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RADIATE ANIMALS
Phyla Cnidaria and Ctenophora
CNIDARIA
Hysdrozoa
Scyphozoa
Cubozoa
Anthozoa (incl. sea anemones, stony corals, gorgonian corals, sea pen)
• More than 9000 species
Cnidaria
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A Fearsome Tiny Weapon
 More highly organized than sponges;
most are sessile.
 Many are effective predators.
 Nematocysts are deadly weapons
requiring only a small stimulus to fire.
 Can kill even very large prey.
The “killer” jellyfish
Eumetazoa
Planes of symmetry
spherical
radial
bilateral
Characteristics of Cnidaria
1. All are aquatic and
mostly marine.
2. Radial or biradial
symmetry forms oral
and aboral ends.
3. The polyp and
medusa forms allow
wider ecological
possibilities.
4. The two body types
are the freeswimming medusae
and the polyps.
Characteristics of Cnidaria cont.
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They have a body, with two layers: epidermis and gastrodermis;
The gastrovascular cavity has a single opening serving as both mouth and anus.
Nematocysts are in epidermis or gastrodermis and abundant on tentacles.
The nerve net may include some sensory organs.
The muscular system has an outer layer of longitudinal fibers and an inner layer of
circular fibers.
Reproduction is either asexual or sexual; a planula larva may be present.
There is no excretory or respiratory system.
Body structure

Body wall
 Epidermis
 Epitheliomuscular cells
 Interstitial cells
 Gland cells
 Cnidocytes
 Sensory Cells
 Nerve Cells
 Gastrodermis
 Nutritive muscular cells
 Gland cells
 Mesoglea
Feeding and Digestion
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Catch food with
nematocysts in tentacles
Mouth opens into
gastrovascular cavity;
mouth may be
surrounded by an
elevated manubrium or
oral lobes
Cnidarians prey on a
variety of organisms,
often larger than
themselves.
Digestion is extracellular
digestion, but
nutritionally, is
intracellular digestion.
Locomotion
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Hydras can move about freely, but
colonial polyps are permanently
attached.
Sea anemones can move on their
basal discs; hydras can move by a
“measuring worm” motion, or float to
the surface on a gas bubble.
Most medusae move freely, or swim
by contracting the bell, expelling
water from the concave oral side.
Cubozoans are strong swimmers.
Nervous System

Sensory Cells
 Mechanoreceptors
(Statocysts, Cnidocilia)
 Photoreceptors
 Chemoreceptors
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Nervous System
 No central nervous system
(not necessary)
 Two nerve nets

