Phylum Cnidaria (radially
symmetric, 2 cell layers in body)
Jellyfish and allies. These alternate 2 phases in their life cycle: the
free-living medusoid phase (“jellyfish”), and a sessile hydroid
phase.
Both feed by capturing planktonic food using tentacles armed with
a cnidarian speciality, the class of stinging cell called nematocysts.
Some are entangling, some inject barbed points to anchor, some
inject toxins.
The main classes are:
• Hydrozoa: various medusoid radiations, often
with several body forms fused into one animal ie
Physalia physalis, the infamous, portugese man
o’war (avoid!).
• Scyphozoa = jellyfish, Aurelia aurita in the
common UK moon jelly (harmless to humans)
• Anthozoa: sessile forms: sea anemones, corals,
sea fans
• Cubozoa: sea wasp
Cnidirians
• Overview: Life Without a Backbone
• Invertebrates
– Are animals that lack a backbone
– Account for 95% of known animal species
Figure 33.1
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Evolutionary Relationships of Cnidaria
The Radiate Animals
Tissue level of organization
• No anterior or posterior
• Terms of direction defined based on the position of the
mouth
• Oral/aboral ends
• Biradial symmetry- single plane, passing through a
central axis, divides the organism into mirror images
• Advantageous b/c sensory receptors evenly distributed.
• Diploblasic tissue level organization- similar cells are
organized into tissues, and all cells are derived from two
embryonical layers (ectoderm and
endoderm/gastrodermis)
gure 33.2
Ancestral colonial
choanoflagellate
Eumetazoa
Bilateria
Chordata
Echinodermata
Other bilaterians (including
Nematoda, Arthropoda,
Mollusca, and Annelida)
Cnidaria
Porifera
• A review of animal phylogeny
Deuterostomia
• Exploring invertebrate diversity
CNIDARIA (10,000 species)
PORIFERA (5,500 species)
A sponge
A jelly
PLACOZOA (1 species)
KINORHYNCHA (150 species)
0.5 mm
250 µm
A placozoan (LM)
PLATYHELMINTHES (20,000 species)
A marine flatworm
ECTOPROCTA (4,500 species)
Figure 33.3Ectoprocts
A kinorhynch (LM)
ROTIFERA (1,800 species)
A rotifer (LM)
PHORONIDA (20 species)
Phoronids
• Cnidarians have radial symmetry, a
gastrovascular cavity, and cnidocytes
• All animals except sponges
– Belong to the clade Eumetazoa, the animals
with true tissues
• Phylum Cnidaria
– Is one of the oldest groups in this clade
• Cnidarians
– Have diversified into a wide range of both
sessile and floating forms including jellies,
corals, and hydras
– But still exhibit a relatively simple diploblastic,
radial body plan
The Body Wall
• Two cellular layers
• Epidermis- epithelio-muscular cells
• Mesoglia- noncellular gel and is abundant
in the Medussa
• Gastrodermis– gland cells for the production and release of
enzymes.
– Flagellated nutitive muscular cells that contain
food vacuoles
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Cnidarian Body Wall
Fig. 9.9
9-7
Phylum Cnidaria- Nematocysts
• Name comes from the presence of
specialized cells used in defense, feeding
and attachment.
• Cnidosytes contain stinging organelles
called Nematocysts.
• Cnidarians are carnivores
– That use tentacles to capture prey
• The tentacles are armed with cnidocytes
– Unique cells that function in defense and the Prey
capture of prey
Tentacle
“Trigger”
Discharge
Of thread
Nematocyst
Figure 33.6
Coiled thread
Cnidocyte
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Cnidocyte Structure and Nematocyst Discharge
Fig. 9.10
9-8
• The basic body plan of a cnidarian
– Is a sac with a central digestive compartment,
the gastrovascular cavity
• A single opening
– Functions as both mouth and anus
• There are two variations on this body plan
– The sessile polyp and the floating medusa
Polyp
Medusa
Mouth/anus
Tentacle
Gastrovascular
cavity
Gastrodermis
Mesoglea
Body
stalk
Epidermis
Tentacle
Mouth/anus
Figure 33.5
Gastrovascular Cavity
• Receives and digest food
• Single opening serves as the mouth and
anus
• Tentacles surrounding the opening aid in
feeding.
