is the primary function of the digestive system?
Learning objectives:
1. Learn the structure and function all of the external features, digestive organs, and excretory
components of the fetal pig.
2. Determine the gender of your pig, and all of the external features that distinguish males and
females.
3. Identify the structure and function of the different types of epithelial tissue that make up the
esophagus and small intestine.
4. Compare the anatomy and function of the major organ systems in three animal phyla: Chordata
(pig), Arthropoda (crayfish), and Annelida (earthworm).
5. Learn how to perform and interpret a t-test
Worksheet 2A, page 2 of 2
PART A: STATISTICAL ANALYSIS OF DATA: t-TESTS
BACKGROUND PART A:
Descriptive Statistics
Last week you measured the body length of the crayfish you dissected, and calculated the mean body
length of all of the crayfish dissected by the students in your lab section. The mean that you
calculated is an important statistic, but it does not provide all of the information contained in the data
set. For example, let’s assume that the mean body length of a crayfish from your lab section was 10.1
cm. This mean does not indicate whether some crayfish were well above the mean (say 14 cm) and
some were well below the mean (say 6 cm), or whether all the crayfish were approximately 10 cm
long. A statistic, called the standard deviation, can provide this information.
The standard deviation is a measure of the amount of variation in a data set. The smaller the
standard deviation is, the more tightly bunched the values are around the mean. Scientists generally
report some measure of dispersion, like a standard deviation, when summarizing their data, to provide
a sense of how much variance there was around the mean.
This type of data can also be presented in graphical form. A histogram is a type of graph in which
the height of each bar corresponds to the number of individuals that display a particular characteristic.
For example, the histogram below depicts the body length of 41 crayfish. From this graph, you can
see that twenty-one crayfish measured between 10 cm and11 cm, whereas only seven crayfish
measured between 9-10 cm, and four crayfish measured between 12 cm and 13 cm. You can also see
that there are no crayfish smaller than 9 cm or larger than 13 cm. On the bottom right-hand corner,
the values for the mean (10.7 cm) and the standard deviation (0.86), and total number of crayfish (41)
are displayed.
Worksheet 2A, page 3 of 2
Hypothesis Testing
There are many times that we would like to determine if two groups of data sets are significantly
different from one another. For example, you probably have a hunch from your observations last
week that female crayfish are slightly larger than male crayfish. But how could we test this in a
rigorous and scientific manner? To do this, we can use a statistical test called the t-test.
The t-test is a statistical test for determining if two means are significantly different from one
another. It is not enough to say two means are "pretty different" or "fairly close." The t-test is
designed to determine if the difference between the means of two groups is due simply to variation in
each data set or to another factor. In our case, we would want to test whether the mean body length of
our two groups of crayfish differed merely because there is inherent variability in crayfish body
length or because gender (male versus female) is a variable that influences the size of a crayfish.
In order to use the t-test, however, we must first consider the concept of a null hypothesis. The null
hypothesis simply states that there is no difference between the means of the two groups. It is
called the null hypothesis because it is hypothesizing that the independent variable (in our case
crayfish gender) does not affect the dependent variable (in our case, body length). In our experiment,
the null hypothesis would be supported if there is no difference in the mean body length of male and
female crayfish. We must test the null hypothesis since we do not know how different the two means
really are.
In science, we can never prove a hypothesis to be true, but we can find evidence to indicate that a
hypothesis is false. How will we know what difference between the mean body lengths of male and
female crayfish is large enough to reject the null hypothesis? Do the means need to differ by 1 cm or
10 cm? To determine this, we will use the t-test to calculate a t-value. This t-value takes into
account both the difference between the means and the standard deviation (variation around each
mean) for each population. After we calculate the t-value, we must compare it to a standard to
determine if the two means are significantly different. In general, a large difference between two
means (especially with small standard deviations) will produce a large t-value.
When you use a computer program to perform a t-test (we will use SPSS in lab today), the software
will automatically calculate a t-value and compare this t-value to a standard to determine a probability
or p-value. Probability (p) refers to the percent of the time when differences between two data sets
are due to random natural variability in the data sets. The p value is expressed in values between 0
and 1. A p-value of 1 means that if you were to perform this experiment 100 times, all 100 of those
times the differences found between the two populations could be attributed to natural variability in
the data sets (and NOT any other factor, such as crayfish gender). A p-value of 0.20 means that, if
you were to perform this experiment 100 times, only 20 of those times the differences detected
between two populations could be attributed to natural variability. This means that, 80 of those times,
the differences found between two populations could be attributed to something other than natural
variability (crayfish gender for our case).
