Cnidarians – Stinging
Celled Animals
Section 28.2
Body Structure
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Radial symmetry
Two layers of cells that form tissues:
 Outer layer from the ectoderm
 Inner layer from the endoderm
Tentacles surround the opening to the gastrovascular
cavity
 Contain cnidocytes – stinging cells – with
nematocysts that have toxins for defense or stinging
prey
Tentacles draw prey into gastrovascular cavity
Food digestion begins with enzymes in the
gastrovascular cavity (extracellular digestion) and ends
inside the cells
Wastes leave through the same opening
Two Body Forms
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2.
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Medusa
Free-floating
Jellylike
Usually umbrella-shaped
Polyp
Tube-like
Attached to rock or other object
Some cnidarians spend whole life in one form
or the other; Other cnidarians alternate
between the two forms at different stages of
their life cycle
Hydrozoans
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The most primitive cnidarians
May be marine or freshwater
Life cycle includes both medusa and polyp
Includes:
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Hydra: individual polyp
Portuguese man-of-war: colony of both medusas
and polyps
Scyphozoans
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Often called true jellyfish
Active predators in the medusa stage
Many also have a small polyp stage
Relatives include:
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Box jellies: with a cube shaped medusa
Comb jellies: tentacles have no nematocysts, only
a sticky substance to trap prey
Anthozoans
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The largest class of Cnidarians
Exist only as polyps
Sea anemones and corals
Many live in a symbiotic relationship with
algae