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Phylum Cnidaria
Hydra, Jellyfish, Sea Anemones,
& Corals
Phylum Cnidaria
1. Soft bodied animals with stinging tentacles
arranged in circles around the mouth
2. Radial Symmetry
3. Specialized cells and tissues, including nervous
system
4. Lifecycles often include two different stages:
sessile polyp and motile medusa
5. 3 Body layers: Ectoderm, Mesoderm, Endoderm
6. Nematocysts – stingers
7. Only one internal cavity, the digestive cavity, with
a mouth, but no anus.
Life Cycle & Polymorphism
 Two basic types
of individuals:
polyps and
medusae.
 Dominance of
Medusa or Polyp
lifecycle varies
species to
species.
General Body Structure
Body Plan
Polyp Stage of Life Cycle
Polyp = adapted
for sedentary or
sessile life.
Tubular body w/
mouth at one end
surrounded by
tentacles.
May live singly or
in colonies.
Colonial polyps
Medusa
 Medusa = jellyfish form, free swimming and sexually
mature form,
bell-shaped or umbrella-shaped bodies,
 Medusa provides dispersal mechanism so don't
compete with parents.
 Mesoglea much thicker than in polyp - constitutes bulk
of animal - makes it buoyant = "jelly" of jellyfish.
Medusa
Nematocysts
1. Hold prey
2. Sticky -- aid in locomotion by attaching tentacles
3. Penetrate and anchor in prey and poison it.
Nematocyst
Nematocysts can be Pretty
Nematocyst poison
Anemone catches goby
 Most dangerous to man is Cubozoan jellyfish, the
sea wasp (Chironex fleckeri). Caused more human
suffering and death off Australian coasts than
Physalia (Portuguese man-of-war) has in any of its
home waters.
 Symptoms range from:
1. burning pain at site of contact
2. lesions and eruptions of various sorts, often severe
enough to leave scars,
3. great pain, fever, and respiratory interference.
4. severe reactions due to shock & allergies
5. "Sting" of most is imperceptible to humans. Problems
usually occur only when repeatedly stung.
Feeding
Tentacles push food through the
mouth and into the gastrovascular
cavity to be broken into small pieces.
Special cells in the gastroderm digest
the food further.
Nutrients travel through body by
diffusion (high  low concentration).
Undigested materials exit the mouth.
Hydra Feeding
Feeding (Symbiotic Relationship)
Cnidarians have a close relationship
with unicellular photosynthetic
partners.
The relationship is widespread and
occurs in many shallow-water
cnidarians.
The photosynthetic protists grow
inside the living cells of the
gastroderm.
The protist takes the CO2 and
metabolic wastes produced by
cnidarians to manufacture oxygen and
other organic compounds.
(photosynthesis)
Respiration and Elimination
 Respire and eliminate
wastes by diffusion
through their body
walls (no organized,
internal excretory
system).
Nervous System
Response
 Lack a centralized nervous system and brain
 Have simple nerves called nerve nets that are
concentrated around the mouth
 Specialized sensory cells transmit information about the
environment to the rest of the nervous system
 Ocelli and statocysts found around the rim of the
medusa’s cell, are groups of sensory cells or simple
organs
 Statocysts help with balance and in determining which
way is up
 Ocelli, or eyespots, detect light
Movement
Sea Anemone Running
Do not have muscle cells
Have epidermal cells that are able to
change shape, controlled by the nervous
system
“Move” by relaxing or contracting
epidermal cells
Contraction of epidermal cells causes the
medusae’s body to close  pushes water
out  jet propulsion
Movement
Reproduction
Moon Jelly life Cycle
 Most can reproduce sexually and asexually
 Polyps can reproduce asexually by budding. Budding
beings with a swelling on the side of the polyp.
 Polyps also bud off tiny polyp .
 Mature medusae reproduce sexually by releasing
gametes into the water.
 Depending on the species, fertilization occurs either in
open water or inside egg-carrying medusa
 zygote grows into a swimming, ciliated larva (Planula)
 Swimming larva then attaches itself to a substrate
 After settling down on a hard surface, changes into
polyp
 Starts a new colony through asexual budding.
Class Hydrozoa
 Short medusa and long polyp stage
 Most polyps grow in branching colonies
(each have specific function)
Exceptional Hydrozoa - Hydra
 Most common in the class
Hydrozoa
 Has no medusa stage
 Locomotion
Can move around by doing
somersaulting movements
 Reproduction
Asexual or sexual
 Sexual produce eggs and
sperm in body wall
• Can be male or female or both
(hermaphrodite)
The Portuguese Man o War wasn’t
just a Ship
Exceptional Hydrozoa – Portuguese
Man-of-War
 Form floating colonies
containing many
specialized types of
polyps
 Types of polyps:
Float
Stinging tentacles 
can sting swimmers
badly
Digestive
Reproductive
Physalia from Jamaica
Portuguese
Man O War
A tenacious
hunter with a
taste for fish
and the poison
to render most
prey helpless
Yikes!
Portuguese Man 'O War
Physalia physalia
Size: to 1 ft (float) and 50 ft (tentacles)
Habitat: floats at the surface
Notes: dangerous
Fortunately, this creature, with its powerful battery of stinging
tentacles, is more common in bays and protected waters than in the
open ocean. The sting of the Man'O'War can vary from extremely
painful to incapacitating to fatal, depending on the severity and the
victim's reaction.
Jellyfish sting to defend themselves.
The Man of War is poisonous to humans.
When a cnidarian
reproduces, the
young hatch into
the mother’s
stomach, then, she
spits them out
Class Scyphozoa – Jellyfish
 Same life cycle as
hydrozoans
 Much longer medusa
stage than polyp
stage
 Most nematocysts are
harmless to humans,
but some may be
painful when stung
Moon Jellyfish
Beached Moon Jellies: the surf has stripped away all the tentacles,
leaving just a harmless dying blob of goo.
Red or
Lion's Mane
Jelly
Avoid these.
Swim away
– swim away
NOW! !
Class Anthozoa – Sea Anemones
& Corals
 Only have polyp stage
 Reproduction
Asexual and sexual
 Sexually – eggs and sperm
released into water
• Zygote becomes ciliated
larva then settles
 Sea anemones
Can live at a range of
depths in the sea
 Corals
Live in shallow tropical water
regions
A closer look on corals
 Produce calcium
carbonate or
limestone skeletons
 Usually colonial
 Many colonies make
coral reefs
 ex. Great barrier Reef
Clown fish live in
anemones, and coral
and they’re immune to
stings!
Fire Coral and Tube Anemone
Ecological Importance
 Cnidarians provide habitat to shrimp, fish, and other small
animals
 Cnidarians have important symbiotic relationships with
other organisms. IE: Clownfish-Anemone, CoralZooxanthellae (algae).
 Medical research -cnidarian nerve toxin is used to help
humans understand nerve cell function.
 Coral reefs: corals are builders of some of the richest and
most complex ecosystems.
 Coral reefs provide protection from wave action.
 Corals are sensitive to changes in light, temperature,
water quality, and salinity. Consequently, coral fossils
provide scientists with information that can be used to
interpret climate and geography of past environments.
 Natural and human impact on coral:
Sponge Bob Jelly Fish Song