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CHAPTER 13 The Cnidar ians
(Radiate Animals)
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13.1. Position and
Contributions
A. Position in AnimalKingdom
1. Both phyla Cnidaria
and Ctenophora make
up the radiate animals.
2. Other eumetazoans
have bilateral
symmetry or their
radial symmetry is
derived from a bilateral
ancestor.
3. Neither Cnidaria nor
Ctenophora have
advanced beyond
tissue level oforganization although
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B. Bio logical Contr ibut ions
1. Both phyla have two well-defined germ layers: ectoderm and
endoderm; mesoderm may be derived from ectoderm.
2. There is an internal body cavity: the gastrovascular cavity.
3. Extracellular digestion occurs in the gastrovascular cavity;
gastrodermal cells accomplish cellular digestion.
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B. Bio logical Con tr ibut ions
1. Both phyla have two well-defined germ
layers: ectoderm and endoderm;
mesoderm may be derived from
ectoderm.
2. There is an internal body cavity: the
gastrovascular cavity.
3. Extracellular digestion occurs in the
gastrovascular cavity; gastrodermal cells
accomplish cellular digestion.
4. Most have tentacles, which are
extensible projections for food capture.
5. Radiates are the simplest animals with
nerve cells; there is no central nervous
system.
6. Radiates are the simplest animals with
sense organs: statocysts and ocelli.
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B. Bio logical Contr ibut ions
1. Both phyla have two well-defined germlayers: ectoderm and endoderm; mesodermmay be derived from ectoderm.
2. There is an internal body cavity: thegastrovascular cavity.
3. Extracellular digestion occurs in thegastrovascular cavity; gastrodermal cellsaccomplish cellular digestion.
4. Most have tentacles, which are extensibleprojections for food capture.
5. Radiates are the simplest animals withnerve cells; there is no central nervoussystem.
6. Radiates are the simplest animals with
sense organs: statocysts and ocelli. 7. Locomotion is by muscular contraction or
ciliary comb plates.
8. The polyp and medusa forms allow widerecological possibilities.
9. Unique features include nematocysts,colloblasts and ciliary comb plates.
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13.2. Phylum Cnidaria
A. Cnidarian Life History
1. Over 9,000 species are in thephylum Cnidaria.
2. Cnidaria have specialized cells(cnidocytes) that contain aspecialized stinging organelle, thenematocyst.
3. Nematocysts are only formed and
used by Cnidarians.
4. Cnidarians originated close to thebase of the metazoan lineage.
5. Today, they are most common inshallow marine environments, some
are freshwater but none areterrestrial.
6. Some ctenophores, molluscs andflatworms eat hydroids and use thestinging nematocysts in their owndefense.
7. Some live symbiotically; algae inreef-building corals are critical to
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B. Characteristics of
Cnidaria
1. All are aquatic and mostlymarine.
2. Radial or biradial
symmetry forms oral and
aboral ends.
3. The two body types arethe free-swimming medusae
and the polyps.
4. Some have an
exoskeleton or endoskeleton
of chitinous, calcareous or
protein components.
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B. Characteristics of Cnidaria
1. All are aquatic and mostly marine.
2. Radial or biradial symmetry forms oral and aboralends.
3. The two body types are the free-swimming medusaeand the polyps.
4. Some have an exoskeleton or endoskeleton ofchitinous, calcareous or protein components.
5. They have a diploblastic body, with two layers:epidermis and gastrodermis; some are triploblastic withan ectomesoderm.
6. The gastrovascular cavity has a single openingserving as both mouth and anus.
7. Special stinging cells called nematocysts are inepidermis or gastrodermis and abundant on tentacles.
8. The nerve net may include some sensory organs.
9. The muscular system has an outer layer oflongitudinal fibers and an inner layer of circular fibers.
10. Reproduction is either asexual or sexual; a planulalarva may be present.
11. There is no excretory or respiratory system.
12. There is no coelomic cavity.
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B. Characteristics of Cnidaria
1. All are aquatic and mostly marine.
2. Radial or biradial symmetry forms oral and aboralends.
3. The two body types are the free-swimming medusaeand the polyps.
4. Some have an exoskeleton or endoskeleton ofchitinous, calcareous or protein components.
5. They have a diploblastic body, with two layers:epidermis and gastrodermis; some are triploblastic withan ectomesoderm.
6. The gastrovascular cavity has a single openingserving as both mouth and anus.
7. Special stinging cells called nematocysts are inepidermis or gastrodermis and abundant on tentacles.
8. The nerve net may include some sensory organs.
9. The muscular system has an outer layer oflongitudinal fibers and an inner layer of circular fibers.
10. Reproduction is either asexual or sexual; a planulalarva may be present.
11. There is no excretory or respiratory system.
12. There is no coelomic cavity.
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B. Characteristics of Cnidaria
12. There is no coelomic cavity.
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C. Form and Function
1. Cnidaria have two basic body
plans.
2. A polyp is a hydroid form.
a. Polyps are an adaptation to
a sedentary life.
b. The body is tubular with the
mouth directed upward andsurrounded by tentacles.
c. The aboral end is attached
to a substrate by a pedal disc.
d. In colonial forms, the polyps
may be specialized for feeding,reproduction or defense.
e. Sea anemones and corals
are all polyps with no medusa
stage.
