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Cnidaria and Ctenophora
Chapter 7, Zoology
Phylum Cnidaria
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Pronounced (ny-dar'e-a).
Name means “like or connected with nettles”.
Approximately 9,000 species.
All aquatic; mostly marine, a few freshwater.
All have radial symmetry.
All have two germ layers (diploblastic); the ectoderm
and endoderm.
Polymorphism, two body types; polyp and medusa.
Phylum Cnidaria
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It's name is derived from the many stinging cells that
they bear called Cnidocytes.
Cnidocytes usually contain stinging organelles called
nematocysts.
The nematocysts function as weapons that deliver
powerful toxins to kill, paralyze, or injure their prey or
enemies.
Cnidocytes and Nematocysts
Ecology of Cnidaria
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Found mostly in shallow,
warm equatorial/tropical
marine habitats.
Hydra and Sea Anenomes
often live symbiotically on
mollusc and crab shells.
Floating medusae can be
found in open ocean
(pelagic) zones and open
lake (limnetic) zones.
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Corals play an important
role in the ecology of coral
reefs by building reefs and
providing a habitat for other
marine organisms.
Dimorphism (Polymorphism)
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Cnidarians exhibit polymorphism, meaning they have
two different body types or plans.
The sedentary or sessile type that lives attached to the
ground are called Polyps.
The mobile type that floats is called a Medusa.
Description of Polyp Body Form
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Cylindrical or tubeshaped body.
Mouth is surrounded
by tentacles.
Mouth leads to the
gastrovascular cavity.
The aboral end is
attached to the
ground.
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Anemone in the
Polyp Body Form
Asexual Reproduction in Polyps
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There are three different ways a polyp can reproduce
asexually.
Budding – A round ball of tissue forms on the side of the
polyp and develops into an adult polyp with a functional
mouth and tentacles.
Fission – When an adult polyp splits in half and develops
into two identical adult clones.
Pedal Laceration – When tissue is torn from the base of
the polyp and develops into a new polyp.
Polyps Budding
Budding Colonies
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If buds stay attached to the polyps, a cnidarian colony
can form.
Budding colonies can share food through a common
gastrovascular cavity.
Different polyps in a colony that specialize and perform
specific tasks are called zooids.
Gastrozooids aid in feeding.
Dactylozooids aid in defense.
Gonozooids aid in reproduction.
The Portugese Man-of-War
Example of colonies of zooids in a
common cnidarian
Description of Medusa
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Medusa are free-swimming or floating.
Medusa are bell-shaped or umbrella-shaped.
The mouth of the medusa (oral side) faces
downward and is surrounded by tentacles.
Both the polyp and medusa have a jelly-like fluid in
the center of their bodies called mesoglea.
The mesoglea is much thicker in medusa making
them buoyant, giving them the name jellyfishes.
Medusa
Cnidarian Movement
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Colonial polyps are stationary
Solitary polyps can move slowly along the surface of the
ground by secreting mucus and sliding along their base.
Medusa can move freely by contracting their body and
by excreting water out their mouth.
Cnidarian Life Cycles
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Life cycles of cnidarians can be complex, but in general
life begins in the form of a larva called a planula.
The planula settles and develops into a an adult polyp.
Depending on the type of cnidarian, the polyp can
remain a polyp for life (like sea anemones and corals)
or the polyp may develop into a medusa (like a jellyfish).
Eventually becoming sexually mature and releasing
gametes (sex cells) that will unite to form a zygote.
Typical Cnidarian Life Cycle
Typical Cnidarian Life Cycle
Cnidarian Digestion
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The tentacles draw the prey into the mouth.
Digestion begins in the gastrovascular cavity.
Digestive enzymes are secreted in the gastrovascular
cavity and the food is broken down into smaller pieces.
After the food is broken down, it gets absorbed by the
cells lining the gastrovascular cavity.
Eventually waste is expelled out the mouth.
Cnidarian Cells and Tissues
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The body of a cnidarian consists of three parts
The outer epidermis (derived from ectoderm)
The inner gastrodermis (derived from the endoderm)
And the middle mesoglea
Cnidarian Cells
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Cnidarians posses a variety of cells in the epidermis and
gastrodermis.
Nutritive-muscular cells – provide support and structure
with a hydrostatic skeleton; additionally they also function as
digestive cells (found in the gastrodermis).
Gland cells – secrete digestive enzymes (gastrodermis).
Epitheliomuscular cells – provide a protective covering and
muscular movement. Contracts tentacles. (epidermis).
