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224 Notes to Instructors
Notes to Instructors
Chapter 32 An Introduction to Animal Evolution
Chapter 33 Invertebrates
What is the focus of this activity?
In studying biodiversity, many students try to memorize each of the phyla and its
What is this activity designed to do?
Activity 32.1/33.1 What can we learn about the evolution of the animal kingdom by
examining modern invertebrates?
This activity is designed to help students understand how studying the existing traits
Activity 32.2/33.2 What factors affect the evolution of organisms as they
become larger?
Activity 27.2 addressed the difference in oxygen needs per m2of membrane surface as
cells increased from about 10 m in diameter (prokaryote) to 100 m in diameter
Answers
Activity 32.1/33.1 What can we learn about the evolution of the
animal kingdom by examining modern invertebrates?
Fill in the chart on the next two pages to organize the major characteristics of key
invertebrate phyla.
Key invertebrate phyla
Characteristics Porifera Cnidaria Platyhelminthes Nematoda Annelida Mollusca Arthropoda Echinodermata
Examples of
organisms
Sponges Hydra, corals,
jellyfish
Planaria, flukes,
tapeworms
(flatworms)
Ascaris,
horsehair worms
(round worms
Earthworms,
leeches, fan
worms
(segmented
worms)
Clams,
oysters, snails
Insects, crabs,
lobsters,
crayfish
Starfish, sea
cucumbers, sea
urchins, sand
dollars
True muscle cells? No No; have
epithelio-
muscular cells
Yes Longitudinally
arranged muscles
only
Longitudinal
and circular
muscles
Increased
complexity of
muscle system
→ →
Key invertebrate phyla
Characteristics Porifera Cnidaria Platyhelminthes Nematoda Annelida Mollusca Arthropoda Echinodermata
Coelom?
Type?
None None None Pseudocoelom,
gut not lined
with mesoderm
embyonically
→ → →
Entercocoelom
formed by
outpocketing
off gut
Ciculatory
system?
Type?
None, primarily
diffusion
Diffusion and
some facilitation
of circulation via
fluid movement
in gastrovascular
Diffusion and some
facilitation of
circulation via fluid
movement in gastro-
vascular cavity.
Diffusion and
some
facilitation of
circulation via
fluid movement
Closed
circulatory
system,
heart(s),
arteries,
Some species
have an open
circulatory
system, and
some have a
All species
have an open
circulatory
system
Species have a
closed circulatory
system
Using the information in the chart and in Chapters 32 and 33 of Campbell Biology,
9th edition, answer the questions.
1. What set of characteristics is shared by all of the invertebrate animal phyla in the
chart?
2. What unique combination of characteristics defines each of the invertebrate phyla
as separate from the other phyla?
If you read down each column, you can determine the characteristics of each
combination of characteristics that is unique to and defines each phylum.
3. If you compare the characteristics of one phylum of the invertebrates with the next,
what key differences separate the groups from each other?
To answer this question, you need to compare the characteristics in a given
column/phylum with those in the next. Those characteristics that change from one
4. a. Looking across the rows, what major trends appear to occur in the evolution
of various organs or organ systems in these animal groups?
This chart is set up to allow you to visualize how the various systems changed in
the evolution of the various phyla. The arrangement also allows you to visualize
b. What developmental evidence is used to link Annelids, Arthropods, and Molluscs
evolutionarily?
Developmentally, Annelids, Arthropods, and Molluscs are all protostomes. They
Activity 32/33.1 227
c. What evidence is used to separate these phyla from the Echinoderms
and Chordates?
Echinoderms and Chordates are deuterostomes. This means that, develop-
d. Does this analysis provide evidence for or against the statement: “Evolution
adds onto or modifies what already exists”? Explain.
As is apparent in the chart, modifications are built upon what already exists. For
5. The chart organizes the major groups of animals based on grade, or shared body
plan features. What changes would you need to make in this organization to reflect
the possible phylogenetic relationships uncovered using molecular evidence?
To answer this:
a. On a separate sheet of paper, redraw the chart to reflect the new phylogenetic
relationships based on molecular evidence.
