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LOWER INVERTEBRATES
Chapter Outline
WHAT ARE ANIMALS?
SPONGES
Sponge Structure and Function
Sponge Size and Body Form
Nutrition and Digestion
CNIDARIANS
Organization of the Cnidarian Body
Stinging Cells
Cnidarian Stings
Types of Cnidarians
Predator-Prey Relationships
Habitat Formation
Symbiotic Relationships
CTENOPHORES
Ctenophore Structure
Reproduction
Ecological Roles of Flatworms
RIBBON WORMS
Ecological Roles of Ribbon Worms
LOPHOPHORATES
Chapter Objectives
• Describe the anatomy and physiology of sponges.
• Recognize the ecological and economic roles of sponges.
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• Recognize the ecological roles of ctenophores.
• Explain bilateral symmetry and its evolution.
Key Terms
invertebrates
vertebrates
sessile
ostia
archaeocytes
spicules
sponging
asconoid
syconoid
leuconoid
suspension feeder
medusa
epidermis
gastrovascular cavity
gastrodermis
hydroid
feeding polyp
(gastrozooid)
reproductive polyp
(gonangium)
scyphozoans
photoreceptors
pedal disk
fission
ctenophores (comb jellies)
comb plates (ctenes)
cephalization
turbellarians
meiofauna
flukes
tapeworms
chemoreceptors
pharynx
Chapter Summary
1. Sponges depend on their ability to filter large amounts of water through their bodies
2. Cnidarians and ctenophores exhibit radial symmetry. Cnidarians have evolved a
highly specialized stinging cell that they use for capturing prey and for defense.
comb plates.
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3. Marine worms display bilateral symmetry and cephalization. These include the
Chapter Outline
I. Characteristics of Animals
A. Invertebrates: animals lacking a vertebral column.
B. Vertebrates: animals with a vertebral column.
II. Sponges
A. Asymmetrical body plan.
B. Sessile: grow attached to the substrate.
C. Body plan.
1. Ostia.
2. Spongocoel.
D. Size and form.
1. Asconoid.
E. Nutrition and digestion.
1. Suspension feeders.
2. Filter feeders.
F. Reproduction.
1. Asexual.
G. Symbiotic relationships.
1. Hosts organism; mutualistic and commensalistic endosymbionts.
H. Boring sponges burrow in coral and mollusc shells, releasing calcium back into the
environment.
I. Commercial uses.
III. Cnidarians
A. Cnidocyte.
B. Cnidarian body plan: radial symmetry.
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C. Cnidocyte.
1. Nematocyst: spear-shaped stinging organelle (cnida).
3. Toxic venom associated with cnidocytes can affect humans.
D. Hydrozoans: class Hydrozoa.
1. Largely colonial; sharing food resources.
2. Dominant polyp phase, reduced medusa phase.
7. Medusae are planktonic, drifting with surface currents.
E. Scyphozoans: class Scyphozoa.
1. Dominant medusae phase, reduced polyp phase.
2. Larger medusae, stronger swimmers—still planktonic.
F. Anthozoans: class Anthozoa.
1. Benthic sessile organisms.
2. Polyp phase only; medusa phase lost.
G. Nutrition and digestion.
1. Digest prey in central gastrovascular cavity.
2. Waste and undigested food are forced back out through the mouth when digestion is
completed.
7. Corals feed on small zooplankton, invertebrates, and fish larvae.
H. Reproduction.
1. External fertilization and external embryonic development.
2. Hydrozoans: asexual polyp phase and sexually reproducing medusa phase.
I. Ecology.
1. Jellyfish are active predators.
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IV. Ctenophores
A. Planktonic, translucent animals with eight rows of comb plates (ctenes) for locomotion.
B. Radially symmetrical.
C. Bioluminescent at night.
F. Ecology.
1. Zooplankton predator; regulates zooplankton population size.
2. Cycle nutrients to larger plankton feeders.
V. Bilateral Symmetry
VI. Flatworms
A. Phylum platyhelminthes.
B. Bilateral symmetry, flattened body with definite anterior and posterior regions.
C. Three groups.
D. Turbellarians.
1. Free-living; freshwater and marine.
2. Mostly benthic, few pelagic species.
3. Meiofauna: small invertebrates associated with benthic sediments.
4. Ventral surface covered with cilia and mucus-secreting cells that are used for
VII. Ribbon Worms
A. Bilaterally symmetrical; ribbon-shaped bodies.
B. Primarily benthic, some deep-water pelagic species.
F. Secretes sticky toxin that immobilizes prey.
G. Some feed on commercially important crab eggs.
H. Ecology.
1. Primary predators of annelids and crustaceans.
VIII. Lophophorates
A. Sessile animals lacking a distinct head region.
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B. Lophophore: feeding structure of ciliated tentacles around the mouth.