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3. A medusa is bell or umbrella-
shaped.
a. The mouth is directed
downward; tentacles may extenddown from the rim of the umbrella.
b. In tetramerous symmetry, body
parts are arranged in fours.
c. Medusae dominate the life oftrue jellyfish, but many have a
polypoid form at the same time.
4. Hydrozoans often have life histories
featuring both stages.
5. Both stages have retained the sac-
like body plan typical of the phylum.
6. Both have three body layers;
medusa has a much thicker mesoglea
than do polyps.
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D. Nematocysts: Stinging Organelles
1. Over 20 different types of nematocysts have been described; they are
important in taxonomy.
2. Nematocysts are tiny capsules made of chitin-like material and containing a
coiled filament. 3. A little lid oroperculum covers the end of the capsule.
4. The inside of the thread may have tiny barbs or spines.
5. The cnidocyte is the cell that produces the nematocyst; it develops from a
cnidoblast.
6. Except in Anthozoa, a modified cilium called a cnidocil functions as a trigger.
7. Both small organic molecules and vibrations sensitize anthozoan cnidocytes.
8. After a nematocyte is discharged, its cnidocyte is absorbed and another
develops.
9. Some lack barbs or poison and rapidly recoil, grasping and holding prey.
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10. Mechanism of NematocystDischarge
a. The cell can generate a highosmotic pressure of 140
atmospheres within thecnidocyte.
b. The osmotic pressure falls asthe hydrostatic pressureincreases.
c. When stimulated, the highinternal osmotic pressure causeswater to rush into the capsule.
d. The operculum opens andrapidly releases the increased
hydrostatic pressure, launchingthe thread.
e. At the everting end of thethread, the barbs point backwardto anchor.
f. Poison may be injected when itpenetrates the prey.
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E. Nerve Net
1. Two nerve nets, one at the base
of epidermis and one at the base
of gastrodermis, interconnect.
2. Nerve impulses move acrosssynapses by neurotransmitters.
3. Unlike higher animals, cnidarian
nerves have neurotransmitters on
both sides of the synapses
allowing transmission in either
direction. 4. Cnidarian nerves lack the
myelin sheath on axons.
5. There is no central nervous
system, but ring nerves serve as
marginal sense organs of
medusae.
6. In some species, a fast network
coordinates swimming
movements, a slower one
coordinates tentacles.
7. This nerve net pattern is alsofound in the digestive systems of
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F. Class Hydrozoa
1. Hydra: A Freshwater Hydrozoan (Fig. 13-6)
a. Hydra are solitary polyps and one of a
few cnidarians found in freshwater.
b. The body is a cylindrical tube; the aboral
end has a basal or pedal disc for
attachment.
c. A ring of hollow tentacles encircles the
mouth.
d. The mouth opens to a gastrovascular
cavity.
e. Undifferentiatedinterstitial cells can
develop into cnidoblasts, sex cells, buds, or
nerve cells, but not epitheliomuscular cells. f. Gland cells on the adhesive disc secrete
an adhesive and sometimes a gas bubble
for floating.
g. Hydras have nematocysts that: 1)
penetrate prey and inject poison; 2) recoiland entangle prey; and 3) secrete an
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H. Class Anthozoa (Fig. 13-21)
1. Sea Anemones
a. They attach to shells, rocks, timber, etc. by pedal discs;some burrow in mud or sand.
b. A crown of tentacles surrounds the flat oral disc.
c. A slit-shaped mouth leads into a pharynx.
d. The siphonoglyph is a ciliated groove that creates thewater current into the pharynx.
e. Currents carry in oxygen and remove wastes, and maintainfluid pressure for a hydrostatic skeleton.
f. When in danger, water is rapidly expelled through pores asthe anemone contracts to a small size.
g. Most anemones can glide slowly on pedal discs; some canswim with limited ability.
h. Escape reactions occur in response to extracts frompredators (e.g. sea stars, nudibranchs).
i. Most harbor symbiotic algae; some have a mutualisticrelationship with hermit crabs.
j. Some damselfishes shelter in sea anemones and have skin
mucus that protects them from triggering nematocysts.
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2. Zoantharian Corals
a. Members of the order Scleractinia are also known as true or stony corals.
b. They are miniature sea anemones that live in calcareous cups they havesecreted. (Fig. 13-26)
c. Instead of a pedal disc, they secrete a limey skeletal cup with scleroseptaprojecting up into the polyp.
d. A sheet of living tissue forms over the coral surface, connecting allgastrovascular cavities. (Fig. 13-28)
3. Coral Reefs
a. Coral reefs have great productivity, rivaled only by tropical rainforests.
b. Living plants and animals are limited to the top layer above the calciumcarbonate deposits.
c. Hermatypic corals and coralline algae form most coral reefs. (Fig. 13-28)
d. These corals require full salinity of seawater and warmth and light, limitingthem to waters between 30 degrees north and south.
e. Microscopic zooanthellae are photosynthetic and begin the food chain andrecycle phosphorus and wastes.
f. These habitats support a diversity of corals and fish.
g. Few nutrients enter or leave the system.
h. Nutrients from fertilizer and sewage threaten coral reefs with excessivealgal growth.
Fig 13 1a
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Fig. 13.1a
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