CV cavity continue
• Simplicity of body plan
• Hydras and other cnidirians don not
require a true circulatory systems
• Body wall two cell thick encloses CV cavity
• Serves both in digestion and distribution of
substances throughout the body
• Extend to tentacles
• The phylum Cnidaria is divided into four
major classes
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Cladogram of Cnidarian Taxonomy
Fig. 9.23
9-17
– Hydrozoa, Scyphozoa, Cubozoa, and
Anthozoa
(a) These colonial polyps are members of
class Hydrozoa.
Figure 33.7a–d
(b) Many species of jellies (class
Scyphozoa), including the
species pictured here, are
bioluminescent. The largest
scyphozoans have tentacles
more than 100 m long
dangling from a bell-shaped
body up to 2 m in diameter.
(c) The sea wasp (Chironex
fleckeri) is a member of
class Cubozoa. Its poison,
which can subdue fish and
other large prey, is more
potent than cobra venom.
(d) Sea anemones and other
members of class Anthozoa
exist only as polyps.
Alterations of Generations
• Typical cnidarian alternates b/w a hydroid
stage (polyp) and a swimming medusa
(jellyfish) stage.
• Polyp – assexual stage
• Medusa- sexual stage
• In some cnidarian classes either the polyp
or the medussa is reduced or missing.
Generalized Cnidarian Life Cycle
Hydrozoans
• Most hydrozoans
– Alternate between polyp and medusa forms
2 Some of the colony’s
polyps, equipped with tentacles,
are specialized for feeding.
3 Other polyps, specialized
for reproduction, lack
tentacles and produce tiny
medusae by asexual budding.
4 The medusae
swim off, grow, and
reproduce sexually.
Reproductive
polyp
1 A colony of
interconnected
polyps (inset,
LM) results
from asexual
reproduction
by budding.
Feeding
polyp
Medusa
bud
MEIOSIS
Gonad
Medusa
SEXUAL
REPRODUCTION
Sperm
Egg
ASEXUAL
REPRODUCTION
(BUDDING)
Portion of
a colony
of polyps
FERTILIZATION
Zygote
Developing
polyp
Mature
polyp
Planula
(larva)
Figure 33.8
1 mm
6 The planula eventually settles
and develops into a new polyp.
5 The zygote develops into a
solid ciliated larva called a planula.
Key
Haploid (n)
Diploid (2n)
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Hydrazoa Obelia Structure and Life Cycle
Fig. 9.12
9-10
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Structure of Gonionemus Medusa
Fig. 9.13b
9-11
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CNIDARIA
Class Hydrozoa
• Feather hydroid
(Halocordyle disticha,
formerly called
Pennaria tiarella),
from Bermuda
• Tiny, white tufts along
lateral branches of
the colony are
individual polyps
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Hydrozoa
• Fire coral (Millepora
complanata),
common in the
Caribbean
• Polyps on surface
and along edges
(“fuzz”) are loaded
with toxic stinging
nematocysts
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Hydrozoa
• 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
Scyphozoans
• In the class Scyphozoa
– Jellies (medusae) are the prevalent form of the life
cycle
– Polyp reduced or absent
– Gametes produced gastrodermally
– Cnidocytes present in gastrodermis and epidermis
– Marine
– Examples- aurelia
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Structure of Scyphozoan Medusa
Fig. 9.15
9-12 Source: After L. H. Hyman, Biology of the Invertebrates, Copyright © 1940 McGraw-Hill Publishing Co.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Aurelia Life History
Fig. 9.16
9-13
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CNIDARIA
Class Scyphozoa
• Sea thimble jellyfish
(Linuche
unguiculata),
Honduras
• Tiny jellyfish (< 1
inch) that swarm in
the spring
• Adults and larvae
(“sea lice”) may cause
a severe skin reaction
in humans
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Scyphozoa
• Upside down jellyfish
(Cassiopea
xamachana) from
Bermuda, with
zooxanthellae
• This specimen is
swimming up to the
surface, where its
reflection is visible
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Scyphozoa
• Jellyfish (Mastigias
sp., family
Rhizostomeae) from
Jellyfish Lake in
Palau, western Pacific
• These jellyfish have
lost their ability to
sting and depend on
zooxanthellae for
nutrition
Photo Copyright © Diane R. Nelson
Anthozoans
• Class Anthozoa includes the corals and
sea anemones
– Which occur only as polyps
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Longitudinal Section of a Stony Coral Polyp
Fig. 9.20
9-15
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Structure of an Anemone
Fig. 9.19
9-14 Charles Lytle and J. E. Wodsedalek, General Zoology Laboratory Guide, 11
th
Hill. Reprinted by permission of The McGraw-Hill Companies.
