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Phylum Cnidaria
Phylum Cnidaria – “stinging cell”
• A.k.a Coelenterata (hollow cavity)
– Radiata (radial symmetry)
Jelly Fishes
Sea Anemones
Corals
One step above sponges…
• Differ from sponges - have
rudimentary organs
• Incomplete digestive & excretory
systems
• No blood or circulatory system
Classification
• 3 Classes
– Hydrozoa = Hydra
– Scyphozoa = Jellyfish
– Anthozoa = Sea anemones & coral
Morphology
• Body wall - 3 layers
• Epidermis (outside)
• Mesoglea (inner
transluscent jelly-like
material)
• Gastrodermis (inside
lining)
Skeletal Structure
• Corals secrete calcium carbonate to
build exoskeleton
• Most sea anemones and hydra use
water pressure from inside body
cavity (hydrostatic pressure) to
maintain body form
Body Features
• Mouth usually surrounded by tentacles
(radial symmetry)
– Tentacles covered in cnidocytes = specialized
stinging cells
• Nematocysts = Shoots like a harpoon from
cnidocyte. Contains deadly toxin.
• Gastrovascular cavity – Hollow digestive
cavity
– Mouth serves as entrance/exit point
Nervous System
• No brain
• Nerve net
– Capable of responding
to stimuli
Habitat
• Aquatic, mostly marine
– Hydra – Freshwater, independent, free
swimming
– Jellyfish – Marine, independent, free
swimming
– Coral & Anemone – Marine, sessile, colonial
Body Forms
• Polyp – present in all 3 classes
• Hollow tube with tentacles
• Can be free swimming (hydrozoa, scyphozoa)
or sessile (anthozoa)
Body Forms
• Medusa – only present
in hydrozoa and
scyphozoa
– Free swimming
• Umbrella shaped with
tentacles
• Mesoglea constitutes
bulk
Reproduction
• Hydrozoa & Scyphozoa –
– Asexual reproduction in polyp form by budding
– Sexual reproduction in medusa form
• Anthozoa –
– No medusa form, but can reproduce asexually
(budding) or sexually
Close-up of
Hydrozaon polyp
Hydrozoan medusa