Phylum Cnidaria includes jellyfish, corals, hydras and other organisms. They have simple diploblastic bodies with radial symmetry. They possess stinging capsules called cnidocytes that aid in defense and prey capture. Their bodies include a gastrovascular cavity with one opening that functions as both a mouth and anus. They exist either as polyps, which are cylindrical and attach to substrates, or medusae, which are bell-shaped and float freely. Some alternate between these forms while others possess only one. The four main classes are Hydrozoa, Scyphozoa, Cubozoa and Anthozoa, which differ in their predominant life stages and habitats.

1. Phylum Cnidaria
A Powerpoint Presentation by
Sophia DiCamillo
and
Will Guiteras

2. Characterization of Cnidaria
● Member of the clade Eumetazoa (true tissues)
● Diversified into range of both sessile and
floating forms
● These include:
○ Jellies
○ Corals
○ Hydras
● Simple, diploblastic bodies; radial symmetry
● Carnivorous
● No brain, rather a nerve net

3. Gastrovascular Cavity
● Definition: A sack with a central digestive
compartment
● A single opening to this cavity serves of dual
function of cavity and anus
● Two types:
○ Sessile Polyp
○ Floating Medusa

4. Gastrovascular Cavity (cont.)
● Polyps: Cylindrical forms that adhere to the
substrate by the aboral end (opposite of
mouth) of the body and extends their
tentacles waiting for prey
○ Example: Hydra
○ Example: Sea anemone
● Medusa: Flattened mouth-down version of
a polyp. Moves freely in the water by a
combination of passive drifting and
contractions of its bell-shaped body.
○ Example: Jellyfish
● Some Cnidarians are just one, while others
have both stages

5. Polyp vs Medusa

6. Cnidocyctes
● Definition: Unique cells that function in
defense and the capture of prey
● Contain cnidae, capsule-like organelles that
are capable of everting and that give the
phylum its name
● Cnidae called nematocysts are stinging
capsules
● Nerves and tissues occur in simplest form
● epidermis - outer layer
● gastrodermis - inner layer

7. Cnidocytes Diagram

8. Class Hydrozoa
● Most alternate between medusa and polyp
form
● Hydras are unusual hydrozoans because
○ Only polyps
○ Found in freshwater
○ Reproduce by budding (asexual)

9. Class Scyphozoa
● Medusa is predominant stage of
scyphozoa
● Coastal scypozoans go through a
small polyp stage
● Open ocean scyphozoans lack the
polyp stage
○ Example: Jellyfish

10. Class Cubozoa
● Name means "cube animals"
● Box-shaped medusa stage
● Complex eyes
● Generally live in tropical oceans
● They are equipped with highly toxic
cnidocytes
○ Example: Sea wasp, one of the deadliest organisms
on Earth
○ The amount of poison in a sea wasp can kill 60
people

11. Class Anthozoa
● Means "flower animals"
○ Exmple: Sea anemone
○ Example: Coral
● Occur exclusively as polyps
● Corals secrete a hard external skeleton of
calcium carbonate; each polyp generation
builds on the skeltal remains of earlier
generations constructing rocks
● Coral reefs provide habitat for a wealth of
other species

12. Cubazoa (box jelliies, sea wasps) All marine; No medusa stage; mostly
Sessile and Colonial