The new chart should show that recent DNA analyses indicate that, among the
b. What specific molecular characteristics/data are being used to determine
evolutionary relationships among animal phyla?
As noted in the text, the new molecular phylogenies “are based on mitochondrial
228 Activity 32/33.1
6. How would your answers to questions 2, 3, and 4 differ (if at all) when the chart is
redrawn and filled in to reflect changes in relationships based on molecular
evidence?
The answers to questions 2 and 3 would not change. Some of the evolutionary trends
7. In biological terms, a group of organisms is said to be successful if it is represented
by a large number of species or if the mass of all the organisms in the group is large.
(In both cases, “large” is determined relative to other groups or organisms.) Given
this definition of success, which of the major groups of animals would you argue is
the most successful? Be sure to provide evidence for your argument.
Answers may vary depending on the criteria chosen. For example, did you choose
success based on the total mass of organisms, the total number of organisms, or the
Activity 32.2/33.2 What factors affect the evolution of organisms
as they become larger?
In the evolution of life on Earth organisms have evolved from single celled to multicelled;
small to large; simpler to more complex. Keep in mind, that this apparent increase in
complexity occurs because evolution adds on to or modifies what already exists.
As you discovered in Activity 27.2, surface-area-to-volume ratios and the need for
organisms to gain or lose substances across their surface areas have put constraints on the
evolution of cell structure and function. These same constraints affect the ability of
multicellular organisms to survive. Given their small size, most of the evolution in
unicellular organisms has involved changes in cell chemistry and/or internal cellular
structure. In contrast the evolution of larger and larger multicellular organisms is
evidenced primarily as changes in both external and internal morphology.
1. A quick review and extension of Activity 27.2: How has small size affected
prokaryotic diversity?
Activity 32.2/33.2 229
a. How do surface-area-to-volume (SA/V) ratios change as the size and shape of
cells and organisms change? To answer this, calculate the SA and Vof a cube
1 mm on a side. Then do the same for cubes that are 2 mm and 4 mm on a side
and compare their SA/V ratios.
230 Activity 32.2/33.2
b. What do the ratios mean?
As linear dimensions increase by 2 times, SA increases by 4 times, but volume
c. Is this graph of the change (in surface area compared to volume) linear or
exponential?
To answer this question, complete the following table and then graph the surface
areas and volumes from this table and the previous table.
Cubes: 1 mm square 2 mm square 4 mm square Ratios
Linear dimension 1 mm 2 mm 4 mm 1:2:4
Cube linear
dimension:
8 mm 16 mm 32 mm 64 mm
Suface area 6 6 6
6 16 16
6 32 32
6 64 64
Activity 32.2/33.2 231
Data:
SA (mm2)V(mm3)
61
24 8
300000
250000
200000
150000
100000
50000
0
0 10000 20000
Surface area
Volume
30000
y = 0.068x1.5
R2 = 1
d. What effect does this relationship between surface area and volume have on the
ability of larger and larger multicellular organisms to support the metabolic needs
of all parts of their bodies, for examples, to supply oxygen needed for cell
respiration?
It is obvious that as organisms become larger, the SA/V ratio decreases
2. In the previous examples, as the “organism” sizes changed, their shapes remained
constant (as cubes). How would changing the shape of the organism affect SA/V
ratios? Use modeling clay or playdough to make a cube that is 2.5 cm (1 inch) on a
side. The cube will obviously have a constant mass or volume.
232 Activity 33.2/33.3
The answers will depend on the methods each student uses to change the shape. For
3. Review the chapters on fungal, animal and plant diversity in your textbook. For each
of the models that you developed to increase SA/V ratios, find an example of a
fungus, plant and/or animal, or a particular organ system, that uses the same method
to increase SA/V ratios and describe it below.
Shape change Effect on SA/V ratio
Model 1
Model 2
Model 3
Proposed model–increase SA/V by:
This type of increase in SA/V ratios can be
found in the following organism(s)/organ
systems:
Activity 33.2/33.3 233