Interstitial cells – stem cells that can develop into a variety
of other cells including cnidocytes, sex cells, buds, nerve
cells etc. (found in the epidermis).
Cnidarian Cells
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Cnidocytes – These cells are the defense mechanism
for the cnidarians.
The cnidocytes contain little spines attached to a coil
called nematocysts that function as weapons and can
injure or paralyze their prey and enemies.
There are three types of nematocysts; Those that
penetrate and inject poison (penetrants), those that
entangle prey (volvents), and those that secrete an
adhesive substance (glutinants)
Nematocysts
Cnidarian Nervous System
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No central nervous system in Cnidarians.
All of the sensory and nerve cells are connected via a
nerve net which is a complex network of cells that are
able to communicate and respond to each other.
Cnidarian Senses
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Some Cnidarians also have some basic sensory
cells that are connected to their nerve net.
Statocysts provide equilibrium and balance.
Ocelli are photosensitive and detect light.
Class Hydrazoa (hi-dro-zo'a)
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Name means “water serpent animal”.
Most Hydrazoa live in marine environments and live in
colonies.
Occasionally Hydrazoa live in freshwater and are
solitary.
Most commonly polyps reproduce asexually and
medusa reproduce sexually
Examples of genera include; Hydra, Obelia, Physalia,
Hydra and Obelia
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Hydra are freshwater
cnidarians that are
solitary and exist in the
polyp body form
Obelia often live in
colonies and exist in
both the polyp and
medusa body form
Solitary Hydra on left
and Obelia colony on
the right
Physalia (Portugese Man-of-War)
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Exist as both medusa
and polyp forms.
These Hydrazoans
form floating colonies.
The nematocysts
secrete a powerful
neurotoxin that can
inflict painful stings.
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This portugese man-ofwar is actually a colony
of hydrazoans acting
as one unit.
Class Scyphozoa (si-fo-zo'a)
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Name means “cup animal”
Includes most of the large common jellyfish
Solitary animals
All scyphozoans are marine
Polyp stage is sometimes absent
Scyphozoans have a bell-shaped medusa filled with
an enlarged mesoglea and they lack a velum.
The velum is a circular membrane surrounding the
cap of a medusa that aids in swimming.
Examples of Scyphozoans genera include; Aurelia,
Cyanea, Cassiopeia, and Rhizostoma
Aurelia and Cyanea
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Aurelia on top “moon jellyfish”.
Feeds on plankton caught in
mucus on its umbrella
Cyanea capillata on bottom.
Called “Sea blubber” by fishermen.
Aka Giant jellyfish or Lion's mane
jellyfish) with a bell diameter of 2
meters and tentacles that can
reach up to 60 or 70 meters long.
Class Cubozoa (ku'bo-zo'a)
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Name means a “cube-shaped animal”.
Solitary animals.
Very little is known about the life cycle of cubozoans.
Medusa are the primary body form.
The bell-shaped medusa have a square or cube-shaped
cross section.
Examples of cubozoa include; Carybdea, Tripedalia,
Chironex, and Chiropsalmus
Class Cubozoa
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Box Jellyfish
(Carybdea
marsupialis)
The Sea Wasp Box Jellyfish (Chironex fleckeri)
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Stings are very painful, dangerous and
sometimes fatal. Found in the waters of
Australia.
Class Anthozoa (an-tho-zo'a)
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Name means “flower animal”.
All polyps, no medusa.
All marine, some colonial, some solitary.
Found in deep, shallow, tropical and polar seas.
Three different subclasses.
Zoantharia – Sea anemones and hard corals
Ceriantipatharia – Tube anemones and thorny corals
Octocorallia – Soft and horny corals such as sea
pens, sea fans, and sea pansies. Corals with an
octomerous arrangement.
Class Anthozoa
Subclass Zoantharia
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Sea Anemones and Hard Corals
Class Anthozoa
Subclass Ceriantipatharia
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Tube Anemones and Thorny Corals
Class Anthozoa
Subclass Octocorallia
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Soft and horny corals
Octocorallians are octomerous meaning they usually have an
arrangement of eight tentacles arranged around the mouth.
Phylum Ctenophora (te-nof'o-ra)
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Name means comb bearing.
Ctenophorans are called “comb jellies”.
All marine, less than 100 species known.
Live mostly in warm, tropical waters.
Eight rows of comblike plates for locomotion.
Radial symmetry like Cnidarians.
Most Ctenophores do not have nematocysts like
Cnidarians. Instead they use sticky glue-like cells called
colloblasts to capture prey.
Comb jellies are also bioluminescent.
Ctenophora (Comb Jellies)