In general, larger t-values correspond to smaller p-values. Small p-values indicate a low probability
that the two populations differ only because of natural variability, demonstrating that the difference
between the two populations is likely due to the effect of another variable (such as crayfish gender).
Thus, the smaller the p-value, the more statistically significant is the difference between means.
Scientists have chosen p = 0.05 as the dividing line between accepting and rejecting the null
hypothesis. Therefore, anytime a p-value is 0.05 or less, you can reject the null hypothesis and
conclude that the difference between the means of the two groups is statistically significant. (Note
Worksheet 2A, page 4 of 2
that we say that difference between the means is significant. It is incorrect to say that the results or
means are significant or not significant. Remember that the t-test is testing differences between
means, not the means themselves.)
If your calculated p-value is higher than 0.05, you cannot reject the null hypothesis and must
conclude that the difference in the means between the two groups is not statistically significant. In
our case, this would mean that any differences in body length between male and female crayfish are
due to natural variability within each data set, and not because of the gender of the animal. In this
situation, the null hypothesis, that male and female crayfish have the same mean body length, is
supported by your data.
PROCEDURE PART A
Follow the directions hanging above the computers in the lab to (1) create a histogram and (2)
perform a t-test to analyze your crayfish body length data.
Worksheet 2A, page 5 of 2
PART B: DIGESTIVE AND EXCRETORY SYSTEMS OF THE
FETAL PIG
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Background on mammalian digestive and excretory systems
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Identification and measurement of multiple anatomical strucutres
PART C: HISTOLOGY
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Background on tissues in mammalian organs
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Observation of tissues on prepared slides
This worksheet is a group assignment. Turn in this worksheet (one copy per group) before you
leave lab today.
1. (2 pts) On the lab computers you will find an SPSS data file that contains data for crayfish body
lengths pooled from every lab section. Use SPSS to create a histogram of the body lengths of all
crayfish. Then create separate histograms for the body lengths for male crayfish and female
crayfish. Attach these histograms to this Worksheet. Report the following values below:
mean crayfish (M + F) body length (cm) ________
std dev___________
mean female body length (cm) ___________
std dev ___________
mean male body length (cm) _____________
std dev ___________
2. (1 pt) What is the null hypothesis that you are testing when you perform a t-test to determine if
male and female crayfish differ in size?
3. (1 pt) Use SPSS to perform a t-test to test whether male and female crayfish differ in body length.
Provide the p-value and an interpretation of this p-value. According to the class data, do male
and female crayfish differ in body length? Explain.
Worksheet 2A, page 6 of 2
BC BIO 1501
Fall 2009
WORKSHEET 2B (INDIVIDUAL): HISTOLOGY AND COMP. ANATOMY (10 PTS)
4. (2 pts) The intestine is tightly coiled inside the pig so that a long structure can fit in a compact
space. If you unraveled this intestine, a very long structure would be revealed. Below find the
gut:body length ratio for a number of different animals of the phylum Chordata. Use your
knowledge about the typical pig diet to propose a potential range of gut:body length ratios for pigs.
Explain your answer.
Animal
Gut:body length ratio*
Type of consumer
Snail
Guinea pig
Tapir
Raccoon
Mole
Sea Lion
6.5:1
5.0:1
4.7:1
1.8:1
2.2:1
1.4:1
Herbivore
Herbivore
Herbivore
Omnivore
Omnivore
Omnivore (but eats mostly animals)
* Data from “Applying the Learning-Cycle Approach to Digestive Systems and the Principles of Structure-Function and
Unity with Diversity” by Ann O. Wilke. Association of Biology Laboratory Educators Proceedings, v 7-8, 1993.
5. (2 pts) Use your knowledge of the process of digestion to propose a plausible explanation as to
why herbivores appear to have a larger gut:body length ratio than omnivores.
6. (2 pts) Your friend has to take a drug test at work to make sure that there are no illegal
substances in her urine. She is a little concerned because she has just eaten a brownie
with an illicit substance (marijuana) baked into it. She knows that if she had smoked
marijuana, the active ingredient (THC) would have been absorbed by her blood, and then
would have been excreted into her urine. However, because she ate the marijuana, she is
not sure if the THC would be excreted in her feces or in her urine. Where do you think
the THC will be excreted? Use one or two sentences to explain your answer.
Worksheet 2A, page 1 of 2