Cnidarians
Kingdom: Animal
Phylum: Cnidaria
Class: Hydrozoa (Hydra & Portuguese man-of-war)
Class: Scyphozoa (Jellyfish)
Class: Anthozoa (Sea anemone, corals)
Phylum Characteristics:
General Characteristics
9000+ species
Marine invertebrates
All aquatic
o Mostly marine, only a few live in fresh water
Body Structure
Radial Symmetry
2 cell layers (transparent), organized into separate tissues
with specific functions
Tentacles surround mouth
Nemocysts (stinging cells) at ends of tentacles
Dissolved oxygen in water enters cells directly (only 2 layers
of cells)
Life cycle
Polyp : tube shape body, mouth surrounded by tentacles,
usually sessile stage
Medusa : body shaped like an umbrella, tentacles hang
down, mobile stage
Digestion
Predators
o plankton, small invertebrates (esp. crustaceans), plastic
o larger jelly fish and sea anemone will eat small fish
Use tentacles with nematocysts to stun/poison prey &
tentacles contract and bring prey into mouth digested in
gastrovascular cavity
Gastrovascular Cavity
o Lining of interior cavity has specialized cells, release
enzymes for digestion over prey
Gas Exchange
Diffusion from water inside and outside organism
Nervous system
Nerve net: no control center (no brain), conducts nerve
impulses from all over body, contraction of tentacles
o Eating & swimming
Skeleton
Some from on outer covering of chitin (partial or nearly
complete, open at oral end)
Reproduction
Asexually : budding, either polyp or medusa stage
Sexually : medusa stage only
Classes
Hydrozoa (no internal division in gastrovascular cavity)
Group: Hydroids
o Hydra
polyp stage dominant
Group: Siphonophores
o Portuguese Man-of-war
Floating colonies
Scyphozoa (four divisions in gastrovascular cavity)
Jelly fish
o Medusa stage dominant
o Found in all oceans, some as deep as 1000m
Anthozoa (division in gastrovascular cavities)
Help build coral reefs
Only polyp stage
Sea anemones
o thought to live for centuries
o live in all oceans (tropical, temperate, artic)
Corals
o Warm ocean water
o Secrete cuplike calcium carbonate, protection
o When polyp dies leaves limestone skeleton
o Adds to the reef, 2mm per year