At base of epidermis

At base of
gastrodermis
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Function: Coordination of
swimming, tentacle and body
retraction
Reproduction
Asexual reproduction by budding
 Sexual reproduction
 in hermaphrodite forms
 in separate male female forms
 Gametes are shed directly into the
water
 Embryo develops into a freeswimming planula larva
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Cnidaria
Class Hydrozoa
 Class Scyphozoa
 Class Cubozoa
 Class Anthozoa
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Class Hydrozoa
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Mostly marine and colonial (asexual polyp and
sexual medusa stage)
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Some freshwater species (Hydra spec.) without
medusa stage
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CNIDARIA
Class Hydrozoa
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Portuguese Man-oWar (Physalia
physalia)
This is a colony with
several types of
polyps: gastrozooids,
gonozooids, and
dactylozooids
Highly toxic
Photo Copyright © Diane R. Nelson
Class Scyphozoa
Larger jellyfish “cup animals” (up to 2m
bell diameter, 70m tentacles)
 E.g. Aurelia aurita (moon jellyfish)
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Life cycle of Aurelia
Class Cubozoa
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Medusa is the
predominant form
In transverse section
the bells are almost
square
Strong swimmers and
voracious predators
(feeding mostly on
fish)
Box jellyfish
The sea wasp
(Chironex fleckery)
stings very dangerous
and sometimes fatal
Class Anthozoa
Polyps with a flower like appearance
“Flower animals”, no medusa stage
 Vary greatly in size
 All marine
 Many are supported by skeletons
 Includes sea anemones, corals (hard corals,
soft corals and horny corals such as sea
fans, sea pens and others)
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Sea anemone
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Larger and heavy
polyps
In coastal waters
Live sometimes
mutualistic with other
animals (e.g. hermit
crabs)
Feed on fish or other
live animals
body contract to small
size when animal is
endangered
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CNIDARIA
Class Anthozoa
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The giant anemone or
purple-tipped
anemone (Condylactis
gigantea) often
harbors cleaning
shrimp among its
tentacles
Common in Bermuda
and the Caribbean
Photo Copyright © Diane R. Nelson
Stony corals
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Miniature sea anemones
that live in calcareous
cups
Epidermis at base of
column secretes a limy
skeletal cup
Polyps retract into safety
of their cup when not
feeding
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CNIDARIA
Class Anthozoa
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Close-up of giant star
coral (Montastrea
cavernosa) during the
day with polyps
retracted
Common in the
Caribbean
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
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Close-up of giant star
coral (Montastrea
cavernosa) with
polyps extended at
night to feed on
plankton
Common in the
Caribbean
Photo Copyright © Diane R. Nelson
Coral Reefs
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Reef-building corals and coralline algae
Take dissolved calcium and carbonate ions from seawater
and precipitate it as limestone (CaCO3) to form reefs
Mutualistic algae (zooxanthellae) live in tissue and are
vital to reef-building corals but depend on light
Reef-building corals rarely live below 30m since there is
not enough light
Reefs begin their growth in shallow water around volcanic
islands . As the islands slowly sink beneath the sea, growth
of the reef is kept up with the rate of sinking (Charles
Darwin)
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CNIDARIA
Class Anthozoa
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Close-up of giant star
coral (Montastrea
cavernosa) during the
day with polyps
retracted
Common in the
Caribbean
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
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Close-up of giant star
coral (Montastrea
cavernosa) with
polyps extended at
night to feed on
plankton
Common in the
Caribbean
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
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Aerial view of coral
reefs in Fiji
Dark blue on right is
deep water
White is the edge of
the reef
Lighter color is the
reef flat
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
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Elkhorn coral
(Acropora palmata),
from the Caribbean
Wide branches are
extended upward,
catching the sunlight
for the zooxanthellae
in the coral tissues
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
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Brain coral (Diploria
strigosa), common in
Bermuda and the
Caribbean
Healthy coral is
golden-brown due to
the presence of
symbiotic
zooxanthellae
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
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Coral aggression
between two stony
corals in Bermuda
Montastrea annularis
(top) and Diploria
strigosa (bottom)
compete for space,
leaving a dead zone
(white) between them
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
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Extended slender
polyps of the stony
coral Goniopora sp.,
found in Fiji and the
western Pacific
Polyps are extended
during the day
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
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Orange cup coral
(Tubastraea coccinea)
from Bonaire in the
Caribbean
These ahermatypic
corals extend their
polyps at night to feed
on plankton
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
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Brain coral (Diploria
strigosa) in Bermuda,
with black band
disease
The black line marks
the cyanophyte alga
(Phormidium
corallyticum) that kills
the coral colony
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
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Coral bleaching in star
coral (Montastrea
annularis) in the
Caribbean
Loss of zooxanthellae
due to higher water
temperatures results in
lighter color
Photo Copyright © Diane R. Nelson
PHYLUM CTENOPHORA
1. This phylum has fewer than 100 species.
2. All are marine species; most prefer warm waters.
3. Ctenophores have eight rows of comblike plates for
locomotion
4. Like cnidarians, they have primary radial symmetry.
5. No nematocysts!
6. Nearly all are free-swimming; only a few creep or are
sessile.
7. They use the ciliated combs to propel themselves forward
8. Many are bioluminescent.
Phylum CTENOPHORA
(Comb jelly fish)
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Tentacles secrete sticky
substance (colloblast cells) to
catch small prey
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When covered with food they
contract and food is wiped onto
the mouth
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Gastrovascular cavity with
pharynx, stomach and canals
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Sensory cells and radial nerve
net
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Locomotion by cilia beating
(comb plates = fused cilia)
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hermaphrodites
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CTENOPHORA
Class Tentaculata
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Comb jelly from
Roatan, Honduras
Does not sting since it
has no nematocysts as
in cnidarians
Bioluminescent
Photo Copyright © Diane R. Nelson