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Phylum Cnidaria. Hydra, Jellyfish, Sea Anemones, & Corals. Phylum Cnidaria. Soft bodied animals with stinging tentacles arranged in circles around the mouth Radial Symmetry Specialized cells and tissues, including nervous system

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phylum cnidaria

Phylum Cnidaria

Hydra, Jellyfish, Sea Anemones, & Corals

phylum cnidaria1
Phylum Cnidaria
  • Soft bodied animals with stinging tentacles arranged in circles around the mouth
  • Radial Symmetry
  • Specialized cells and tissues, including nervous system
  • Lifecycles often include two different stages: sessile polyp andmotile medusa
  • 3 Body layers: Ectoderm, Mesoderm, Endoderm
  • Nematocysts – stingers
  • Only one internal cavity, the digestive cavity, with a mouth, but no anus.
life cycle polymorphism
Life Cycle & Polymorphism
  • Two basic types of individuals: polyps and medusae.
  • Dominance of Medusa or Polyp lifecycle varies species to species.

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.
  • 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.
  • Hold prey
  • Sticky -- aid in locomotion by attaching tentacles
  • Penetrate and anchor in prey and poison it.
nematocyst poison
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:
    • burning pain at site of contact
    • lesions and eruptions of various sorts, often severe enough to leave scars,
    • great pain, fever, and respiratory interference.
    • severe reactions due to shock & allergies
    • "Sting" of most is imperceptible to humans. Problems usually occur only when repeatedly stung.
  • 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.
feeding symbiotic relationship
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.


respiration and elimination
Respiration and Elimination
  • Respire and eliminate wastes by diffusion through their body walls (no organized, internal excretory system).
  • 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

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

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
Class Hydrozoa
  • Short medusa and long polyp stage
  • Most polyps grow in branching colonies (each have specific function)
exceptional hydrozoa hydra
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)
exceptional hydrozoa portuguese man of war
Exceptional Hydrozoa – PortugueseMan-of-War
  • Form floating colonies containing many specialized types of polyps
  • Types of polyps:
    • Float
    • Stinging tentacles  can sting swimmers badly
    • Digestive
    • Reproductive
portuguese man o war
Portuguese Man O War

A tenacious hunter with a taste for fish and the poison to render most prey helpless



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 younghatch into the mother’s stomach, then, she spits them out

class scyphozoa jellyfish
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

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
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
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!

ecological importance
Ecological Importance
  • Cnidarians provide habitat to shrimp, fish, and other small animals
  • Cnidarians have important symbiotic relationships with other organisms. IE: Clownfish-Anemone, Coral-Zooxanthellae (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: