Phylum Cnidaria
Jelly Fishes
Sea Anemones
Corals
Phylum Cnidaria
• A.k.a Coelenterata or Radiata
• Differ from sponges - have
rudimentary organs
Morphology
• Body wall - 3
layers
• Epidermis
• Gastrodermis
• Mesoglea
Body plans
• Sessile - Polyp (hydranth or
hydroid)
• Cylindrical stalk and oral end with tentacles
Body Plan
• Free swimming Medusa
• Bell shaped
• Mesoglea
constitutes
bulk
Cell Types
• Epithelio-muscle cells
• Cover and protect
• Can contract (muscular properties)
• Interstitial cells
• Production of gamets
• Mucous secreting cells (Gland cells)
• Nerve cells
• Cnidocytes
Cnidocyte
• On Tentacles
• Defense and feeding
• Nematocyst
• Cnidocil - trigger
Nervous System
• First time
• Nerve net
• 2 nets
• Associated with epidermis and gastrodermis
Classification
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3 Classes
Hydrozoa
Scyphozoa
Anthozoa
Hydrozoa
• Polymorphic
• Polyp - Hydra
• Medusa - Obelia
• Simple gastrovascular cavity - no
septa
• No nematocysts in digestive cavity
• Solitary or colonial
Close-up of Hydroid
Hydroid medusa
Composition of Colonies
• 3 types of individuals
• Gastrozooid - responsible for
feeding also numerous
• Gonozooid - sexual reproduction
• Dactylozooids - defense and food
capture
Life Cycles
• 3 basic types
• Obelia - both polyp and medusa
stage
• Hydra - only polyp
– Asexual reproduction - budding
• Physalia - Colony attached to float
called Pneumatophore
Scyphozoa
• Exclusively marine
• Medusa dominant stage
• Much larger in size compared to
hydrozoan medusae
Scyphozoa
• Mesoglea makes up large portion of
body
• Gastrovascular cavity divided by septa
• complex system of canals in medusa to
help move food and oxygen
• Muscular system on outer edge of
medusa
• Very toxic - can kill in 30 secs or less
Sense organs
• 8 compound marginal sense
organs
– 2 sensory pits - chemoreception
– 1 ocellus (eye spot) - light sensing
– tactile structures - touch sensitive
– 1 statocyst - orientation and balance
Reproduction
Anthozoa
• None have medusa
- Only Polyp
• Hence Sessile
• Several Septa in
Gastro-vascular
cavity
• Attached to
substrate by Pedal
Disk
Phylum - Ctenophora
Combjellies or Sea Walnuts
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Offshoot of Medusa like Cnidarian
Transparent animals
Never colonial
Free swimming
No nematocysts instead have
COLLOBLASTS – adhesive cells
Characteristics
• 8 rows of fused cilia
(combs)
• Nervous system –
nerve net
• Single sense organ
– Statocyst
• Cydippid larva
• Bioluminescence