C. Three groups.
1. Phoronids: phylum Phoronida.
D. Ecology.
1. Filter feeders.
Suggestions for Presenting the Material
1. The key to getting students interested in this material is to help them establish a
2. Ask students to work in groups to summarize the characteristics of each taxonomic
group and list one interesting fact that they learned about each group. Let the
collected during group work.
Classroom Discussion Ideas
1. Explain the difference between bilateral and radial symmetry. What are the benefits
of these different types of symmetry?
2. For creatures like sponges, what are the advantages of reproducing sexually and
asexually?
3. Explain the differences between the Scyphozoan, or true jellyfish, and the
Ctenophores, or comb jellies.
Videos, Animations, and Websites
Videos
The Shape of Life. (PBS, 2002)
Treasures of the Great Barrier Reef. (PBS, Nova series)
This program covers coral reefs and the threats against them.
Coral Reef Multimedia Project. (web videos)
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Animations
Invertebrate Animations—University of Alberta Zoology.
A collection of animations on the anatomy and behavior of various marine invertebrates.
Websites
Arkive.
University of California Museum of Paleontology.
A good overview of jellyfish, with links to other cnidarian groups. Good photos of
cnidarians.
Suggested Answers to End of Chapter Questions
Multiple Choice
1. c. Sponges reproduce only sexually.
2. c. feed
3. a. capture prey
Matching
1. b.
6. c.
Short Answer
1. How does a sponge’s body structure affect its size?
The body wall of the sponge is highly folded to increase the surface area used for
2. What role do boring sponges play in marine environments?
Members of the family Clionidae (class Demospongia), known as boring sponges
3. What are the advantages of bilateral symmetry?
Bilateral symmetry allows for the development of an anterior (head) end, called
4. Describe the ecological role of meiofauna.
Meiofauna are microscopic animals that live in the ocean bottom’s sediments.
These organisms are heterotrophic, feeding on bacteria, fungi, other microscopic
5. Describe how sponges feed and reproduce.
Sponges feed by creating a water current flowing through their bodies, bringing in
bacteria, plankton, and suspended organic matter. The incurrent water enters tiny
6. Describe how the cnidarian stinging cell (cnidocyte) functions.
The cnidocyte produces a highly specialized microstructure called the nematocyst.
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7. Why is radial symmetry advantageous to a sessile organism?
A sessile animal does not move through the environment, so there is no need to
8. Distinguish between hydrozoans and scyphozoans.
Hydrozoans, or hydroids (class Hydrozoa), are mainly colonial organisms either in
9. What is a lophophore and how does it function in feeding?
A lophophore (LOHF-uh-fohr) is a ciliated, horseshoe-shaped coil of tentacles that
10. How do armed ribbon worms capture prey?
An armed nemertean captures prey by everting its proboscis (a hollow tube) out
Thinking Critically
1. Development of beachfront property frequently increases the amount of sediment in
coastal water. How would you expect this to affect sponges and hydrozoans living in
the shallow water close to shore?
Since coastal development tends to increase the sediment load in the water
column, the amount of sunlight penetrating the water decreases, decreasing the
phytoplankton biomass. Fewer phytoplankton means lower zooplankton
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2. Based on the information in this chapter, construct a food web that includes
meiofauna, marine worms, and larger predators such as fishes.
Primary producers would be phytoplankton, seaweeds, and sea grasses.
Zooplankton consumes phytoplankton and small pieces or organic matter.
3. Explain with examples how symbiotic relationships can allow marine animals to live
in habitats where they normally could not survive.
Corals and hydrocorals are found in tropical waters with lower nutrients. Low
nutrient levels in the water produce clear water but low levels of phytoplankton.
Suggested InfoTrac® Articles
“Jellies on a Roll: Jellyfish Are ‘Blooming’ around the World and for Many Marine
Ecosystems, that Means Trouble.” Estabrook, B. Audubon, (2002).