The phylum Cnidaria includes sea jellies, hydra, coral colonies, sea anemones, sea pens, and sea fans. They are characterized by having radial symmetry, two germ layers, a jelly-like mesoglea layer, an acoelomate body cavity, and specialized stinging cells called cnidocytes. Many species have both a sessile polyp stage and a free-swimming medusa stage. Cnidarians play important ecological roles through food sources, habitat, symbiosis, and wave protection.

1. Phylum Cnidaria
SEA JELLIES, HYDRA, CORAL
COLONIES, SEA ANEMONES

2. Sea Jellies

3. Hydra

4. Coral Colonies

5. Brain coral

6. Sea Anemones

7. Sea Pens

8. Sea Fans

9. Phylum Cnidaria
Over 10,000 known species
Microscopic – 4 meter diameter
Solitary or colonial

10. Phylum Cnidaria Characteristics
Body Type: Radial Symmetry
Body Organization: Diploblastic
Two germ layers (endoderm and
ectoderm)
Mesoglea: Jelly layer between endo
and ectoderm, not a true tissue layer
Body Cavity: Acoelomate

11. Phylum Cnidaria Characteristics
Body Plan: Many have both body plans
in their life cycle
Polyp – Attached to substrate, sessile
Medusa – Free swimming

12. Phylum Cnidaria Characteristics
Ecological Roles
Food source
Home/shelter
Symbiosis with algae/protists
Wave break to protect shore lines

14. Phylum Cnidaria Characteristics
Digestive System
Sac body: Single
opening (mouth/anus)
Central gastrovascular
cavity
Specialized stinging
cells (cnidocytes)
contain coiled fibre
(nematocyst)trap or
paralyze prey

16. This Phylum has nematocysts for feeding &
defense:

17. Phylum Cnidaria Characteristics
Reproduction
Sexual
Mostly dioecious (sexes separate)
External fertilization
Egg and sperm form planula larva
Asexual
Budding

19. Phylum Cnidaria Characteristics
Nervous System
Display coordinated movement
Nerve net over entire body
Sensory cells concentrated near mouth
detect light, chemicals, or touch.

20. Phylum Cnidaria Characteristics
Circulation & Respiration: Open system
Body contractions create currents to
circulate materials through GV cavity.
Diffusion moves substances in and out of
cells
Excretion: Solid wastes expelled through
mouth opening

21. Classification of Cnidarians
Class Schyphozoa “Jelly Fish”
Majority of life cycle as free swimming
medusa form
Ex. Aurelia aurita
(Moon Jelly)

22. Classification of Cnidarians
Class Anthozoa Anemones, Corals
No medusa stage.
Polyps solitary or colonial
All marine
Ex. Anthoplera elegantissima
(Aggregating Anemone)

23. Corals
Corals are colonies of polyps contained
in a hard calcium carbonate skeleton

24. Classification of Cnidarians
Class Hydrozoa Hydroids
Alternation of generations between polyp
and medusa
Colonial Polyps - Polymorphic
Ex. Obelia sp. / Physalia physalis
 Man of war jellyfish