Characteristics of Phylum Cnidaria
• Radial Symmetry; typically quadramous
• Diploblastic (two germ layers during development) –
ectoderm and endoderm
• Adult body with an outer cellular layer (epidermis or
ectodermis) and an inner cellular layer (gastrodermis)
with a non-cellular layer (mesoglea) in between
• Tissue level of organization – some specialized tissues
present
• Possess unique stinging or adhesive structures (cnidae)
found in specialized cells (cnidocytes); most common
cnidae called nematocysts
Characteristics of Phylum Cnidaria
• Musculature formed largely by myoepithelial
cells (=epitheliomuscular cells)
• Simple nerve net forms a “nervous system”
• Alternation of generation with an asexually
reproducing polyp stage and a sexually
reproducing medusa stage; variations of this
strategy exist throughout the Cnidaria
• Typically have a ciliated, motile planula larvae
Phylum Cnidaria
• Two adult forms in life cycle
– Polyp
– Medusa
• Alternation of Generation
Hydra - Representative Cnidarian
Reproduction
• Asexually by budding
– Only in polyp stage
• Sexual Reproduction
– In polyp if only stage
– In medusa if alternation of generation
– In medusa if is only stage
Alternation of Generation
Class Hydrozoa
• When alternation of generation polyp
dominant stage
• Many have polyp only
• Solitary and colonial
• Specialized polyps in colonial and A of G
Polyp Stage Only
Medusa Stage Only
Gonionemus
Class Scyphozoa
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When AG, medusa is dominant stage
Many are medusa only
None are polyp only
Large medusa, 2 – 40 cm; one species
reaches 2 m diameter
• Some capable of lethal stings
Class Anthozoa
• Exclusively polyp in life cycle
• Largest group - >6,000 species
• Soltary and colonial polyps
Cnidarian Evolution
• Which came first, the polyp or medusa?
• Brooks (1886) proposed a theory still
widely accepted
• Ancestral cnidarian was a medusa
– Evolved planula
– Evolved Actinula larva
Actinula Larva
• The actinula took up a benthic existence
• Why would it do that?
– Extend larval life
– Exploit new food supply
– Enhance capacity for asexual reproduction
• Suppression of the medusa in Hydrozoa
• Later colonialism and polymorphism
Scyphozoa and Anthozoa
• Probably a common ancestor. Both have
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Cells in mesoglea
Cnidocytes in gastrodermis
Septate gastrovascular cavity
Gastrodermal gonds
All are lacking in hydrozoa
• Scyphozoa retained the dominant medusa stage
or lost polyp stage (why?)
• Anthozoa suppressed medusa, colonialism
evolved later (solitary fossils)
Velalla – sail-by-the-wind sailor