ed., Complete Version, New York, McGraw-
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CNIDARIA
Class Anthozoa
• Sea fan (Subergorgia
sp.) from the Solomon
Islands
• This is a colony, with
reticulated branches
in one plane, at right
angles to the
prevailing current
Photo Copyright © Diane R. Nelson
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CNIDARIA
Class Anthozoa
• Gorgonian sea fan
(Plexuaridae) on a
reef wall in Fiji
• Individual polyps in
the colony filter
plankton from the
water column
Photo Copyright © Diane R. Nelson
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CNIDARIA
Class Anthozoa
• Polyp of a tree fern or
palm coral (Clavularia
sp.) with pinnate
tentacles, in the
Solomon Islands
• Large sheets of these
polyps are attached
by a common stolon
to the substrate
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
• Divaricate tree coral
(Dendronephthya
sp.), from Fiji
• These bushy or
tree-like soft corals
(alcyonaceans) are
found only in the
Pacific
Photo Copyright © Diane R. Nelson
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CNIDARIA
Class Anthozoa
• Close-up of an
alcyonacean tree
coral
(Dendronephthya sp.)
• Spicules, the small
white rod-shaped
structures, are
embedded in the
tissue and aid in
supporting the colony
Photo Copyright © Diane R. Nelson
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
• Corky sea finger
(Briareum
asbestinum), common
in the Caribbean
• Sea rod in front has
polyps extended,
while rod in back has
polyps retracted,
revealing purple color
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CNIDARIA
Class Anthozoa
• Sea anemone
(Heteractis sp.) from
Fiji
• Note column of
anemone and
tentacles along
margin
• This anemone may
be host to symbiotic
anemonefish
Photo Copyright © Diane R. Nelson
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CNIDARIA
Class Anthozoa
• 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
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CNIDARIA
Class Anthozoa
• 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
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CNIDARIA
Class Anthozoa
• 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
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CNIDARIA
Class Anthozoa
• 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
• Coral bleaching in
star coral (Montastrea
annularis) in the
Caribbean
• Golden brown color
indicates healthy
tissue
• White area has lost
zooxanthellae
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Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
• 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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
• Close-up of the raised
corallites of
Caribbean elliptical
star coral
(Dichocoenia stokesii)
in the daytime
• Polyps are extended
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
• Close-up of giant star
coral (Montastrea
cavernosa) with
polyps extended at
night to feed on
plankton
• Common in the
Caribbean
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Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA
Class Anthozoa
• Sea pen (Pteroeides
sp.) from the Solomon
Islands
• This is a colony that
lives anchored in the
sand, turning at right
angles to the
prevailing current
Photo Copyright © Diane R. Nelson
Class Cubozoa
A few a lethal to humans - NEVER EVER
swim with box jellies (sea wasps)
Characteristics of Ctenophora
1)Radially or biradial Symmetrical.
2)Body multicellular, few tissues, some organs and
organelles.
3)Body contains an internal cavity and a mouth and
anal pores.
4)Swims by means of plates of cilia (the combs)
5)Reproduction mostly sexual as hermaphrodites,
occasionally asexual.
6)Has a well developed subepidermal nerve net.
7)Has a distinct larval stage which is planktonic.
8)Lives in marine environments.
9)All are carnivorous.
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Ctenophore Mnemiopsis
Fig. 9.22
9-16
Ctenophora Anatomy
• Comb Jellies
Ctenophora